NormalizeHEADmain

49 files changed, 17 insertions, 4905 deletions

diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..d7b82bc
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,4 @@
+*.txt text eol=lf
+*.htm text eol=lf
+*.html text eol=lf
+*.md text eol=lf
diff --git a/LICENSE.txt b/LICENSE.txt
new file mode 100644
index 0000000..6312041
--- /dev/null
+++ b/LICENSE.txt
@@ -0,0 +1,11 @@
+This eBook, including all associated images, markup, improvements,
+metadata, and any other content or labor, has been confirmed to be
+in the PUBLIC DOMAIN IN THE UNITED STATES.
+
+Procedures for determining public domain status are described in
+the "Copyright How-To" at https://www.gutenberg.org.
+
+No investigation has been made concerning possible copyrights in
+jurisdictions other than the United States. Anyone seeking to utilize
+this eBook outside of the United States should confirm copyright
+status under the laws that apply to them.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..7b3a948
--- /dev/null
+++ b/README.md
@@ -0,0 +1,2 @@
+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #65512 (https://www.gutenberg.org/ebooks/65512)
diff --git a/old/65512-0.txt b/old/65512-0.txt
deleted file mode 100644
index 4aefb86..0000000
--- a/old/65512-0.txt
+++ /dev/null
@@ -1,2273 +0,0 @@
-The Project Gutenberg eBook of Lasers, by Hal Hellman
-
-This eBook is for the use of anyone anywhere in the United States and
-most other parts of the world at no cost and with almost no restrictions
-whatsoever. You may copy it, give it away or re-use it under the terms
-of the Project Gutenberg License included with this eBook or online at
-www.gutenberg.org. If you are not located in the United States, you
-will have to check the laws of the country where you are located before
-using this eBook.
-
-Title: Lasers
-
-Author: Hal Hellman
-
-Release Date: June 4, 2021 [eBook #65512]
-
-Language: English
-
-Produced by: Stephen Hutcheson and the Online Distributed Proofreading
-             Team at https://www.pgdp.net 
-
-*** START OF THE PROJECT GUTENBERG EBOOK LASERS ***
-
-
-
-
-
-                                 Lasers
-
-
-                             by Hal Hellman
-
-
-                     U.S. ATOMIC ENERGY COMMISSION
-                   Division of Technical Information
-                    _Understanding the Atom Series_
-
-                        ATOMIC ENERGY COMMISSION
-                        UNITED STATES OF AMERICA
-
-
-The Understanding the Atom Series
-
-Nuclear energy is playing a vital role in the life of every man, woman,
-and child in the United States today. In the years ahead it will affect
-increasingly all the peoples of the earth. It is essential that all
-Americans gain an understanding of this vital force if they are to
-discharge thoughtfully their responsibilities as citizens and if they
-are to realize fully the myriad benefits that nuclear energy offers
-them.
-
-The United States Atomic Energy Commission provides this booklet to help
-you achieve such understanding.
-
-                                                  {Edward J. Brunenkant}
-                                          Edward J. Brunenkant, Director
-                                       Division of Technical Information
-
-  UNITED STATES ATOMIC ENERGY COMMISSION
-  Dr. Glenn T. Seaborg, Chairman
-  James T. Ramey
-  Wilfrid E. Johnson
-  Dr. Clarence E. Larson
-
-    [Illustration: LASERS]
-
-                                                          by Hal Hellman
-
-
-
-
-                                CONTENTS
-
-
-  INTRODUCTION                                                         1
-  THE ELECTROMAGNETIC SPECTRUM                                         5
-  RADIO WAVES                                                          9
-  LIGHT AND THE ATOM                                                  14
-  WHAT’S SO SPECIAL ABOUT COHERENT LIGHT?                             19
-  CONTROLLED EMISSION                                                 25
-  A LASER IS BORN                                                     29
-  LASING—A NEW WORD                                                   32
-  SOME INTERESTING APPLICATIONS                                       34
-  A MULTITUDE OF LASERS                                               42
-  COMMUNICATIONS                                                      48
-  A LASER IN YOUR FUTURE?                                             52
-  SUGGESTED REFERENCES                                                53
-
-
-                 United States Atomic Energy Commission
-                   Division of Technical Information
-
-           Library of Congress Catalog Card Number: 68-60742
-                            1968; 1969(rev.)
-
-    [Illustration: _Nothing about lasers is more astonishing than their
-    ability to produce holograms, under arrangements such as shown
-    above. Two laser beams (of different colors) emerge from the curtain
-    (rear). They are optically combined (left center) and the combined
-    beam is then divided by prisms, mirrors and lenses so that part of
-    it shines on the figurines (foreground) and part on the square
-    holographic plate (right center). When the plate is developed (like
-    an ordinary photographic film), it will seem to have only a dull
-    gray surface until it is viewed with spatially coherent light (such
-    as from a laser or a beam through a pinhole) shining through it.
-    Then an amazing, multi-colored, three-dimensional image of the
-    figurines will be visible. (See page 19 and Figure 13.)_]
-
-    [Illustration: LASERS]
-
-                                                          By HAL HELLMAN
-
-
-
-
-                              INTRODUCTION
-
-
-The transistor burst upon the electronic scene in the 1950s. Almost
-overnight the size of new models of radios, television sets, and a host
-of other electronic devices shrank like deflating balloons. Suddenly the
-hard-of-hearing could carry their sound amplifiers in their ears.
-Teenagers could listen to favorite music wherever they went. Everywhere
-we turned the transistor was making its mark. There was even a proposal
-before Congress to require that every home have a transistor radio in
-case of emergency.
-
-The next development to fire the imagination of scientists and engineers
-was the laser—an instrument that produces an enormously intense
-pencil-thin beam of light. Most of us have heard so much about this
-invention it seems hard to believe that the first one was built only a
-few years ago. We were told that the laser was going to have an even
-greater effect on our lives than the transistor. It was going to replace
-everything from dentists’ drills to electric wires. The whole world, it
-seemed, eventually would be nothing but a gigantic collection of lasers
-that would do everything anyone wanted. Roads would be blazed through
-jungles at one sweep; our country would be safe once and for all from
-intercontinental ballistic missiles; cancer would be licked; computers
-would be small enough to carry in a purse; and so on and on.
-
-Yet for the first couple of years the laser seemed able to do nothing
-but blaze holes in razor blades for TV commercials. Somehow the device
-never seemed to emerge from the laboratory, prompting one cynic to call
-it “an invention in search of an application”.
-
-Many of the wild claims came from misunderstandings on the part of the
-press, others from exaggerations by a few manufacturers who wanted free
-publicity. But with even less exotic devices than lasers, the road from
-the laboratory to the marketplace may often be long and hard. Price,
-efficiency, reliability, convenience—these are all factors that must be
-considered. It soon became clear that with something as new as the
-laser, much improvement was necessary before it could be used in science
-and medicine, and even more before it could be used in industry.
-
-It now seems, however, that the turning point has been reached. We have
-seen laser equipment put on the market for performing delicate surgery
-on the eye, spot-welding tiny electronic circuits (Figure 1), and
-controlling machine tools with amazing accuracy (Figure 2).
-
-    [Illustration: Figure 1 _A commercial laser microwelder. A
-    microscope is needed for accurate placement of beam energy._]
-
-The pace is quickening. At least a dozen manufacturers have announced
-that they are designing laser technology into their products. These are
-not laboratory experiments but practical products for measurement and
-testing, and for industrial, military, medical, and space uses. The
-Army, for example, has announced that it will purchase its first
-equipment for use in the field: a portable, highly accurate range finder
-for artillery observation.
-
-Still, this hardly accounts for the $100,000,000 spent in one recent
-year on laser research and development by some 500 laboratories in the
-United States. The U. S. Government alone has spent about $25,000,000 on
-laser research in a single year. Dozens, and perhaps hundreds, of other
-applications are on the fire—simmering or boiling as the case may be.
-Some require particular technical innovations such as greater power or
-higher efficiency. Others are entirely new applications. One of the most
-exciting of these is holography (pronounced ho LOG ra phy).
-
-Holography involves a completely different approach to photography. In
-addition to more immediate applications in microscopy, information
-storage and retrieval, and interferometry, it promises such bonuses as
-3-dimensional color movies and TV someday.
-
-You have to see the holographic process in operation to believe it. One
-moment you are looking at what appears to be an underexposed or lightly
-smudged photographic plate. Then suddenly a true-to-life image of the
-original object springs into being behind the negative—apparently
-suspended in midair! Not only is the full effect of “roundness” and
-depth there, but you can also see anything lying behind the object’s
-image by moving your head, exactly as if the original scene containing
-the object were really there.
-
-Still another important field of application is that of communications.
-Perhaps because it is less spectacular than burning holes in razor
-blades, we haven’t heard as much about it. Yet there are probably more
-physicists and engineers working on adapting the laser for use in
-communications than on any other single laser project.
-
-The reason for this is the fact that existing communications facilities
-are becoming overloaded. Space on transoceanic telephone lines is
-already at a premium, with waiting periods sometimes running into hours.
-Radio “ham” operators have been threatened with loss of some of their
-best operating frequencies to meet the demand of emerging nations of
-Africa for new channels. Television programs must compete for space on
-cross-country networks with telephone, telegraph, and transmission of
-data. The increasing use of computers in science, business, and industry
-will strain our facilities still further. Communication satellites will
-help, but they will not give us the whole answer; and much development
-work remains to be done on satellites.
-
-    [Illustration: Figure 2 _Precision control of a machine tool by
-    laser light._]
-
-Why the interest in the laser for communications? In a recent experiment
-all seven of the New York TV channels were transmitted over a single
-laser beam. In terms of telephone conversations, one laser system could
-theoretically carry 800,000,000 conversations—four for each person in
-the United States.
-
-In this booklet we shall learn what there is about the laser that gives
-it so much promise. We shall investigate what it is, how it works, and
-the different kinds of lasers there are. We begin by discussing some of
-the more familiar kinds of radiation, such as radio and microwaves,
-light and X rays.
-
-
-
-
-                      THE ELECTROMAGNETIC SPECTRUM
-
-
-Some 85% of what man learns comes to him through his vision in response
-to the medium of light. Yet, ironically, it wasn’t until the end of the
-17th century that he first began to get an inkling of what light really
-is. It took the great scientific genius Isaac Newton to show that
-so-called white light is really a combination of all the colors of the
-rainbow. A few years later the Dutch astronomer Christiaan Huygens
-introduced the idea that light is a wave motion, a concept finally
-validated in 1803 when the British physician Thomas Young ingeniously
-demonstrated interference effects in waves. Thus it was finally realized
-that the only difference between the various colors of light was one of
-wavelength.
-
-For light was indeed found to be a wave phenomenon, no different in
-principle from the water waves you have seen a thousand times. If you
-stand at the seashore, you can easily count the number of waves that
-approach the shore in a minute. Divide that number by 60 and you have
-the frequency of the wave motion in the familiar unit, cycles-per-second
-(cps).[1]
-
-You would have to count pretty quickly to do this for light, however.
-Light waves vibrate or oscillate at the rate of some 400 million million
-times a second. That’s the vibration rate of waves of red light; violet
-results from vibrations that are just about twice that fast.
-
-With frequencies of this magnitude, discussion and handling of data and
-dimensions are cumbersome and rather awkward. Fortunately there is
-another approach. Let’s look again at our ocean waves. We see that there
-is a regularity about them (before they begin to break on the shore).
-The distance from one crest to the next is significant and is called the
-_wavelength_. Water waves are measured in feet, and in comparable units
-light waves are recorded in ten-millionths of an inch—again a very
-cumbersome number. Scientists therefore use the metric system[2] and
-have standardized a unit called the angstrom[3], which is equal to the
-one-hundred-millionth part of a centimeter (10⁻⁸ cm). Thus we find, as
-shown in Figure 3, that the visible light range runs from the violet at
-about 4000 angstroms to red at about 7000 angstroms.
-
-    [Illustration: Figure 3 _The visible light spectrum ranges between
-    approximately 4000 and 7000 angstroms._]
-
-                                Wavelength
-                               (Angstroms)
-
-               Violet           4000-4300
-               Blue             4300-5000
-               Green            5000-5600
-               Yellow           5600-5800
-               Orange           5800-6100
-               Red              6100-7000
-
-At roughly the same time that the wavelength of light was being
-determined, the German-British astronomer William Herschel performed an
-interesting experiment. He held a thermometer in turn in the various
-colors of light that had been spread out by an optical prism. As he
-moved the thermometer from the violet to the red, the temperature
-reading rose—and it continued to rise as he moved the instrument
-_beyond_ the red area, where no prismatic light could be seen.
-
-Thus Herschel discovered infrared rays (the kind of heat we get from the
-sun) adjoining the visible red light, and at the same time found that
-they were merely a continuation of the visible spectrum. Shortly
-thereafter, ultraviolet rays were found on the other end of the visible
-light band.
-
-One of the most fascinating movements in science has been the constant
-expansion since then of both ends of the radiating-wave spectrum. The
-result has come to be called the _electromagnetic spectrum_, which, as
-we see in Figure 4, encompasses a wide variety of apparently different
-kinds of radiation. Above the visible band (the higher frequencies), we
-find ultraviolet light, X rays, gamma rays, and some cosmic rays; below
-it are infrared radiation, microwaves, and radio waves. Only a small
-proportion of the total spectrum is occupied by the visible band.
-Another point of interest is the inverse relationship between wavelength
-and frequency. As one goes up the other goes down.[4]
-
-    [Illustration: Figure 4 _Visible light region spans a tiny portion
-    of the total electromagnetic spectrum._]
-
-  Frequency (cps)                                            Wavelength
-                                                             Angstroms
-
-                                 Cosmic rays
-             10²²                                            0.0001
-                                                             0.001
-             10²⁰                 Gamma rays                 0.01
-                                                             0.1
-             10¹⁸                   X rays                   1
-                                                             10
-             10¹⁶           Ultraviolet radiation            100
-                                                             1,000
-                                Visible light
-             10¹⁴                                            10,000
-                              Infrared radiation             100,000
-
-                                                             Angstroms
-
-                                                             0.01
-             10¹²              Millimeter waves              0.1
-             10¹⁰             Microwaves, radar              1
-                                                             10
-              10⁸              TV and FM radio               100
-                                  Short wave                 1,000
-              10⁶                  AM radio                  10,000
-                         Low frequency communications        100,000
-     10,000 = 10⁴                                            1,000,000
-
-These many kinds of rays and waves vary tremendously in the ways they
-interact with matter. But they are all part of a single family. The only
-difference among them, as with the colors of the rainbow, lies in their
-wavelengths. In a few cases, as we shall see later, the mode of
-generation is also different.
-
-The band of radiation stretching from the infrared to cosmic rays has
-been, up to now, largely the concern of physical scientists. Because of
-their high frequencies, these radiations are generally handled, when
-calculations or measurements must be made, in terms of wavelength. Radio
-and microwaves[5], on the other hand, have been more in the domain of
-communications engineers and are more likely to be discussed in terms of
-frequency. Thus it is that your radio is marked off in kilocycles, or
-thousands of cycles per second, while light is described as radiation in
-the 4000 to 7000 angstrom band.
-
-The relative newness of the various radiations has kept scientists busy
-learning about them and, as information and experience have accumulated,
-putting them to work.
-
-
-
-
-                              RADIO WAVES
-
-
-One of the first of the newly discovered electromagnetic radiations to
-be put to work was the radio wave, which is characterized by long
-wavelength and low frequency.[6] The low frequency makes it relatively
-easy to produce a wave having virtually all its power concentrated at
-one frequency.
-
-The advantage of this capability becomes obvious after a moment’s
-thought. Think for example of a group of people lost in a forest. If
-they hear sounds of a search party off in the distance, all likely will
-begin to shout in various ways for help. Not a very efficient process,
-is it? But suppose all the energy going into the production of this
-noise could be concentrated in a single shout or whistle. Clearly, their
-chances of being found would be much improved.
-
-    [Illustration: Figure 5 _(a) Temporally coherent radiation. (b)
-    Temporally incoherent radiation._]
-
-The single frequency capability of radio waves has been given the name
-_temporal coherence_ (or coherence in time) and is illustrated in Figure
-5. Part _a_ shows a single sine wave, the common way of representing
-electromagnetic radiation, and particularly _temporally coherent
-radiation_. In _b_ we see what _temporally incoherent radiation_ (such
-as the mixed sounds of the stranded party) would look like.
-
-It was on Christmas Eve 1906 that music and speech came out of a radio
-receiver for the first time. Today the sight of someone walking, riding,
-or studying with an earpiece plugged into a transistor radio is common.
-But the early radio enthusiasts _had_ to wear earphones because it takes
-considerable power to activate a loudspeaker and the received signal was
-quite weak. Some means of increasing, or amplifying, the signal was
-needed if the process was to advance beyond this primitive stage.[7]
-
-The use of vacuum tube or electron tube amplifiers is so widespread that
-it is unnecessary to explain their operations here in any detail. It is
-important that the principle of amplification be understood, however.
-The input or information wave causes the grid to act as a sort of faucet
-as shown in Figure 6. That is, it controls the flow of electrons (the
-current in the circuit) from cathode to anode. A weak signal can
-therefore cause a similar, but much stronger, signal to appear in the
-circuit. The larger signal is subsequently used to power a loudspeaker
-in the radio set.
-
-    [Illustration: Figure 6 _Amplification by a three-element vacuum
-    tube._]
-
-  Power source
-  Cathode
-  Grid
-    Input wave
-  Anode
-  Output wave
-
-The amplification principle can be applied in another equally important
-way. Once a signal gets started in the circuit, part of it can be _fed
-back into the input_ of the circuit. Thus the signal is made to go
-“round and round”, continuously regenerating itself. The device has
-become an _oscillator_, that is, a frequency generator that produces a
-steady and temporally coherent wave. The frequency of the wave can be
-rigidly controlled by suitable circuitry.
-
-The oscillator plays a vital part in radio transmission, for a
-transmitter beams energy continuously, not just when sound is being
-carried. The oscillator generates what is called a “carrier wave”.
-Information, such as speech or music, is carried in the form of audio
-(detectable-by-ear) frequencies, which ride “piggyback” on the carrier
-wave. In other words, the carrier wave is _modulated_, or varied, in
-such a way that it can carry meaningful information. The familiar
-expressions AM and FM, for example, stand for Amplitude Modulation and
-Frequency Modulation—two different ways of impressing information on the
-carrier wave. Figure 7 shows a basic and an amplitude- (or height-)
-modulated wave.
-
-    [Illustration: Figure 7 _(a) Unmodulated radio wave._ _(b)
-    Amplitude-modulated wave carries information._]
-
-The electron tube made its giant contribution to radio, television, and
-other electronic devices by making it possible to generate, detect, and
-amplify radio waves.
-
-Because radio waves are easily controlled, something useful can be done
-with them. Suppose we set up five radio transmitters, all beaming at the
-same frequency. The waves might look like those shown in Figure 8.
-Although the waves are temporally (or time) coherent, they are out of
-step, and not _spatially coherent_. But since good control is possible
-in radio circuits, we can force each antenna to radiate in _phase_ (that
-is, in step) with the others, thus producing fully coherent radiation
-(Figure 8).
-
-    [Illustration: Figure 8 _(a) Spatially incoherent radiation._ _(b)
-    Spatially coherent radiation._]
-
-Such a process can increase the radiation _power_ to an almost unlimited
-degree. But it does nothing to solve the problem of the limited total
-carrying capacity of the radio spectrum.
-
-The most obvious and best way out of this difficulty is to raise the
-operating frequencies into the higher frequency bands. There are two
-reasons for this. First, it is clear that the wider the frequency band
-(the number of frequencies available) with which we work, the greater
-the number of communication channels that can be created.
-
-But second, and more important, the higher the frequency of the wave,
-the greater is its information-carrying capacity. In almost the same way
-that a large truck can carry a bigger load than a small one, the greater
-number of cycles per second in a high frequency wave permits it to carry
-more information than a low frequency wave.
-
-However, high frequencies must be generated in different ways than low
-frequency waves are; they require special equipment to handle them.
-Radio waves are transmitted by causing masses of free electrons to
-oscillate or swing back and forth in the transmitting antenna. (Any time
-electrons are made to change their speed or direction they radiate
-electromagnetic energy.)
-
-Each kind of oscillator has some limit to the frequencies at which it
-can operate. The three-element electron tube has been successfully
-developed to oscillate at frequencies up to, but not including, the
-vibration rate of the microwave region. Here ordinary tubes have trouble
-for the unexpected reason that free electrons are just too slow in their
-reactions to oscillate as rapidly as required in microwave transmission.
-
-To overcome this obstacle, two new types of electron tubes were
-developed: the klystron in 1938 and the traveling-wave tube some 10
-years later. These lifted operation well up into the microwave region;
-it was the klystron that made wartime radar possible. Today many
-communication links depend heavily upon microwave frequencies.
-
-At this point in our story we have a situation where low temporally
-coherent radio waves and microwaves can be generated, but nothing of
-higher frequency. Communications engineers have gazed wistfully, but
-almost hopelessly, at light waves, whose frequencies are millions of
-times higher than radio waves. Thus, just by way of example, some 15
-million separate TV channels could operate in the frequency range
-between red and orange in the visible band.
-
-What, then, is the problem?
-
-Why is light so much more difficult to handle?
-
-
-
-
-                           LIGHT AND THE ATOM
-
-
-Since light waves have such high frequencies, a different mode of
-generation comes into play. We can no longer count on the controlled
-movement of free electrons _outside_ atoms and molecules. Rather, light
-and all the radiations in the higher frequencies are generated by the
-movement of electrons _inside_ atoms and molecules.
-
-Let us review momentarily the modern, albeit highly simplified,
-conception of an atom. Remember that no one has yet seen one. We
-describe the atom on the basis of how it acts, as well as how it reacts
-to things scientists do to it.
-
-For the present purpose, the best model we have of the atom is that of a
-miniature solar system, with a nucleus or heavy part at the center and a
-cloud of electrons dashing around the nucleus in fixed orbits.
-
-The term “fixed orbits” is used advisedly.
-
-Our planet moves in a certain orbit around the sun. If we attached a
-large enough rocket to the earth we theoretically _could_ move it closer
-to or farther away from the sun. In the atom, we have learned, this
-cannot be done. An electron can only exist in one of a certain number of
-fixed orbits; different kinds of atoms have different numbers of orbits.
-
-We might think in terms of an elevator that can only stop at the various
-floors of an apartment building. Each upper floor is like an orbit of
-the electron. But you get nothing for nothing in the world of physics,
-and just as it takes energy to raise an elevator to a higher floor, it
-takes energy to move an electron to an outer orbit.
-
-Hence the atom is said to be raised to higher _energy_ levels when an
-electron is nudged to an outer orbit. The energy input can be of many
-different kinds. Examples are heat, pressure, electrical current,
-chemical energy, and various forms of electromagnetic radiation. If too
-much energy is put into the elevator it goes flying out the roof. If too
-much energy is put into the atom, one or more of its electrons will go
-flying out of the atom. This is called _ionization_, and the atom, now
-minus one of its negative electrons and therefore positively charged, is
-called a positive _ion_.
-
-But if the _right_ amount of energy is put into the atom, one of its
-electrons will merely be raised to a higher energy level. Shown in
-Figure 9, for instance, are the ground state (Circle No. 1) and two
-possible higher energy levels. As you can see there are three possible
-transitions.
-
-    [Illustration: Figure 9 _Schematic representation of the electron
-    orbits and energy levels of an atom. Each circle represents a
-    separate possible orbit and each arrow a possible energy level
-    difference._]
-
-The higher energy levels are abnormal, or excited, states, however, and
-the electron will shortly fall back to its normal (ground state) orbit
-(assuming some other electron has not fallen into it first). In order
-for the electron to do this (go back to its normal orbit), it must give
-off the energy it has acquired. This it does in the form of
-electromagnetic radiation.
-
-The energy difference between the two levels will determine what kind of
-radiation is emitted, for there is a direct correlation between energy
-and frequency.[8] If the energy difference between the two levels is
-such that the frequency of emitted radiation is roughly between 10¹⁴ and
-10¹⁵ cycles per second, we see the radiation as light. When more energy
-is added, the radiation emerges as ultraviolet or X rays. In other words
-the higher the energy difference, the higher the frequency, and vice
-versa. Thus it is that cosmic rays, with the highest frequencies known
-to man, can pass right through us as if we weren’t there.
-
-This simple picture of energy levels and associated frequencies doesn’t
-quite hold for ordinary white light, however. Such light is generally
-produced by a process called incandescence, which results from the
-heating of a material until it glows. True, the atoms of the
-incandescent material are being raised to higher energy levels by
-chemical energy (as in fire), electricity (light bulb), or nuclear
-energy (the sun). In a hot solid, however, the explanation becomes more
-complicated. Many different electronic configurations are possible and
-the differences in energy among the various levels (which can be many
-more than the three shown in Figure 9) vary only slightly from one
-another. The result is a wide band of radiation.
-
-Thus, while the incandescent electric bulb is a great advance over fire,
-it is still a very inefficient source of light. Because it depends upon
-incandescence, a considerable portion of the electrical input goes into
-the production of unwanted heat, for the bulb’s filament radiates in the
-infrared as well as the visible region.
-
-For providing illumination, the fluorescent tube is far more efficient
-than the incandescent lamp: a 40-watt fluorescent tube gives as much
-light as a 150-watt incandescent light. This is because its radiation is
-more controlled, operating more in accord with our description of
-electronic energy levels. Hence more of its output is in the desired
-visual region of the spectrum.
-
-In certain types of lighting, particular energy level changes may
-predominate, leading to the characteristic colors of neon tubes and
-vapor lamps. Although the resulting radiation bandwidth is narrow enough
-in these devices to appear as a definite color instead of the broad
-spectrum we know as white, it is still quite broad. In other words, the
-radiation is still frequency incoherent—and it is still spatially
-incoherent.
-
-To understand this, let us return for a moment to the group of radio
-antennas we showed in Figure 8. All of them, you will recall, could be
-made to radiate in phase. In the production of light, however, each
-antenna is replaced by a single atom!
-
-This creates two problems. First, because the energy stored in the atom
-is quite small, it comes out not as a continuous wave but as a tiny
-packet of radiation—a _photon_.[9] It has an effect more like the hack
-of an ax than the buzz of a power saw.
-
-Second, atoms are notoriously “individualistic”. When a batch of atoms
-in a material has been raised to higher energy levels there is no way to
-know in what order, or in what direction, they will release their
-energy.
-
-This kind of process is called _spontaneous emission_, since each atom
-“makes up its own mind”. All we know is that within a certain period of
-time—a short period, to be sure—a certain percentage of these higher
-energy atoms will release their photons.
-
-    [Illustration: Figure 10 _Ordinary light is a jumble of frequencies,
-    directions, and phases._]
-
-What we have, then, is incoherent radiation—a jumble of frequencies (or
-colors), directions, and phases. Such light, symbolized in Figure 10,
-works well enough in lighting up this page, but is almost worthless as a
-carrier of information (and in other ways, as we shall see shortly).
-About the best that can be done with it is to turn it on and off in a
-sort of visual Morse code, which is exactly what is done on the blinker
-communication systems sometimes used for ship-to-ship communication.
-
-In other words, ordinary light cannot be modulated as radio waves can.
-
-It is of interest to note, however, that ordinary white light _can_ be
-made coherent, to some extent, but at a very high cost in the intensity
-of the light. For example, we might first pass the light through a
-series of filters, each of which would subtract some portion of the
-spectrum, until only the desired wavelength came through. As can be seen
-in Figure 11, only a small fraction of the original light would be left.
-
-    [Illustration: Figure 11 _Obtaining coherent radiation the hard way.
-    Filters and pinhole block all but a small amount of the original
-    radiation._]
-
-    Incoherent
-  Filters
-    Coherent in time
-  Pinhole
-    Coherent in time and space
-
-We would then have monochromatic (one color) light, which is temporally
-coherent radiation, but it would still be spatially incoherent. In our
-diagram, we show three monochromatic waves. If we then passed this light
-through a tiny pinhole as shown, most of these few remaining waves would
-be blocked; the ones that got through would be pretty much in step. (In
-a similar manner, a true point source of light would produce spatially
-coherent radiation; but, as in the process described here, there
-wouldn’t be very much of it.)
-
-We have, finally, obtained coherent light.
-
-The important thing about the laser is that, by its very nature, it
-produces coherent light automatically.
-
-Now....
-
-
-
-
-                WHAT’S SO SPECIAL ABOUT COHERENT LIGHT?
-
-
-So desirable are the qualities of coherent light that the complicated
-filtering process described above has actually been used. For example,
-one British experimenter, Dennis Gabor, used it in the 1940s in an
-attempt to make a better microscope. But so great was the effort, and so
-meager the resulting light, that this project was abandoned.
-
-In the course of Dr. Gabor’s experiments, however, he did manage to make
-a special kind of picture, using coherent light, which he called a
-_hologram_. He derived the name from two Greek words meaning a _whole
-picture_. We shall see why in a moment.
-
-Ordinary black and white photographs merely record darks and lights, or
-the intensity of the illumination, thereby providing a scale of grays,
-nothing more. But because waves of coherent light consistently maintain
-their relative spacing, they can be used to record additional
-information, namely the distance from objects.
-
-For example, if we shine a beam of coherent (laser) light between two
-objects we can, knowing the light wavelength, determine the distance
-between them to a high degree of accuracy. The basic idea is diagramed
-in Figure 12. It can be seen that the number of waves times the
-wavelength gives the precise distance (to within 1 wavelength of light)
-from the laser source to each object. But this would be a difficult
-process to implement.
-
-A better way, and one that is already in operation, is to use
-conventional methods to measure the approximate distance and use the
-laser beam for precise or fine measurement. In the device shown in
-Figure 2, the beam is split into two parts. One part is kept in the
-instrument itself to act as a reference. The other is aimed at a
-reflector, which sends it back to a detector in the main device, where
-it is automatically compared with the reference beam. If the two beams
-are in phase (that is, if their crests are superimposed), the waves
-combine and produce a high intensity beam at the detector. As the
-reflector moves closer to or farther away from the laser source the beam
-intensity decreases and then increases again as the wave crests move in
-and out of phase. The instrument counts the changes and displays the
-distance the reflector moves, as a function of the wavelengths, on the
-control cabinet meters.
-
-    [Illustration: Figure 12 _Principle of distance measurement using
-    coherent light. Wavelength times number of waves gives precise
-    distance between laser and object._]
-
-  Distance to be measured
-  Laser
-  Object No. 2
-  1 Wavelength
-  Object No. 1
-
-Since the Word For the Interaction of the Waves in a System Like This
-Is “Interference”, the Measurement Process Is Called _interferometry_
-(Pronounced in Ter Fer Om E Try). Although Not New, It Can Now
-Be Applied For the First Time in Machine Tool Applications,
-Providing the Accuracy Needed in This Age of Space Technology and
-Microminiaturization. Measurements With a Laser Interferometer Can
-Be Made With an Accuracy of 0.5 Part Per Million at Distances Up to
-200 Inches. Such Precision Was Previously Unheard of in a Machine
-Shop Environment, Having Been Limited to Laboratory Measurements, and
-Only at A Range of a Few Inches. Under Similar Laboratory Conditions,
-Measurements by Laser Interferometry Now Detect Movements of 10⁻¹¹
-Centimeter, a Distance Approaching the Dimensions of an Atomic Nucleus.
-
-Now let us suppose we expand the laser beam as shown on page 22, and,
-with the aid of a mirror, direct part of it (the reference beam) at a
-photographic plate. The remaining portion of the diverging beam is used
-to illuminate the object to be photographed. Some of this light (the
-object beam) is reflected toward the plate and carries with it
-information about the object, as indicated by the wavy line. In the
-region where these two beams intersect, interference occurs, and a
-sample of this interference is recorded within the photographic
-emulsion. Where two crests meet a dark spot is recorded; where the waves
-are out of phase the processed plate is clear. The result is a hologram,
-a complex pattern of “fringes”, characteristic of the contour and light
-and dark areas of the object, as well as its distance from the plate.
-These fringes have the ability to diffract light rays. When light from a
-laser, or a point source of white light, is directed at the hologram
-from the same direction as the reference beam, part of the light is
-“bent” so that it appears to come from the place once occupied by the
-object. The result is a remarkably realistic 3-dimensional image.
-
-There, in a nutshell, is the incredible new technique of holography. The
-extreme order of laser light is illustrated by the regularity of the
-dots on the cover of this booklet.
-
-This strange kind of light provides us with yet other advantages.
-Indeed, one of the most important is the fact that the energy of the
-laser is not being sprayed out in all directions. All of it is
-concentrated in the narrow beam that emerges from the device. And it
-_stays_ narrow. Laser light has already been shone on the moon, the beam
-spreading out to only a few miles in traveling there from earth. The
-best optical searchlight beam would spread wider than the moon itself,
-thus dissipating its energy.
-
-It is for this reason, as well as its temporal coherence, that laser
-light is being considered for communications. A narrow beam is
-particularly important for space communications because of the long
-distances involved.
-
-But it is also possible to focus laser light as no light has ever been
-focused before. At close range a laser beam can be focused down to a
-circle just a few wavelengths across, concentrating its energy and
-making it possible to drill holes only 0.0002 inch in diameter. The
-photo on page 52 shows the exquisite control that can be exercised.
-
-Let us see what this focusability means in terms of power. Consider, by
-way of analogy, a dainty 100-pound lady in a pair of spike-heeled shoes.
-As she takes a step, her weight will be concentrated on one of those
-heels. If the area of the heel is, say, one quarter of a square inch (½
-× ½ inch), the pressure exerted on the poor tile or carpet rises to 400
-pounds per square inch (4 × 100) and if the heel is only ¼ inch on a
-side, the pressure will be 1600 pounds per square inch!
-
-    [Illustration: Making and Viewing a Hologram]
-
-  MAKING A HOLOGRAM
-    Object
-    Object beam
-    Holographic plate
-    Mirror
-    Reference beam
-    Laser
-  VIEWING A HOLOGRAM
-    Hologram
-    Image
-    Eye
-    Coherent light source
-
-What we are getting at, of course, is the fact that the coherence of the
-laser beam permits it to be concentrated into a tiny area. Thus whatever
-total energy is being sent out by the laser can be concentrated to the
-point where its effective energy is tremendous. The sun emits some 6500
-watts per square centimeter. Laser beams have already reached 500
-_million_ watts per square centimeter.
-
-But the power of the laser does not derive solely from its ability to be
-focused. Even an unfocused beam is several times more powerful than the
-sun’s output (per square centimeter).
-
-    [Illustration: Figure 13 _The typical hologram, looks like a
-    geometric design, but it contains more information than would an
-    ordinary photograph. The images below, made from a hologram, show
-    the detail, apparent solidity, and parallax effect of the
-    reconstructed light waves. The parallax effect is the ability to see
-    around the objects just as one could if they were really there. (See
-    frontispiece.)_]
-
-    [Illustration: Model tank]
-
-    [Illustration: Tank, from another angle]
-
-The crucial difference between the sun’s light or any ordinary kind of
-light and laser light lies in the extent to which the emission of energy
-can be controlled. In the production of ordinary light the atoms, as we
-know, emit spontaneously, or in an uncontrolled fashion. But if the
-atoms could be forced to take in the proper amount of energy, store it,
-and release it when we wanted them to, we would have _stimulated_,
-rather than spontaneous, emission.
-
-This, however, is practically the same as the amplification principle we
-discussed earlier. In that case, a small radio signal is jacked up into
-a large one by stimulating an available power source to release its
-energy at the same wavelength and in step with the smaller signal.
-
-The question is, how can we do this with light?
-
-
-
-
-                          CONTROLLED EMISSION
-
-
-The laser and its parent, the maser, can be traced back half a century
-to its theoretical beginnings. The great physicist Albert Einstein is
-most widely known for his work in relativity. But he did early and
-important work on that other gigantic 20th century scientific
-achievement, the quantum theory.[10] In one of his papers, published
-first in Zurich, Switzerland, in 1916, Einstein showed that controlled
-emission of light energy could be obtained from an atom under certain
-conditions. When an atom or molecule has somehow had its energy level
-raised, the release of this stored energy could be stimulated by
-subjecting the atom or molecule to a small “shot” of electromagnetic
-radiation of the proper frequency.
-
-Einstein wrote that when such a photon of energy caused an electron to
-drop from a higher to a lower orbit, the electron would emit another
-photon of the same frequency and in the same direction as the one that
-hit it.[11] In other words, the energy of the emitted photon would be
-added to that of the photon that stimulated the emission in the first
-place. Here, _potentially_, was light amplification. The three major
-factors, absorption of energy, spontaneous emission, and stimulated
-emission are diagrammed in Figure 14.
-
-There the matter lay for more than 30 years.
-
-In 1951 Charles H. Townes, then on the Columbia University faculty, was
-interested in ways of extending to still higher frequencies the range of
-microwaves available for use in communications and in other scientific
-applications. Townes and other scientists who were interested in the
-problem were to meet in Washington, D. C., on the 26th of April. The
-night before the meeting he slept in a small Washington hotel; but he
-awoke at 5:30—pondering, pondering the high frequency problem.
-
-He dressed and took a walk, then sat on a park bench and savored the
-beauty of azaleas in bloom. But all the while his mind was running over
-the various aspects of the problem.
-
-    [Illustration: Figure 14 _An atom can release absorbed energy
-    spontaneously or it can be stimulated to do so._]
-
-                                     Before    After
-
-                   Excited state      —–—       —•—
-  Absorption ~~→
-                   Relaxed state      —•—       —–—
-                   Excited state      —•—       —–—
-  Spontaneous emission
-                   Relaxed state      —–—       —•—     ~~→
-                   Excited state      —•—       —–—
-  Stimulated emission ~~→
-                   Relaxed state      —–—       —•—     ~~→
-                                                        ~~→
-
-Suddenly the answer came to him.
-
-Normally more of the molecules in any substance are in low-energy states
-than in high ones. He would change the natural balance and create a
-situation with an abnormally large number of high-energy molecules. Then
-he would stimulate them to emit their energy by nudging them with
-microwaves. Here was amplification.
-
-He could even take some of the emitted radiation and feed it back into
-the device to stimulate additional molecules, thereby creating an
-oscillator. This _feedback_ arrangement, he knew, could be carried out
-in a cavity, which would resonate (just like an organ pipe) at the
-proper frequency. The resonator would be a box whose dimensions were
-comparable with the wavelength of the radiation, that is, a few
-centimeters on a side.
-
-On the back of an envelope he figured out some of the basic
-requirements. Three years, and many experiments, later the maser
-(_m_icrowave _a_mplification by _s_timulated _e_mission of _r_adiation)
-was a reality. The original maser was a small metal box into which
-excited ammonia molecules were added. When microwaves were beamed into
-the excited ammonia the box emitted a pure, strong beam of high
-frequency microwaves, far more temporally coherent than any that had
-ever been achieved before. The output of an ammonia maser is stable to
-one part in 100 billion, making it an extremely accurate atomic
-“clock”.[12] Moreover, the amplifying properties of masers have been
-found to be very useful for magnifying faint radio signals from space,
-and for satellite communications.
-
-Ammonia gas was chosen for the first maser because molecules of ammonia
-have two individual energy states that are separated by a gap
-corresponding in frequency to 23,870 megacycles (23,870 million cycles)
-per second. Ammonia molecules also react to a nonuniform electric field
-in ways that depend on their energy level: low-level molecules can be
-attracted and high-level ones repelled by the same field. Thus it is
-possible to separate the low-energy molecules from the high, and to get
-the excited molecules into the cavity without too much trouble.
-
-This procedure for getting the majority of atoms or molecules in a
-container into a higher energy state, is called _population inversion_
-and is basic to the operation of both masers and lasers.
-
-It should be noted that two Russians, N. G. Basov and A. M. Prokhorov,
-were working along similar lines independently of Townes. In 1952 they
-presented a paper at an All-Union (U.S.S.R.) Conference, in which they
-discussed the possibility of constructing a “molecular generator”, that
-is, a maser. Their proposal, first published in 1954, was in many
-respects similar to Townes’s. In 1955, Basov and Prokhorov discussed, in
-a short note, a new way to obtain the active atomic systems for a maser,
-a method that turned out to be of great importance.
-
-Thus on October 29, 1964, the Nobel Prize in Physics was awarded, not
-only to Townes, but to Basov and Prokhorov as well. The award was for
-fundamental work in the field of quantum electronics, which has led to
-the construction of oscillators and amplifiers based on the “aser”
-principle.
-
-
-
-
-                            A LASER IS BORN
-
-
-Following the maser development, there was much speculation about the
-possibility of extending the principle to the optical region. Indeed the
-first lasers—_l_ight _a_mplification by _s_timulated _e_mission of
-_r_adiation—were called “optical masers”.
-
-The difficulty, of course, was that optical wavelengths are so
-tiny—about ¹/₁₀,₀₀₀ that of microwaves. The maser principle depended
-upon a physical resonator, a box a few centimeters (or even millimeters)
-in length. But at millimeter wavelengths, such resonators are already so
-small that they are hard to make accurately. Making a box ¹/₁,₀₀₀ that
-size was out of the question. Another approach was necessary.
-
-In 1958 A. L. Schawlow of Bell Telephone Laboratories and Dr. Townes
-outlined the theory and proposed a structure for an optical maser. They
-suggested that resonance could be obtained by making the waves travel
-back and forth along a relatively long, thin column of amplifying
-substance that had parallel reflectors at the ends.
-
-After their theory of the optical maser had been published, the race to
-build the first actual device began in earnest. The winner, in 1960, was
-Dr. T. H. Maiman, then with Hughes Aircraft Company. (He is now
-president of Maiman Associates.) The active substance he used was a
-single crystal of ruby, with the ends ground flat and silvered.
-
-Ruby is an aluminum oxide in which a small fraction of the aluminum
-atoms in the molecular structure, or lattice, have been replaced with
-chromium atoms. These atoms absorb green and blue light and hence impart
-a red color to the ruby. The chromium atoms can be boosted from their
-ground state into excited states when they absorb the green or blue
-light. This process, by which population inversion is achieved, has been
-given the name pumping.[13]
-
-Pumping in a crystal laser is generally achieved by placing the ruby rod
-within a spiral flash lamp (Figure 15) that operates like those used in
-high-speed (stroboscopic) photography. When the lamp is flashed, a
-bright beam of red light emerges from the ruby, shining out through one
-end, which has been only partially silvered.
-
-    [Illustration: Figure 15 _A ruby laser system._]
-
-  Ruby
-  Flash lamp
-  Partially silvered end
-  Laser output
-  Power
-  Cooling
-
-The duration of this flash of red light is quite brief, lasting only
-some 300 millionths of a second, but it is very intense. In the early
-lasers, such a flash reached a peak power of some 10,000 watts.
-
-When Maiman’s device was successfully built and operating, a public
-relations expert was called in to help introduce this revolutionary
-device to the world. He took one look at the laser and decided that it
-was too small and insignificant looking and would not photograph well.
-Looking around the lab, he spotted a larger laser and decided that that
-one was better.
-
-Dr. Maiman informed him in his best scientific manner that laser action
-had not been achieved with that one. But the world of promotion won out,
-and Dr. Maiman allowed the larger device to be photographed on the
-assumption—or was it hope?—that he would be able to get it to operate in
-the future. (He did.)
-
-The device shown in Figure 16 is the true first laser. The all-important
-crystal rod is seen at the center. These crystals, incidentally, must be
-quite free of extraneous material; hence they are artificially “grown”,
-as shown in Figure 17. The single large crystal is formed as it is
-pulled slowly from the “melt”, after which it is ground to size and
-polished.
-
-    [Illustration: Figure 16 _Dr. Maiman’s first laser. Output was
-    10,000 watts._]
-
-    [Illustration: Figure 17 _An exotic crystal of the garnet family is
-    “grown” from a melt at a temperature of 3400°F._]
-
-
-
-
-                           LASING—A NEW WORD
-
-
-Now we can begin to put together the various processes and equipment we
-have been discussing separately. Perhaps the best way to do this is to
-look again at the word _laser_ and recall its meaning: _l_ight
-_a_mplification by _s_timulated _e_mission of _r_adiation. Our objective
-is to create a powerful, narrow, coherent beam of light. Let us see how
-to do this.
-
-In Figure 18 we imagine a laser crystal containing many atoms in the
-ground state (white dots) and a few in the excited state (black dots).
-Pumping light (wavy arrows in _a_) raises most of the atoms to the
-excited state, creating the required population inversion.
-
-    [Illustration: Figure 18 _Sequence of operations in a solid crystal
-    laser. (a) Pumping light raises many atoms to excited state. (b)
-    Lasing begins when a photon is spontaneously emitted along the axis
-    of the crystal. This stimulates other atoms in its path to emit. (c)
-    The resulting wave is reflected back and forth many times between
-    the ends of the crystal and builds in intensity until finally it
-    flashes out of the partially silvered end._]
-
-  (a)
-    Ruby crystal
-    Pumping light
-    Atom in ground state
-    Excited atom
-    Partial reflecting mirror
-    Full reflecting mirror
-  (b)
-    Excited atom emits photon parallel to axis
-  (c)
-
-_Lasing_ begins when an excited atom spontaneously emits a photon
-parallel to the axis of the crystal (_b_). (Photons emitted in other
-directions merely pass out of the crystal.) The photon stimulates
-another atom in its path to contribute a second photon, in step, and in
-the same direction.
-
-This process continues as the photons are reflected back and forth
-between the ends of the crystal. (We might think of lone soldiers
-falling into step with a column of marching men.) The beam builds up
-until, when amplification is great enough (_c_), it flashes out through
-the partially silvered mirror at the right—a narrow, parallel,
-concentrated, coherent beam of light, ready for....
-
-
-
-
-                     SOME INTERESTING APPLICATIONS
-
-
-Application of lasers can be divided into two broad categories: (1)
-commercial, industrial, military, and medical uses, and (2) scientific
-research. In the first case, lasers are used to do something that has
-been done in another way up to now (but not as well). Sometimes a laser
-solves a particular problem. For example, one of the first applications
-was in eye surgery, for “welding” a detached retina. The laser is
-particularly useful here because laser light can penetrate transparent
-objects such as the eye’s lens (Figure 19), eliminating the need to make
-a cut into the eye.
-
-    [Illustration: Figure 19 _Diagram of human eye showing laser beam
-    focused on retina._]
-
-  Cornea
-  Lens
-  Optic Nerve
-  Beam angle
-  Fovea centralis
-  Iris
-  Image
-  Retina
-
-Surgeons have long wanted a better technique for treating extremely
-small areas of tissue. A laser beam, focused into a small spot, performs
-perfectly as a lilliputian surgical knife. An additional advantage is
-that the beam, being of such high intensity, can also sterilize or
-cauterize tissue as it cuts.
-
-The narrowness of the laser beam has made it ideal for applications
-requiring accurate alignment. Perhaps the ultimate here is the
-2-mile-long linear accelerator built by Stanford University for the
-United States Atomic Energy Commission. “Arrow-straight” would not have
-been nearly good enough to assure expected performance. A laser beam was
-the only technique that could accomplish the incredible task of keeping
-the ⅞ inch bore of the accelerator straight along its 2-mile length. A
-remote monitoring system, based on the same laser beam, tells operators
-when a section of the accelerator has shifted out of line (due for
-example to small earth movements) by more than about ¹/₃₂ inch—and
-identifies the section.[14]
-
-Figure 20 shows the 2-mile-long “klystron gallery” that generates the
-power for kicking the high-energy particles down the tube. The gallery
-parallels the accelerator housing and lies 25 feet beneath it (Figure
-21). The large tube houses the optical alignment system and supports the
-smaller accelerator tube above. Target patterns dropped into the large
-tube at selected points produce an interference pattern at the far end
-of the tube similar to the one in Figure 13. Precise alignment of the
-tube is achieved by aiming the laser at the center dot of the pattern.
-Then the section that is out of line is physically moved until the dot
-appears in the proper place at the other end of the tube. It is the
-extreme coherence of the laser beam that makes this technique possible.
-
-Having heard that laser light has bored through steel and is being used
-in microwelding, some have asked whether the laser will ever be used to
-weld bridge members and other structural girders. This is missing the
-whole point of the laser: It would be like washing your floor with a
-toothbrush (even one with extra stiff bristles)! There would be no
-advantage to using lasers for large-scale welding; present equipment for
-this operation is quite satisfactory and far less wasteful of input
-power. The sensible approach is to use lasers where existing processes
-leave something to be desired.
-
-Until the advent of the laser, for example, there was no good way to
-weld wires 0.001 inch in diameter. Nor was there a good way to bore the
-tiny hole in a diamond that is used as a die for drawing such fine wire.
-It used to take 2 days to drill a single diamond. With laser light the
-operation takes 2 minutes—and there is no problem with rapid wear of a
-cutting tool.
-
-So much for the first category of application. In the second category,
-namely use of the laser as a scientific tool, we enter a more
-theoretical domain. Here we use coherent light as an extension of
-ourselves, to probe into and to look at the world around us.
-
-    [Illustration: Figure 20 _A laser beam was used (and continues to be
-    used) for precise alignment of Stanford University’s 2-mile-long
-    linear accelerator. This view shows the aboveground portion during
-    construction._]
-
-Much experimental science is a matter of cooling, heating, grinding,
-squeezing, or otherwise abusing matter to see how it will react. With
-each new tool—ultrafast centrifuges, high- and low-pressure and
-extreme-temperature chambers, intense magnetic fields, atomic
-accelerators and so on—more has been learned about this still-puzzling
-world.
-
-Since coherent light is something new, we can do things to matter that
-have not been done before, and see how it reacts. The laser is being
-used to investigate many problem areas in biology, chemistry, and
-physics. For example, sound waves of extremely high frequency can be
-generated in matter by subjecting it to laser light. These intense
-vibrations may have profound effects on materials.
-
-    [Illustration: Figure 21 _Subterranean view of Stanford accelerator
-    housing. Alignment optics (laser systems) are housed in the large
-    tube, which also acts as support for the smaller accelerator tube
-    above it._]
-
-    [Illustration: Figure 22 _Laser beam spot as observed at the end of
-    the accelerator._]
-
-In the chemical field the sharp beam and monochromatic energy of the
-laser hold great promise in the exploration of molecular structure and
-the nature of chemical reactions. Chemical reactions usually are set off
-by heat, agitation, electricity, or other broadly applied means. None of
-these energizers allow the fine control that the laser beam does. Its
-extremely fine beam can be focused to a tiny spot, thus allowing
-chemical activity to be pinpointed. But there is a second advantage: The
-monochromaticity of coherent light also makes it possible to control the
-energy (in addition to the intensity) of the beam accurately by simply
-varying the wavelength. Thus it may be possible, for instance, to cause
-a reaction in one group of molecules and not in another.
-
-One application in chemistry that holds great promise is the use of
-laser energy for causing specific chemical reactions such as those
-involved in the making of plastics. Bell Telephone Laboratory scientists
-have changed the styrene monomer (a “raw” plastic material) to its final
-state, polystyrene, in this way. The success of these and similar
-experiments elsewhere opens for exploration a vast area of molecular
-phenomena.
-
-In another scientific application, the laser is being used more and more
-as a teaching tool. Coherence is a concept that formerly had to be
-demonstrated by diagrams, formulas, and inference from experiments. The
-laser makes it possible to see coherence “in action”, along with many of
-the physical effects that result from it. Such phenomena as diffraction,
-interference, the so-called Airy disc patterns, and spatial harmonics,
-always difficult to demonstrate to students in the abstract, can now be
-seen quite concretely.
-
-Other interesting things can also be seen more plainly now. At the Los
-Alamos Scientific Laboratory, laser light is being used to “look” at
-plasmas; the result of one such look is shown in Figure 23. Plasmas are
-ionized gaseous mixtures. Their study lies at the heart of a constant
-search by atomic scientists for a self-sustained, controlled fusion
-reaction that can be used to provide useful thermonuclear power. This
-kind of reaction provides the almost unlimited energy in the sun and
-other stars. It is more efficient and releases less radioactivity than
-the other principal nuclear process, fission, which is used in
-atomic-electric power plants.[15]
-
-    [Illustration: Figure 23 _Shadowgraph of deuterium discharge taken
-    in laser light. Turbulence of the plasma is clearly seen._]
-
-Westinghouse Electric Corporation scientists, on the other hand, have
-used the concentrated energy of the laser, not to look at, but to
-_produce_ a plasma (Figure 24). They blasted an aluminum target the size
-of a pinhead with a laser beam, thereby vaporizing it and creating a
-plasma. The calculated temperature in the electrically charged gas was
-3,000,000° centigrade. This is pretty hot, but still not hot enough for
-a thermonuclear reaction.
-
-    [Illustration: Figure 24 _Plasma heating by laser light._]
-
-  Diamagnetic loop
-  Laser beam
-  Vacuum chamber
-  Magnetic field
-  Magnetic coils
-  Electrostatic probe
-  Plasma
-  Lens
-  Mirror
-  To vacuum pump
-  Camera
-
-The temperature of a plasma necessary to sustain a thermonuclear
-reaction is so high (above 10,000,000°C) that any material is vaporized
-instantly on coming into contact with it. The only means developed so
-far to contain the plasma is an intense magnetic field, or “magnetic
-bottle”; containment has been accomplished for only a few thousandths of
-a second at most. The objective of the Westinghouse research, which was
-supported by the Atomic Energy Commission, was to study in detail the
-interaction of the plasma with a magnetic field.
-
-We do not have room to describe more applications in detail, but it may
-be interesting to list a few other uses of lasers—some commercial and
-some still experimental:
-
-
-—Earthquake prediction.
-
-—Measurement of “tides” in the earth’s crust under the sea.
-
-—Laser gyroscopes.
-
-—Highly accurate velocity measurement (useful in certain assembly line
-  and continuous manufacturing processes).
-
-—Scanner for analyzing photographs of bubble chamber tracks and
-  astronomical phenomena.
-
-—Computer output and storage systems; perhaps even complete optical data
-  processing systems.
-
-—Lightning-fast printing devices.
-
-—High-speed photography (Figure 25).
-
-—Missile tracking and accurate alignment of antennas.
-
-—Automatic flaw spotter for big radio antennas.
-
-—Aircraft landing aid for poor weather conditions.
-
-—Fast, painless dental drill.
-
-—Cancer research.
-
-
-    [Illustration: Figure 25 _Twenty-two caliber bullet and its shock
-    wave are photographed from the image produced by a doubly exposed
-    laser hologram. The original hologram was exposed twice by a ruby
-    laser within half a thousandth of a second as the bullet sped past
-    at 2½ times the speed of sound._]
-
-
-
-
-                         A MULTITUDE OF LASERS
-
-
-It is almost self-evident that no single device, even one as incredible
-as the laser, could accomplish all the feats mentioned in the preceding
-paragraphs. After all, some of these applications require high power but
-not extremely high monochromaticity, while in others the reverse may be
-true. Yet, by its very nature, any laser produces a beam with one, or at
-the most a few, wavelengths, and many different materials would be
-needed to provide the many different wavelengths required for all the
-tasks listed.
-
-Also, the first laser was a pulsed device. Light energy was pumped in
-and a bullet of energy emerged from it. Then the whole process had to be
-repeated. Pulsed operation is fine for spot-welding and for applications
-such as radar-type rangefinding, where pulses of energy are normally
-used anyway. With lasers smaller objects can be detected than when using
-the usual microwaves. But a pulsed process is not useful for
-communications. In other words, pulsing is good for certain applications
-but not for others.
-
-And of course solid crystals are difficult to manufacture. Hence, it was
-natural for laser pioneers to look hopefully at gases. Gas lasers would
-be easier to make—simply fill a glass tube with the proper gas and seal
-it.
-
-But other advantages would accrue. For one thing the relatively sparse
-population of emitting atoms in a gas provides an almost ideally
-homogeneous medium. That is, the emitting atoms (corresponding to
-chromium in the ruby crystal) are not “contaminated” by the lattice or
-host atoms. Since only active atoms need be used, the frequency
-coherence of a gas laser would probably be even better than that of the
-crystal laser, they reasoned.
-
-It was less than a year after the development of the ruby laser that Ali
-Javan of Bell Telephone Laboratories proposed a gas laser employing a
-mixture of helium and neon gases. This was an ingeniously contrived
-partnership whereby one gas did the energizing and the other did the
-amplifying. Gas lasers now utilize many different gases for different
-wavelength outputs and powers and provide the “purest” light of all. An
-additional advantage is that the optical pumping light could be
-dispensed with. An input of radio waves of the proper frequency did the
-job very nicely.
-
-But most significant of all, Javan’s gas laser provided the first
-continuous output. This is commonly referred to as CW (continuous wave)
-operation. The distinction between pulsed and CW operation is like the
-difference between baking one loaf of bread at a time and putting the
-ingredients in one end of a baking machine and having a continuous loaf
-emerge at the other.
-
-When a non-expert thinks of a laser, he is apt to think of
-power—blinding flashes of energy—as illustrated in Figure 26. As we
-know, this is only a small part of the capability of the laser.
-Nevertheless, since lasers are often specified in terms of power output
-it may be well to discuss this aspect.
-
-The two units generally used are _joules_ and _watts_. You are familiar
-with a watt and have an idea of its magnitude: think, for example, of a
-15-watt or a 150-watt bulb. A watt is a unit of _power_; it is the rate
-at which (electrical) work is being done.
-
-    [Illustration: Figure 26 _High power is demonstrated as a laser beam
-    blasts through metal chain._]
-
-The joule is a unit of _energy_ and can be thought of as the total
-capacity to do work. One joule is equivalent to 1 watt-second, or 1 watt
-applied for 1 second. But it can also mean a 10-watt burst of laser
-light lasting 0.1 second, or a billion watts lasting a billionth of a
-second.
-
-In general, the crystal (ruby) lasers are the most powerful, although
-other recently introduced materials, such as liquids (see Figure 27) and
-specially prepared glass, are providing competition. With proper
-auxiliary equipment, bursts of several _billion_ watts have been
-achieved; but the burst lasts only about 100 millionths of a second. For
-certain uses, that’s just what is wanted: a highly concentrated burst of
-energy that does its work without giving the material being “shot” a
-chance to heat up and spread the energy, perhaps damaging adjacent
-areas.
-
-    [Illustration: Figure 27 _Active substance for a modern liquid laser
-    is made in an uncomplicated 10-minute procedure. Bluish powder of
-    the rare earth, neodymium, is dissolved in a solution of selenium
-    oxychloride and sealed in a glass tube._]
-
-Since the joule gives a measure of the total energy in a laser burst it
-is not applicable to CW output. Power in this area began low—in the
-milliwatt (one thousandth of a watt) region—but has been creeping up
-steadily. A recent gas laser utilizing carbon dioxide has already
-reached 550 watts of continuous infrared radiation. This is the giant
-44-footer shown in Figure 28. An advantage of gas (and liquid) lasers is
-that they can be made just about as large as one wishes. By way of
-comparison, the smallest gas laser in use is shown in Figure 29.
-
-    [Illustration: Figure 28 _A giant 44-foot gas laser produces 550
-    watts of continuous power and is expected to reach 1000 watts.
-    Glowing of the tube comes from gas discharge, not from laser light,
-    which is in the infrared region and cannot be seen._]
-
-One of the least satisfactory aspects of the laser has been its
-notoriously low efficiency. For a while the best that could be
-accomplished was about 1%. That is, a hundred watts of light had to be
-put in to get 1 watt of coherent light out. In gas lasers the efficiency
-was even lower, ranging from 0.01% to 0.1%.
-
-In gas lasers this was no great problem since high power was not the
-objective. But with the high-power solid lasers, pumping power could be
-a major undertaking. A high-power laser pump built by Westinghouse
-Research Laboratories handles 70,000 joules. In more familiar terms, the
-peak power input while the pump is on is about 100,000,000 watts. For a
-brief instant this is roughly equal to all the electrical power needs of
-a city of 100,000 people.
-
-Two relatively new developments have changed the efficiency levels. One,
-the carbon dioxide gas laser, is quite efficient, with the figure having
-passed 15%. The second is the injection, or semiconductor laser, in
-which efficiencies of more than 40% have been obtained. Unless
-unforeseen difficulties arise this figure is expected to continue to
-rise to a theoretical maximum of close to 100%.
-
-    [Illustration: Figure 29 _A miniature gas laser produces continuous
-    output in visible red region._]
-
-The semiconductor laser is to solid and gas lasers what the transistor
-was to the vacuum tube; all the functions of the laser have been packed
-into a tiny semiconductor crystal. In this case, electrons and “holes”
-(vacancies in the crystal structure that act like positive charges)
-accomplish the job done by excited atoms in the other types. That is,
-when they are stimulated they fall from upper energy states to lower
-ones, and emit coherent radiation in the process. Aside from this the
-principle of operation is the same.
-
-The device itself, however, is vastly different. For one thing it is
-about the size of this letter “o” (Figure 30). For another, it is
-self-contained; since it can convert electric current directly into
-laser light—the first time this has been possible—an external pumping
-source is not required. This makes it possible to modulate the beam by
-simply modulating the current. (A different approach has been to
-modulate a magnetic field around the device. This, it turns out, can
-also be done with some newer solid crystal lasers.)
-
-An additional advantage offered by the semiconductor laser is
-simplicity. There are no gases or liquids to deal with, no glassware to
-break, and no mirrors to align. Although it will not deliver high power,
-it can already deliver enough CW power for certain communications
-purposes. Its simplicity, efficiency, and light weight make it ideal for
-use in space.
-
-    [Illustration: Figure 30 _A tiny injection laser works in infrared
-    region. The beam is visible because photo was taken with infrared
-    film. The laser itself is a tiny crystal of gallium arsenide inside
-    the metal mount being held between the fingers._]
-
-
-
-
-                             COMMUNICATIONS
-
-
-Future deep space missions are expected to require extremely high data
-transmission rates (on the order of a million bits[16] per second) to
-relay the huge quantities of scientific and engineering information
-gathered by the spacecraft. Higher data rates are necessary to increase
-both the total capacity and the speed of transmission. By comparison,
-the Mariner-4 spacecraft that sent back TV pictures of Mars had a data
-rate of only eight bits per second—a hundred thousand times too small
-for future missions. The use of lasers would mean that results could be
-transmitted to earth in seconds instead of the 8 hours it took for the
-photos to be sent from Mariner-4.
-
-One of the problems to be solved in using lasers for deep space
-communication, oddly enough, is that of pointing accuracy. Since the
-beam of laser energy is narrow, it would be possible for the radiation
-to miss the earth altogether and be lost entirely unless the laser were
-pointed at the receiver with extreme precision. Aiming a gun at a target
-50 yards away is one thing; aiming a laser from an unmanned spacecraft
-100 million miles away is quite another. It is believed, however, that
-present techniques can cope with the problem.
-
-Another peculiarity of laser communication is that it will probably be
-accomplished faster and more readily in space than here on earth.
-Powerful though laser light may be, it is light and is therefore impeded
-to some extent by our atmosphere even under good conditions. Data
-transmissions of 20 and 30 miles have already been accomplished in good
-weather with lasers.
-
-But if you have ever tried to force a searchlight beam or shine
-automobile headlights through heavy fog, rain, or snow, you will
-appreciate the magnitude of the problem under these conditions. The use
-of infrared frequencies helps to some extent, since infrared is somewhat
-more penetrating, but the poor-weather problem is a serious one.
-
-A possible solution is the use of “light pipes”, similar to the wave
-guides already in use for certain microwave applications over short
-distances. But as often happens, new developments create new needs; how,
-for example, can we get the laser beam to stay centered in the pipe and
-follow curves? A series of closely spaced lenses, about 1000 per mile,
-probably would accomplish this, but too much light would be lost by
-scattering from the many lens surfaces.
-
-Scientists are experimenting with a new kind of “lens”, one that uses
-variations in the density of gases to focus and guide the beam
-automatically. Since there are no surfaces in the path of the light
-beam, and since the gas is transparent and free of turbulence, the laser
-beam is not appreciably weakened or scattered as it travels through the
-pipe.
-
-    [Illustration: Figure 31 _Laser light beam being guided through a
-    “light pipe” by a gas “lens”. Heating coil (lower left) or mixture
-    of gases (lower right) are two possible ways of maintaining proper
-    density gradient in the gas._]
-
-Figure 31 shows how the gas focusing principle might be used to guide a
-beam through a curving pipe. The shading represents the density of the
-gas. Several means have been developed to keep the gas denser in the
-center than around the outside. When the pipe curves, the light beam
-starts moving off the axis of the pipe. The gas then acts like a prism,
-deflecting the light beam in the direction of the curvature of the
-“prism”.
-
-In communication between distant space and earth, a light pipe might be
-a little cumbersome; hence it may prove necessary to set up an
-intermediate orbiting relay station that will, particularly in cases of
-poor weather, intercept the incoming laser beam and convert it to radio
-frequencies that can penetrate our atmosphere with greater reliability.
-
-Powering space-borne lasers will, of course, be a problem. Indeed one of
-the major unsolved problems in production of spacecraft and long-term
-satellites is the provision of an adequate supply of power. Fuel cells
-and solar cells have helped but do not give the whole answer.[17]
-
-One other approach has already been developed: a sun-pumped laser.
-Sunlight focused onto the side of the laser (see Figure 32) provides the
-pumping power, enabling the device to put out 1 watt of continuous
-infrared radiation, enough for special space applications. Descendents
-of this device could produce visible light if this is deemed desirable.
-
-Another approach, using _chemical lasers_, is even more intriguing and
-may have greater consequences. Chemical lasers will derive their energy
-from their internal chemistry rather than from the outside. A mixture of
-two chemicals may be all that is needed to initiate laser action aboard
-a spacecraft or satellite. (Chemical lasers also offer the promise of
-even greater concentrations of power than have been achieved heretofore,
-which may make them useful in plasma research.)
-
-With all these possibilities, it may still be that spacecraft will need
-more power than is available on board. The narrow beam of the laser
-offers one more fascinating possibility, especially in the case of
-satellites relatively near earth. The light of a laser might actually be
-used to beam energy to a receiver, either for immediate use or storage.
-It would then become possible to “refuel” satellites at will, giving
-them much greater capabilities.
-
-If available laser power is great enough, laser beams might even be used
-to push satellites back into their proper orbits when they begin to
-wander off course, as they almost invariably do after a while.
-
-    [Illustration: Figure 32 _Artist’s rendering of sun-pumped laser as
-    it would operate in space. The sun’s rays are collected by a
-    parabolic reflector and are focused on the laser’s surface by two
-    cylindrical mirrors._]
-
-  Sun
-  Parabolic Collector
-  Hyperbolic-cylindric secondary mirror
-  Semi-circular-cylindric tertiary mirror
-  Laser beam
-
-
-
-
-                        A LASER IN YOUR FUTURE?
-
-
-Atomic energy, only a scientific dream a few short years ago, is now
-providing needed power in many parts of the world. In the same way, the
-laser, also an atomic phenomenon, has made its way out of the laboratory
-and into the fields of medicine, commerce, and industry. If it hasn’t
-touched your life as yet, you need only be patient. It will.
-
-Indeed the most exciting probability of all is that lasers undoubtedly
-will change our lives in ways we cannot even conceive of now.
-
-    [Illustration: Figure 33 _Tiny hole drilled in paper clip
-    demonstrates remarkable capability of laser beam. Paper clip is 1¼
-    inches long. Hole (top) was drilled by the laser microwelder shown
-    in Figure 1._]
-
-
-
-
-                          SUGGESTED REFERENCES
-
-
-Books
-
-  _ABC’s of Masers and Lasers_, Allan H. Lytel, Howard W. Sams and
-          Company, Inc., Publishers, Indianapolis, Indiana 46206, 1966,
-          96 pp., $2.25.
-  _The Laser: Light That Never Was Before_, Ben Patrusky, Dodd, Mead and
-          Company, New York 10016, 1966, 128 pp., $3.50.
-  _Masers and Lasers_, Manfred Brotherton, McGraw-Hill Book Company, New
-          York 10036, 1964, 224 pp., $8.50.
-  _Masers and Lasers_, H. Arthur Klein, J. B. Lippincott Company,
-          Philadelphia, Pennsylvania 19105, 1963, 184 pp., $3.95.
-  _The Story of the Laser_, John M. Carroll, E. P. Dutton and Company,
-          Inc., New York 10003, 1964, 181 pp., $3.95.
-  _Quantum Electronics: The Fundamentals of Transistors and Lasers_,
-          John R. Pierce, Doubleday and Company, Inc., New York 10017,
-          1966, 138 pp., $1.25.
-  _Lasers and Their Applications_, Kurt R. Stehling, The World
-          Publishing Company, Cleveland, Ohio 44102, 1966, 192 pp.,
-          $6.00.
-  _Understanding Lasers and Masers_, Stanley Leinwoll, Hayden Book
-          Companies, New York 10011, 1964, 96 pp., $1.95.
-  _Atomic Light: Lasers_, Richard B. Nehrich, Jr., Glenn I. Voran, and
-          Norman F. Dessel, Sterling Publishing Company, Inc., New York
-          10016, 1967, 136 pp., $3.95.
-
-
-Articles—General and Historical
-
-  Advances in Optical Masers, A. L. Schawlow, _Scientific American_,
-          209: 34 (July 1963).
-  The Evolution of the Physicist’s Picture of Matter, P. A. M. Dirac,
-          _Scientific American_, 208: 45 (May 1963).
-  Filling in the Blanks in the Laser’s Spectrum, F. M. Johnson,
-          _Electronics_, 39: 82 (April 18, 1966).
-  The Amateur Scientist—How a persevering amateur can build a gas laser
-          in the home, C. L. Stong, _Scientific American_, 211: 227
-          (September 1964).
-  The Amateur Scientist—Homemade Laser, C. L. Stong, _Scientific
-          American_, 213: 108 (December 1965).
-  The Amateur Scientist—How to make holograms and experiment with them
-          or with ready-made holograms, C. L. Stong, _Scientific
-          American_, 216: 122 (February 1967).
-  The Maser, James P. Gordon, _Scientific American_, 199: 42 (December
-          1958).
-  The Quantum Theory: Early Years to 1923, Karl Darrow, _Scientific
-          American_, 186: 47 (March 1952).
-  Laser’s Bright Magic, T. Meloy, _National Geographic Magazine_, 130:
-          858 (December 1966).
-  Infrared and Optical Masers (original paper), A. L. Schawlow and C. H.
-          Townes, _Physical Review_, 112: 1940 (December 15, 1958).
-  Laser Market Enters Era of Practicality, W. Mathews, _Electronic
-          News_, 11: 1 (April 18, 1966).
-  Lasers, A. K. Levine, _American Scientist_, 51: 14 (March 1963).
-  Lasers, A. L. Schawlow, _Science_, 149: 13 (July 2, 1965).
-  Lasers and Coherent Light, A. L. Schawlow, _Physics Today_, 17: 28
-          (January 1964).
-  The Laser’s Dazzling Future, L. Lessing, _Fortune_, 67: 138 (June
-          1963).
-  Optical Masers, A. L. Schawlow, _Scientific American_, 204: 52 (June
-          1961).
-  Optical Pumping, A. L. Bloom, _Scientific American_, 202: 72 (October
-          1960).
-  Research on Maser-Laser Principle Wins Nobel Prize in Physics, J. P.
-          Gordon, _Science_, 146: 897 (November 13, 1964).
-  Resource Letter MOP-1 on Masers (Microwave through Optical) and on
-          Optical Pumping, H. W. Moos, _American Journal of Physics_,
-          32: 589 (August 1964), extensive bibliography. Available from
-          American Institute of Physics, 335 East 45th Street, New York
-          10017. Enclose stamped return envelope.
-  Advances in Holography, K. S. Pennington, _Scientific American_, 218:
-          40 (February 1968).
-  Applications of Laser Light, D. R. Herriott, _Scientific American_,
-          219: 140 (September 1968).
-  Holography for the Sophomore Laboratory, R. H. Webb, _American Journal
-          of Physics_, 36: 62 (January 1968).
-  Laser Light, A. L. Schawlow, _Scientific American_, 219: 120
-          (September 1968).
-  The Modulation of Laser Light, D. F. Nelson, _Scientific American_,
-          218: 17 (June 1968).
-
-
-Articles—Special Subjects
-
-  Biological Effects of High Peak Power Radiation, S. Fine et al., _Life
-          Sciences_, 3: 209 (1964).
-  The Interaction of Light with Light, J. A. Giordmaine, _Scientific
-          American_, 210: 38 (April 1964).
-  Chemical Lasers, George C. Pimental, _Scientific American_, 214: 32
-          (April 1966).
-  Color Laser Stores Data, J. Eberhart, _Science News_, 90: 51 (July 23,
-          1966).
-  Communication by Laser, Stewart E. Miller, _Scientific American_, 214:
-          19 (January 1966).
-  Guidelines for Selecting Laser Materials, R. H. Hoskins, _Electronic
-          Design_, 13: _M_29 (July 19, 1965).
-  Holography: The Picture Looks Good, J. Blum, _Electronics_, 39: 139
-          (April 18, 1966).
-  How Dangerous Are Lasers?, L. H. Dulberger, _Electronics_, 35: 27
-          (January 26, 1962).
-  Injection Lasers, R. W. Keyes, _Industrial Research_, 6: 46 (October
-          1964).
-  Laser Potential in Deep-Space Link Grows, B. Miller, _Aviation Week
-          and Space Technology_, 84: 71 (January 31, 1966).
-  Laser Retinal Photocoagulator, N. S. Kapany et al., _Applied Optics_,
-          4: 517 (May 1965).
-  Laser Welding in Electronic Circuit Fabrication, J. P. Epperson,
-          _Electrical Design News_ (EDN), 10: 8 (October 1965).
-  The Light That Slices Inch into Millionths, (use of interferometry in
-          industry), _Steel_, 158: 38 (February 28, 1966).
-  The Optical Heterodyne—Key to Advanced Space Signaling, S. Jacobs,
-          _Electronics_, 36: 29 (July 12, 1963).
-  Photography by Laser, E. N. Leith and J. Upatnieks, _Scientific
-          American_, 212: 24 (June 1965).
-  Liquid Lasers, Alexander Lempicki and Harold Samelson, _Scientific
-          American_, 216: 81 (June 1967).
-  Plasma Experiments with a 570-kJ Theta-Pinch, F. C. Yahoda, et al.,
-          _Journal of Applied Physics_, 35: 2351 (August 1964).
-  A Sun-Pumped CW One-Watt Laser, C. G. Young, _Applied Optics_, 5: 993
-          (June 1966).
-  3-D Image Made at Home, _Science News_, 90: 185 (10 September 1966).
-  Scanning with Lasers, Robert A. Myers, _International Science and
-          Technology_, 65: 40 (May 1967).
-
-
-Booklets
-
-  _Applications of Lasers to Information Handling_, The Perkin-Elmer
-          Corporation, Norwalk, Connecticut 06852, 1966, 32 pp., free.
-          Reprint of five talks given by company personnel.
-  _Laser Interferometer_, Airborne Instruments Laboratory, Division of
-          Cutler-Hammer, Inc., Deer Park, Long Island, New York 11729,
-          1965, 20 pp., free. Collection of article reprints.
-  _Laser: The New Light_, Bell Telephone Laboratories, Murray Hill, New
-          Jersey 07971, 19 pp., free. Full color, nontechnical brochure
-          presents some background, principles, and applications of the
-          laser.
-
-    [Illustration: _Argon laser, which emits high-power blue-green beam
-    continuously, has application in signal processing, communications,
-    and spectroscopy. This unit is being beamed through prisms that
-    separate its several discrete wavelengths of light, displayed on
-    card at left foreground._]
-
-
-
-
-                               FOOTNOTES
-
-
-[1]Sometimes referred to as _hertz_ (abbreviated Hz), for the 19th
-    Century German physicist Heinrich Hertz; 1000 Hz = 1000 cps.
-
-[2]Devised in France and officially adopted there in 1799, the metric
-    system uses the meter as the basic unit of length and has been
-    proposed for all measurements in this country.
-
-[3]Named for the Swedish physicist Anders J. Angstrom.
-
-[4]The wavelength, indicated by the Greek letter λ (lambda) is related
-    to frequency (f) in the proportion λ (in meters) = 300,000,000/f.
-    (The number 300,000,000 is the velocity of light in meters per
-    second.)
-
-[5]Microwaves are radio waves with frequencies above 1000 megacycles per
-    second.
-
-[6]Ten to 30,000,000 kilocycles per second; this is low in the
-    electromagnetic spectrum, but not low in terms of the radio
-    spectrum, which has a low-frequency classification of its own.
-
-[7]Primitive as early radios were by today’s standards, they brought a
-    new era to communication at the time. Unmodulated CW (continuous
-    wave) transmissions and crystal receivers were used to summon
-    rescuers in the _Titanic_ disaster of 1912, for example.
-
-[8]Energy = h (Planck’s constant) × frequency. Planck’s constant is the
-    energy of 1 quantum of radiation, and equals 6.62556 × 10⁻²⁷
-    erg-sec.
-
-[9]Each photon carries 1 _quantum_ of radiation energy, which is a unit
-    equal to the product of the radiation frequency and Planck’s
-    constant (see footnote page 15).
-
-[10]Einstein was awarded the Nobel Prize in 1921 for his 1905
-    explanation of the photoelectric effect (in terms of quanta of
-    energy) and _not_ for his relativity theory.
-
-[11]Einstein’s theoretical explanation applies in the case of
-    stimulation of a single atom. In practical stimulation,
-    directionality is enhanced by stimulating many atoms in phase.
-
-[12]An atomic clock is a device that uses the extremely fast vibrations
-    of molecules or atomic nuclei to measure time. These vibrations
-    remain constant with time, consequently short intervals can be
-    measured with much higher precision than by mechanical or electrical
-    clocks.
-
-[13]The 1966 Nobel Prize in Physics was awarded to Prof. Alfred Kastler
-    of the University of Paris for his research on optical pumping and
-    studies on the energy levels of atoms.
-
-[14]See _Accelerators_, a companion booklet in this series, for a full
-    account of the Stanford “Atom Smasher”.
-
-[15]For descriptions of fission and fusion processes, see _Controlled
-    Nuclear Fusion_, _Nuclear Reactors_, and _Nuclear Power Plants_,
-    other booklets in this series.
-
-[16]A bit is a digit, or unit of information, in the binary
-    (base-of-two) system used in electronic data transmission systems.
-
-[17]See _SNAP_, _Nuclear Space Reactors_ and _Power from Radioisotopes_,
-    other booklets in this series, for descriptions of nuclear sources
-    of power for space.
-
-
-This booklet is one of the “Understanding the Atom” Series. Comments are
-invited on this booklet and others in the series; please send them to
-the Division of Technical Information, U. S. Atomic Energy Commission,
-Washington, D. C. 20545.
-
-Published as part of the AEC’s educational assistance program, the
-series includes these titles:
-
-  _Accelerators_
-  _Animals in Atomic Research_
-  _Atomic Fuel_
-  _Atomic Power Safety_
-  _Atoms at the Science Fair_
-  _Atoms in Agriculture_
-  _Atoms, Nature, and Man_
-  _Books on Atomic Energy for Adults and Children_
-  _Careers in Atomic Energy_
-  _Computers_
-  _Controlled Nuclear Fusion_
-  _Cryogenics, The Uncommon Cold_
-  _Direct Conversion of Energy_
-  _Fallout From Nuclear Tests_
-  _Food Preservation by Irradiation_
-  _Genetic Effects of Radiation_
-  _Index to the UAS Series_
-  _Lasers_
-  _Microstructure of Matter_
-  _Neutron Activation Analysis_
-  _Nondestructive Testing_
-  _Nuclear Clocks_
-  _Nuclear Energy for Desalting_
-  _Nuclear Power and Merchant Shipping_
-  _Nuclear Power Plants_
-  _Nuclear Propulsion for Space_
-  _Nuclear Reactors_
-  _Nuclear Terms, A Brief Glossary_
-  _Our Atomic World_
-  _Plowshare_
-  _Plutonium_
-  _Power from Radioisotopes_
-  _Power Reactors in Small Packages_
-  _Radioactive Wastes_
-  _Radioisotopes and Life Processes_
-  _Radioisotopes in Industry_
-  _Radioisotopes in Medicine_
-  _Rare Earths_
-  _Research Reactors_
-  _SNAP, Nuclear Space Reactors_
-  _Sources of Nuclear Fuel_
-  _Space Radiation_
-  _Spectroscopy_
-  _Synthetic Transuranium Elements_
-  _The Atom and the Ocean_
-  _The Chemistry of the Noble Gases_
-  _The Elusive Neutrino_
-  _The First Reactor_
-  _The Natural Radiation Environment_
-  _Whole Body Counters_
-  _Your Body and Radiation_
-
-A single copy of any one booklet, or of no more than three different
-booklets, may be obtained free by writing to:
-
-           USAEC, P. O. BOX 62, OAK RIDGE, TENNESSEE    37830
-
-Complete sets of the series are available to school and public
-librarians, and to teachers who can make them available for reference or
-for use by groups. Requests should be made on school or library
-letterheads and indicate the proposed use.
-
-Students and teachers who need other material on specific aspects of
-nuclear science, or references to other reading material, may also write
-to the Oak Ridge address. Requests should state the topic of interest
-exactly, and the use intended.
-
-In all requests, include “Zip Code” in return address.
-
-
-                Printed in the United States of America
-USAEC Division of Technical Information Extension, Oak Ridge, Tennessee
-
-
-
-
-                          Transcriber’s Notes
-
-
-—Silently corrected a few typos.
-
-—Retained publication information from the printed edition: this eBook
-  is public-domain in the country of publication.
-
-—In the text versions only, text in italics is delimited by
-  _underscores_.
-
-
-
-*** END OF THE PROJECT GUTENBERG EBOOK LASERS ***
-
-Updated editions will replace the previous one--the old editions will
-be renamed.
-
-Creating the works from print editions not protected by U.S. copyright
-law means that no one owns a United States copyright in these works,
-so the Foundation (and you!) can copy and distribute it in the
-United States without permission and without paying copyright
-royalties. Special rules, set forth in the General Terms of Use part
-of this license, apply to copying and distributing Project
-Gutenberg-tm electronic works to protect the PROJECT GUTENBERG-tm
-concept and trademark. Project Gutenberg is a registered trademark,
-and may not be used if you charge for an eBook, except by following
-the terms of the trademark license, including paying royalties for use
-of the Project Gutenberg trademark. If you do not charge anything for
-copies of this eBook, complying with the trademark license is very
-easy. You may use this eBook for nearly any purpose such as creation
-of derivative works, reports, performances and research. Project
-Gutenberg eBooks may be modified and printed and given away--you may
-do practically ANYTHING in the United States with eBooks not protected
-by U.S. copyright law. Redistribution is subject to the trademark
-license, especially commercial redistribution.
-
-START: FULL LICENSE
-
-THE FULL PROJECT GUTENBERG LICENSE
-PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
-
-To protect the Project Gutenberg-tm mission of promoting the free
-distribution of electronic works, by using or distributing this work
-(or any other work associated in any way with the phrase "Project
-Gutenberg"), you agree to comply with all the terms of the Full
-Project Gutenberg-tm License available with this file or online at
-www.gutenberg.org/license.
-
-Section 1. General Terms of Use and Redistributing Project
-Gutenberg-tm electronic works
-
-1.A. By reading or using any part of this Project Gutenberg-tm
-electronic work, you indicate that you have read, understand, agree to
-and accept all the terms of this license and intellectual property
-(trademark/copyright) agreement. If you do not agree to abide by all
-the terms of this agreement, you must cease using and return or
-destroy all copies of Project Gutenberg-tm electronic works in your
-possession. If you paid a fee for obtaining a copy of or access to a
-Project Gutenberg-tm electronic work and you do not agree to be bound
-by the terms of this agreement, you may obtain a refund from the
-person or entity to whom you paid the fee as set forth in paragraph
-1.E.8.
-
-1.B. "Project Gutenberg" is a registered trademark. It may only be
-used on or associated in any way with an electronic work by people who
-agree to be bound by the terms of this agreement. There are a few
-things that you can do with most Project Gutenberg-tm electronic works
-even without complying with the full terms of this agreement. See
-paragraph 1.C below. There are a lot of things you can do with Project
-Gutenberg-tm electronic works if you follow the terms of this
-agreement and help preserve free future access to Project Gutenberg-tm
-electronic works. See paragraph 1.E below.
-
-1.C. The Project Gutenberg Literary Archive Foundation ("the
-Foundation" or PGLAF), owns a compilation copyright in the collection
-of Project Gutenberg-tm electronic works. Nearly all the individual
-works in the collection are in the public domain in the United
-States. If an individual work is unprotected by copyright law in the
-United States and you are located in the United States, we do not
-claim a right to prevent you from copying, distributing, performing,
-displaying or creating derivative works based on the work as long as
-all references to Project Gutenberg are removed. Of course, we hope
-that you will support the Project Gutenberg-tm mission of promoting
-free access to electronic works by freely sharing Project Gutenberg-tm
-works in compliance with the terms of this agreement for keeping the
-Project Gutenberg-tm name associated with the work. You can easily
-comply with the terms of this agreement by keeping this work in the
-same format with its attached full Project Gutenberg-tm License when
-you share it without charge with others.
-
-1.D. The copyright laws of the place where you are located also govern
-what you can do with this work. Copyright laws in most countries are
-in a constant state of change. If you are outside the United States,
-check the laws of your country in addition to the terms of this
-agreement before downloading, copying, displaying, performing,
-distributing or creating derivative works based on this work or any
-other Project Gutenberg-tm work. The Foundation makes no
-representations concerning the copyright status of any work in any
-country other than the United States.
-
-1.E. Unless you have removed all references to Project Gutenberg:
-
-1.E.1. The following sentence, with active links to, or other
-immediate access to, the full Project Gutenberg-tm License must appear
-prominently whenever any copy of a Project Gutenberg-tm work (any work
-on which the phrase "Project Gutenberg" appears, or with which the
-phrase "Project Gutenberg" is associated) is accessed, displayed,
-performed, viewed, copied or distributed:
-
-  This eBook is for the use of anyone anywhere in the United States and
-  most other parts of the world at no cost and with almost no
-  restrictions whatsoever. You may copy it, give it away or re-use it
-  under the terms of the Project Gutenberg License included with this
-  eBook or online at www.gutenberg.org. If you are not located in the
-  United States, you will have to check the laws of the country where
-  you are located before using this eBook.
-
-1.E.2. If an individual Project Gutenberg-tm electronic work is
-derived from texts not protected by U.S. copyright law (does not
-contain a notice indicating that it is posted with permission of the
-copyright holder), the work can be copied and distributed to anyone in
-the United States without paying any fees or charges. If you are
-redistributing or providing access to a work with the phrase "Project
-Gutenberg" associated with or appearing on the work, you must comply
-either with the requirements of paragraphs 1.E.1 through 1.E.7 or
-obtain permission for the use of the work and the Project Gutenberg-tm
-trademark as set forth in paragraphs 1.E.8 or 1.E.9.
-
-1.E.3. If an individual Project Gutenberg-tm electronic work is posted
-with the permission of the copyright holder, your use and distribution
-must comply with both paragraphs 1.E.1 through 1.E.7 and any
-additional terms imposed by the copyright holder. Additional terms
-will be linked to the Project Gutenberg-tm License for all works
-posted with the permission of the copyright holder found at the
-beginning of this work.
-
-1.E.4. Do not unlink or detach or remove the full Project Gutenberg-tm
-License terms from this work, or any files containing a part of this
-work or any other work associated with Project Gutenberg-tm.
-
-1.E.5. Do not copy, display, perform, distribute or redistribute this
-electronic work, or any part of this electronic work, without
-prominently displaying the sentence set forth in paragraph 1.E.1 with
-active links or immediate access to the full terms of the Project
-Gutenberg-tm License.
-
-1.E.6. You may convert to and distribute this work in any binary,
-compressed, marked up, nonproprietary or proprietary form, including
-any word processing or hypertext form. However, if you provide access
-to or distribute copies of a Project Gutenberg-tm work in a format
-other than "Plain Vanilla ASCII" or other format used in the official
-version posted on the official Project Gutenberg-tm website
-(www.gutenberg.org), you must, at no additional cost, fee or expense
-to the user, provide a copy, a means of exporting a copy, or a means
-of obtaining a copy upon request, of the work in its original "Plain
-Vanilla ASCII" or other form. Any alternate format must include the
-full Project Gutenberg-tm License as specified in paragraph 1.E.1.
-
-1.E.7. Do not charge a fee for access to, viewing, displaying,
-performing, copying or distributing any Project Gutenberg-tm works
-unless you comply with paragraph 1.E.8 or 1.E.9.
-
-1.E.8. You may charge a reasonable fee for copies of or providing
-access to or distributing Project Gutenberg-tm electronic works
-provided that:
-
-* You pay a royalty fee of 20% of the gross profits you derive from
-  the use of Project Gutenberg-tm works calculated using the method
-  you already use to calculate your applicable taxes. The fee is owed
-  to the owner of the Project Gutenberg-tm trademark, but he has
-  agreed to donate royalties under this paragraph to the Project
-  Gutenberg Literary Archive Foundation. Royalty payments must be paid
-  within 60 days following each date on which you prepare (or are
-  legally required to prepare) your periodic tax returns. Royalty
-  payments should be clearly marked as such and sent to the Project
-  Gutenberg Literary Archive Foundation at the address specified in
-  Section 4, "Information about donations to the Project Gutenberg
-  Literary Archive Foundation."
-
-* You provide a full refund of any money paid by a user who notifies
-  you in writing (or by e-mail) within 30 days of receipt that s/he
-  does not agree to the terms of the full Project Gutenberg-tm
-  License. You must require such a user to return or destroy all
-  copies of the works possessed in a physical medium and discontinue
-  all use of and all access to other copies of Project Gutenberg-tm
-  works.
-
-* You provide, in accordance with paragraph 1.F.3, a full refund of
-  any money paid for a work or a replacement copy, if a defect in the
-  electronic work is discovered and reported to you within 90 days of
-  receipt of the work.
-
-* You comply with all other terms of this agreement for free
-  distribution of Project Gutenberg-tm works.
-
-1.E.9. If you wish to charge a fee or distribute a Project
-Gutenberg-tm electronic work or group of works on different terms than
-are set forth in this agreement, you must obtain permission in writing
-from the Project Gutenberg Literary Archive Foundation, the manager of
-the Project Gutenberg-tm trademark. Contact the Foundation as set
-forth in Section 3 below.
-
-1.F.
-
-1.F.1. Project Gutenberg volunteers and employees expend considerable
-effort to identify, do copyright research on, transcribe and proofread
-works not protected by U.S. copyright law in creating the Project
-Gutenberg-tm collection. Despite these efforts, Project Gutenberg-tm
-electronic works, and the medium on which they may be stored, may
-contain "Defects," such as, but not limited to, incomplete, inaccurate
-or corrupt data, transcription errors, a copyright or other
-intellectual property infringement, a defective or damaged disk or
-other medium, a computer virus, or computer codes that damage or
-cannot be read by your equipment.
-
-1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
-of Replacement or Refund" described in paragraph 1.F.3, the Project
-Gutenberg Literary Archive Foundation, the owner of the Project
-Gutenberg-tm trademark, and any other party distributing a Project
-Gutenberg-tm electronic work under this agreement, disclaim all
-liability to you for damages, costs and expenses, including legal
-fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
-LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
-PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE
-TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
-LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
-INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
-DAMAGE.
-
-1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
-defect in this electronic work within 90 days of receiving it, you can
-receive a refund of the money (if any) you paid for it by sending a
-written explanation to the person you received the work from. If you
-received the work on a physical medium, you must return the medium
-with your written explanation. The person or entity that provided you
-with the defective work may elect to provide a replacement copy in
-lieu of a refund. If you received the work electronically, the person
-or entity providing it to you may choose to give you a second
-opportunity to receive the work electronically in lieu of a refund. If
-the second copy is also defective, you may demand a refund in writing
-without further opportunities to fix the problem.
-
-1.F.4. Except for the limited right of replacement or refund set forth
-in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO
-OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT
-LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE.
-
-1.F.5. Some states do not allow disclaimers of certain implied
-warranties or the exclusion or limitation of certain types of
-damages. If any disclaimer or limitation set forth in this agreement
-violates the law of the state applicable to this agreement, the
-agreement shall be interpreted to make the maximum disclaimer or
-limitation permitted by the applicable state law. The invalidity or
-unenforceability of any provision of this agreement shall not void the
-remaining provisions.
-
-1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the
-trademark owner, any agent or employee of the Foundation, anyone
-providing copies of Project Gutenberg-tm electronic works in
-accordance with this agreement, and any volunteers associated with the
-production, promotion and distribution of Project Gutenberg-tm
-electronic works, harmless from all liability, costs and expenses,
-including legal fees, that arise directly or indirectly from any of
-the following which you do or cause to occur: (a) distribution of this
-or any Project Gutenberg-tm work, (b) alteration, modification, or
-additions or deletions to any Project Gutenberg-tm work, and (c) any
-Defect you cause.
-
-Section 2. Information about the Mission of Project Gutenberg-tm
-
-Project Gutenberg-tm is synonymous with the free distribution of
-electronic works in formats readable by the widest variety of
-computers including obsolete, old, middle-aged and new computers. It
-exists because of the efforts of hundreds of volunteers and donations
-from people in all walks of life.
-
-Volunteers and financial support to provide volunteers with the
-assistance they need are critical to reaching Project Gutenberg-tm's
-goals and ensuring that the Project Gutenberg-tm collection will
-remain freely available for generations to come. In 2001, the Project
-Gutenberg Literary Archive Foundation was created to provide a secure
-and permanent future for Project Gutenberg-tm and future
-generations. To learn more about the Project Gutenberg Literary
-Archive Foundation and how your efforts and donations can help, see
-Sections 3 and 4 and the Foundation information page at
-www.gutenberg.org
-
-Section 3. Information about the Project Gutenberg Literary
-Archive Foundation
-
-The Project Gutenberg Literary Archive Foundation is a non-profit
-501(c)(3) educational corporation organized under the laws of the
-state of Mississippi and granted tax exempt status by the Internal
-Revenue Service. The Foundation's EIN or federal tax identification
-number is 64-6221541. Contributions to the Project Gutenberg Literary
-Archive Foundation are tax deductible to the full extent permitted by
-U.S. federal laws and your state's laws.
-
-The Foundation's business office is located at 809 North 1500 West,
-Salt Lake City, UT 84116, (801) 596-1887. Email contact links and up
-to date contact information can be found at the Foundation's website
-and official page at www.gutenberg.org/contact
-
-Section 4. Information about Donations to the Project Gutenberg
-Literary Archive Foundation
-
-Project Gutenberg-tm depends upon and cannot survive without
-widespread public support and donations to carry out its mission of
-increasing the number of public domain and licensed works that can be
-freely distributed in machine-readable form accessible by the widest
-array of equipment including outdated equipment. Many small donations
-($1 to $5,000) are particularly important to maintaining tax exempt
-status with the IRS.
-
-The Foundation is committed to complying with the laws regulating
-charities and charitable donations in all 50 states of the United
-States. Compliance requirements are not uniform and it takes a
-considerable effort, much paperwork and many fees to meet and keep up
-with these requirements. We do not solicit donations in locations
-where we have not received written confirmation of compliance. To SEND
-DONATIONS or determine the status of compliance for any particular
-state visit www.gutenberg.org/donate
-
-While we cannot and do not solicit contributions from states where we
-have not met the solicitation requirements, we know of no prohibition
-against accepting unsolicited donations from donors in such states who
-approach us with offers to donate.
-
-International donations are gratefully accepted, but we cannot make
-any statements concerning tax treatment of donations received from
-outside the United States. U.S. laws alone swamp our small staff.
-
-Please check the Project Gutenberg web pages for current donation
-methods and addresses. Donations are accepted in a number of other
-ways including checks, online payments and credit card donations. To
-donate, please visit: www.gutenberg.org/donate
-
-Section 5. General Information About Project Gutenberg-tm electronic works
-
-Professor Michael S. Hart was the originator of the Project
-Gutenberg-tm concept of a library of electronic works that could be
-freely shared with anyone. For forty years, he produced and
-distributed Project Gutenberg-tm eBooks with only a loose network of
-volunteer support.
-
-Project Gutenberg-tm eBooks are often created from several printed
-editions, all of which are confirmed as not protected by copyright in
-the U.S. unless a copyright notice is included. Thus, we do not
-necessarily keep eBooks in compliance with any particular paper
-edition.
-
-Most people start at our website which has the main PG search
-facility: www.gutenberg.org
-
-This website includes information about Project Gutenberg-tm,
-including how to make donations to the Project Gutenberg Literary
-Archive Foundation, how to help produce our new eBooks, and how to
-subscribe to our email newsletter to hear about new eBooks.
diff --git a/old/65512-0.zip b/old/65512-0.zip
deleted file mode 100644
index e13459e..0000000
--- a/old/65512-0.zip
+++ /dev/null
diff --git a/old/65512-h.zip b/old/65512-h.zip
deleted file mode 100644
index 41bf3d4..0000000
--- a/old/65512-h.zip
+++ /dev/null
diff --git a/old/65512-h/65512-h.htm b/old/65512-h/65512-h.htm
deleted file mode 100644
index 9792e04..0000000
--- a/old/65512-h/65512-h.htm
+++ /dev/null
@@ -1,2632 +0,0 @@
-<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
-<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
-<head>
-<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
-<meta http-equiv="Content-Style-Type" content="text/css" />
-<meta name="viewport" content="width=device-width, initial-scale=1.0" />
-<title>Lasers, by Hal Hellman&mdash;a Project Gutenberg eBook</title>
-<meta name="author" content="Hal Hellman" />
-<meta name="pss.pubdate" content="1969" />
-<link rel="coverpage" href="images/cover.jpg" />
-<link rel="spine" href="images/spine.jpg" />
-<link rel="schema.DC" href="http://dublincore.org/documents/1998/09/dces/" />
-<meta name="DC.Title" content="Lasers" />
-<meta name="DC.Language" content="en" />
-<meta name="DC.Format" content="text/html" />
-<meta name="DC.Created" content="1969" />
-<meta name="DC.Creator" content="Hal Hellman" />
-<meta name="DC.Creator" content="U.S. Atomic Energy Commission" />
-<style type="text/css">
-/* == GLOBAL MARKUP == */
-body, table.twocol tr td  { margin-left:2em; margin-right:2em; }   /* BODY */
-.box     { border-style:double; margin-bottom:2em; max-width:30em; margin-right:auto; margin-left:auto; margin-top:2em; clear:both; }
-.box div.box  { border-style:solid; margin-right:auto; margin-left:auto; max-width:26em; }
-div.box h1 { text-indent:0; margin-top:0; margin-right:0em;
-            margin-left:0em; width:100%; color:white; font-size:250%;
-            background-color:black; font-family:sans-serif; }
-.box p   { margin-right:1em; margin-left:1em; }
-.box dl  { margin-right:1em; margin-left:1em; }
-h1, h2, h5, h6, .titlepg p { text-align:center; clear:both; text-indent:0; }   /* HEADINGS */
-h2                { margin-top:2em; margin-bottom:1em; font-size:120%;
-                    font-family:sans-serif; text-align:left; }
-h2 .small         { font-size:100%; }
-h2+h2             { margin-top:3.5em; }
-h1                { margin-top:3em; }
-h1 .likep         { font-weight:normal; font-size:50%; }
-h3                { margin-top:2em; text-align:left; font-size: 100%;
-                    font-family:sans-serif; clear:both; }
-h4, h5            { font-size:100%; text-align:right; clear:right; }
-h6                { font-size:100%; }
-h6.var            { font-size:80%; font-style:normal; }
-.titlepg          { margin-left:auto; margin-right:auto; border-style:double; clear:both; }
-span.chaptertitle { font-style:normal; display:block; text-align:center; font-size:150%; text-indent:0; }
-.tblttl           { text-align:center; text-indent:0;}
-.tblsttl          { text-align:center; font-variant:small-caps; text-indent:0; }
-
-pre sub.ms { width:4em; letter-spacing:1em; }
-pre        { margin-top:1em; margin-bottom:1em; }
-table.fmla { text-align:center; margin-top:0em; margin-bottom:0em; margin-left:0em; margin-right:0em; }
-table.inline, table.symbol { display: inline-table; vertical-align: middle; }
-td.cola           { text-align:left; vertical-align:100%; }
-td.colb           { text-align:justify; }
-
-p, blockquote, div.p, div.bq { text-align:justify; }                              /* PARAGRAPHS */
-div.p, div.bq   { margin-top:1em; margin-bottom:1em; }
-blockquote, .bq { margin-left:1em; margin-right:0em; }
-.verse    { font-size:100%; }
-p.indent  {text-indent:2em; text-align:left; }
-p.tb, p.tbcenter, verse.tb, blockquote.tb      { margin-top:2em; clear:both; }
-
-span.pb, div.pb, dt.pb, p.pb                   /* PAGE BREAKS */
-{ text-align:right; float:right; margin-right:0em; clear:right; }
-div.pb { display:inline; }
-.pb, dt.pb, dl.toc dt.pb, dl.tocl dt.pb, dl.undent dt.pb, dl.index dt.pb    { text-align:right; float:right; margin-left: 1.5em;
-         margin-top:.5em; margin-bottom:.5em; display:inline; text-indent:0;
-         font-size:80%; font-style:normal; font-weight:bold;
-         color:gray; border:1px solid gray;padding:1px 3px; }
-div.index .pb  { display:block; }
-.bq  div.pb, .bq span.pb  { font-size:90%; margin-right:2em; }
-
-div.img, body a img  {text-align:center; margin-left:auto; margin-right:auto; margin-top:2em; margin-bottom:2em; clear:right; }
-img         { max-width:100%; height:auto; }
-
-sup, a.fn { font-size:75%; vertical-align:100%; line-height:50%; font-weight:normal; }
-h3   a.fn { font-size:65%; }
-a.fn      { font-style:normal; }
-sub { font-size:75%; }
-.center, .tbcenter       { text-align:center; clear:both; text-indent:0; }                  /* TEXTUAL MARKUP */
-span.center              { display:block; }
-table.center { clear:both; margin-right:auto; margin-left:auto; }
-table.center tr       { font-family:sans-serif; font-size:85%; }
-table.center tr td.l,
-table.center tr th.l  {text-align:left; margin-left:0em; }
-table.center tr td.j  {text-align:justify; }
-table.center tr td.ltab { text-align:left; width:1.5em; }
-table.center tr td.t  {text-align:left; text-indent:1em; }
-table.center tr td.t2 {text-align:left; text-indent:2em; }
-table.center tr td.r, table.center tr th.r  {text-align:right; }
-table.center tr th.rx  { width:4.5em; text-align:right; }
-table.center tr th    {vertical-align:bottom; }
-table.center tr td    {vertical-align:top; }
-table.inline, table.symbol { display: inline-table; vertical-align: middle; }
-
-p           { clear:left; }
-.small, .lsmall      { font-size:90%; }
-.smaller    { font-size:80%; }
-.smallest   { font-size:67%; }
-.larger     { font-size:150%; }
-.large      { font-size:125%; }
-.xlarge     { font-size:150%; }
-.xxlarge    { font-size:200%; }
-.gs         { letter-spacing:1em; }
-.gs3        { letter-spacing:2em; }
-.gslarge    { letter-spacing:.3em; font-size:110%; }
-.sc         { font-variant:small-caps; font-style:normal; }
-.cur        { font-family:cursive; }
-.unbold     { font-weight:normal; }
-.xo         { position:relative; left:-.3em; }
-.over       { text-decoration: overline; display:inline; }
-hr          { width:20%; margin-left:40%; }
-hr.dwide    { margin-top:0; margin-bottom:0; width:90%; margin-left:5%; clear:right; }
-hr.double   { margin-top:0; margin-bottom:0; width:100%; margin-left:0; margin-right:0; }
-hr.f        { margin-top:0; margin-bottom:0; width:100%; margin-left:0; }
-.jl         { text-align:left; }
-.jr, .jri   { text-align:right; min-width:2em; display:inline-block; float:right; }
-.pcap .jri  { font-size:80%; }
-.jr1        { text-align:right; margin-right:2em; }
-h1 .jr      { margin-right:.5em; }
-.ind1       { text-align:left;  margin-left:2em; }
-.u          { text-decoration:underline; }
-.hst        { margin-left:2em; }
-.hst2       { margin-left:4em; }
-.rubric     { color:red; }
-.blue       { color:blue;   background-color:white; }
-.purple     { color:purple; background-color:white; }
-.green      { color:green;  background-color:white; }
-.yellow     { color:yellow; background-color:white; }
-.orange     { color:#ffa500; background-color:white; }
-.brown      { color:brown; background-color:white; }
-.white      { color:white; background-color:black; margin-left:1em; margin-right:1em; max-width:28em; }
-.cnwhite    { color:white; background-color:black; min-width:2em; display:inline-block;
-              text-align:center; font-weight:bold; font-family:sans-serif; }
-.cwhite     { color:white; background-color:black; text-align:center; font-weight:bold;
-              font-family:sans-serif; }
-ul  li      { text-align:justify; }
-u.dbl       { text-decoration:underline; }
-.ss         { font-family:sans-serif; font-weight:bold; }
-.ssn        { font-family:sans-serif; font-weight:normal; }
-p.revint    { margin-left:2em; text-indent:-2em; }
-.box p.revint  { margin-left:3em; }
-p.revint2   { margin-left:5em; text-indent:-3em; }
-p.revint2 .cn { min-width:2.5em; text-indent:0; text-align:left; display:inline-block; margin-right:.5em; }
-i .f        { font-style:normal; }
-.b          { font-weight:bold; }
-.i          { font-style:italic; }
-.f          { font-style:italic; font-weight:bold; }
-
-dd.t           { text-align:left; margin-left: 5.5em; }
-dl.toc         { clear:both; margin-top:1em; }                    /* CONTENTS (.TOC) */
-dl.toc  dt.center   { text-align:center; clear:both; margin-top:3em; margin-bottom:1em; text-indent:0;}
-.toc  dt      { text-align:right; clear:both; }
-.toc  dt.just { text-align:justify; margin-left:2em; margin-right:2em; }
-.toc  dd      { text-align:right; clear:both; }
-.toc  dd.ddt, .toc dd.t  { text-align:right; clear:both; margin-left:4em; }
-.toc  dd.ddt2,.toc dd.t2 { text-align:right; clear:both; margin-left:5em; }
-.toc  dd.ddt3 { text-align:right; clear:both; margin-left:6em; }
-.toc  dd.ddt4 { text-align:right; clear:both; margin-left:7em; }
-.toc  dd.ddt5 { text-align:right; clear:both; margin-left:8em; }
-.toc  dd.note { text-align:justify; clear:both; margin-left:5em; text-indent:-1em; margin-right:3em; }
-.toc  dt .xxxtest {width:17em; display:block; position:relative; left:4em; }
-.toc  dt  a,
-.toc  dd  a,
-.toc dt span.left,
-.toc dt span.lsmall,
-.toc dd span.left { text-align:left; clear:right; float:left; }
-.toc  dt  a  span.cn { width:4em; text-align:right; margin-right:.7em;  float:left; }
-.toc  dt.sc   { text-align:right;  clear:both; }
-.toc  dt.scl  { text-align:left;   clear:both; font-variant:small-caps; }
-.toc  dt.sct  { text-align:right;  clear:both; font-variant:small-caps; margin-left:1em; }
-.toc  dt .jl, .toc dd .jl   { text-align:left;  float:left;  clear:both; font-variant:normal; }
-.toc  dt.scc  { text-align:center; clear:both; font-variant:small-caps; text-indent:0; }
-.toc  dt span.lj, span.lj { text-align:left; display:block; float:left; }
-.toc  dd.center { text-align:center; text-indent:0; }
-dd.tocsummary {text-align:justify; margin-right:2em; margin-left:2em; }
-dd.center .sc  {display:block; text-align:center; text-indent:0; }
-/* BOX CELL */
-td.top { border-top:1px    solid; width:.5em; height:.8em; }
-td.bot { border-bottom:1px solid; width:.5em; height:.8em; }
-td.rb  { border:1px solid; border-left:none;  width:.5em; height:.8em;  }
-td.lb  { border:1px solid; border-right:none; width:.5em; height:.8em;  }
-td span.cellt   { text-indent:1em; }
-td span.cellt2  { text-indent:2em; }
-td span.cellt3  { text-indent:3em; }
-td span.cellt4  { text-indent:4em; }
-
-/* INDEX (.INDEX) */
-dl.index   { clear:both; }
-.index dt  { margin-left:2em; text-indent:-2em; text-align:left; }
-.index dd  { margin-left:4em; text-indent:-2em; text-align:left; }
-.index dd.t  { margin-left:6em; text-indent:-2em; text-align:left; }
-.index dt.center {text-align:center; text-indent:0; }
-  dl.indexlr     { clear:both; margin-left:auto; margin-right:auto;
-  max-width:20em; text-align:right; }
-  dl.indexlr dt  { clear:both; text-align:right; }
-  dl.indexlr a   { clear:both; float:right; text-align:right; }
-  dl.indexlr dt span { text-align:left; display:block; float:left; }
-  dl.indexlr dt.center {text-align:center; text-indent:0; }
-.ab, .ab1, .ab2 {
-font-weight:bold; text-decoration:none;
-border-style:solid; border-color:gray; border-width:1px;
-margin-right:0px; margin-top:5px; display:inline-block; text-align:center; text-indent:0; }
-.ab { width:1em; }
-.ab2 { width:1.5em; }
-a.gloss { background-color:#f2f2f2; border-bottom-style:dotted; text-decoration:none; border-color:#c0c0c0; color:inherit; }
-                                                           /* FOOTNOTE BLOCKS */
-div.notes p  { margin-left:1em; text-indent:-1em; text-align:justify; }
-
-dl.undent dd    { margin-left:3em; text-indent:-2em; text-align:justify; }
-dl.undent dt    { margin-left:2em; text-indent:-2em; text-align:justify; clear:both; }
-dl.undent dd.t  { margin-left:4em; text-indent:-2em; text-align:justify; }
-dl.undent dd.t2 { margin-left:5em; text-indent:-2em; text-align:justify; }
-                                                           /* POETRY LINE NUMBER */
-.lnum   { text-align:right; float:right; margin-left:.5em; display:inline; }
-
-.hymn     { text-align:left; }                                  /*  HYMN AND VERSE: HTML  */
-.verse    { text-align:left; margin-top:1em; margin-bottom:1em; margin-left:0em; }
-.versetb  { text-align:left; margin-top:2em; margin-bottom:1em; margin-left:0em; }
-.originc  { text-align:center; text-indent:0; }
-.subttl   { text-align:center; font-size:80%; text-indent:0; }
-.srcttl   { text-align:center; font-size:80%; text-indent:0; font-weight:bold; }
-p.lc      { text-indent:0; text-align:center; margin-top:0; margin-bottom:0; }
-p.t0, p.l { margin-left:4em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.lb                 { margin-left:4em;  text-indent:-3em; margin-top:2em; margin-bottom:0; text-align:left; }
-p.tw, div.tw, .tw    { margin-left:1em;  text-indent:-1em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t,  div.t,  .t     { margin-left:5em;  text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t2,  div.t2,  .t2  { margin-left:6em;  text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t3,  div.t3,  .t3  { margin-left:7em;  text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t4,  div.t4,  .t4  { margin-left:8em;  text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t5,  div.t5,  .t5  { margin-left:9em;  text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t6,  div.t6,  .t6  { margin-left:10em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t7,  div.t7,  .t7  { margin-left:11em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t8,  div.t8,  .t8  { margin-left:12em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t9,  div.t9,  .t9  { margin-left:13em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t10, div.t10,.t10  { margin-left:14em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t11, div.t11,.t11  { margin-left:15em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t12, div.t12,.t12  { margin-left:16em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t13, div.t13,.t13  { margin-left:17em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t14, div.t14,.t14  { margin-left:18em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.t15, div.t15,.t15  { margin-left:19em; text-indent:-3em; margin-top:0; margin-bottom:0; text-align:left; }
-p.lr, div.lr, span.lr { display:block; margin-left:0em; margin-right:1em; margin-top:0; margin-bottom:0; text-align:right; }
-dt.lr                { width:100%; margin-left:0em; margin-right:0em; margin-top:0; margin-bottom:0; margin-top:1em; text-align:right; }
-dl dt.lr a           { text-align:left; clear:left; float:left; }
-
-.fnblock   { margin-top:2em; margin-bottom:2em; }
-.fndef, p.fn     { text-align:justify; margin-top:1.5em; margin-left:1.5em; text-indent:-1.5em; }
-.fndef p.fncont, .fndef dl { margin-left:0em; text-indent:0em; }
-.fnblock div.fncont { margin-left:1.5em; text-indent:0em; margin-top:1em; text-align:justify; }
-.fnblock dl  { margin-top:0; margin-left:4em; text-indent:-2em; }
-.fnblock dt  { text-align:justify; }
-dl.catalog dd { font-style:italic; }
-dl.catalog dt { margin-top:1em; }
-.author { text-align:right; margin-top:0em; margin-bottom:0em; display:block; }
-
-dl.biblio  dt        { margin-top:.6em; margin-left:2em; text-indent:-2em; text-align:justify; clear:both; }
-dl.biblio  dt  div   { display:block; float:left; margin-left:-6em; width:6em; clear:both; }
-dl.biblio  dt.center { margin-left:0em; text-align:center; text-indent:0; }
-dl.biblio  dd        { margin-top:.3em; margin-left:3em; text-align:justify; font-size:90%; }
-p.biblio             { margin-left:2em; text-indent:-2em; }
-.clear     { clear:both; }
-p.book     { margin-left:2em; text-indent:-2em; }
-p.review   { margin-left:2em; text-indent:-2em; font-size:80%; }
-p.pcap     { margin-left:0em; text-indent:0; text-align:justify; margin-top:0; font-size:110%; }
-p.pcapc    { margin-left:4.7em; text-indent:0em; text-align:justify; }
-dl.pcap    { font-family:sans-serif; font-size:85%; margin-left:2em; }
-span.attr  { font-size:80%; font-family:sans-serif; }
-span.pn    { display:inline-block; width:4.7em; text-align:left; margin-left:0; text-indent:0; }
-</style>
-</head>
-<body>
-<div style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of Lasers, by Hal Hellman</div>
-<div style='display:block; margin:1em 0'>
-This eBook is for the use of anyone anywhere in the United States and
-most other parts of the world at no cost and with almost no restrictions
-whatsoever. You may copy it, give it away or re-use it under the terms
-of the Project Gutenberg License included with this eBook or online
-at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you
-are not located in the United States, you will have to check the laws of the
-country where you are located before using this eBook.
-</div>
-
-<div style='display:table; margin-bottom:1em;'>
-  <div style='display:table-row'>
-    <div style='display:table-cell; padding-right:0.5em'>Title:</div>
-    <div style='display:table-cell'>Lasers</div>
-  </div>
-</div>
-<div style='display:table; margin-bottom:1em;'>
-<div style='display:table-row'>
-  <div style='display:table-cell; padding-right:0.5em'>Author:</div>
-  <div style='display:table-cell'>Hal Hellman</div>
-</div>
-</div>
-<div style='display:block; margin:1em 0'>Release Date: June 4, 2021 [eBook #65512]</div>
-<div style='display:block; margin:1em 0'>Language: English</div>
-<div style='display:table; margin-bottom:1em;'>
-  <div style='display:table-row'>
-    <div style='display:table-cell; padding-right:0.5em; white-space:nowrap;'>Produced by:</div>
-    <div style='display:table-cell'>Stephen Hutcheson and the Online Distributed Proofreading Team at https://www.pgdp.net </div>
-  </div>
-</div>
-<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK LASERS ***</div>
-<div id="cover" class="img">
-<img id="coverpage" src="images/cover.jpg" alt="Lasers" width="1000" height="1554" />
-</div>
-<div class="box">
-<h1>Lasers</h1>
-<p class="center"><span class="ss">by Hal Hellman</span></p>
-<p class="tbcenter"><span class="ss">U.S. ATOMIC ENERGY COMMISSION
-<br />Division of Technical Information
-<br /><i>Understanding the Atom Series</i></span></p>
-<p class="center smallest"><span class="ss">ATOMIC ENERGY COMMISSION
-<br />UNITED STATES OF AMERICA</span></p>
-</div>
-<div class="pb" id="Page_i">i</div>
-<h3 id="c1">The Understanding the Atom Series</h3>
-<p>Nuclear energy is playing a vital role in the life of every
-man, woman, and child in the United States today. In the
-years ahead it will affect increasingly all the peoples of the
-earth. It is essential that all Americans gain an understanding
-of this vital force if they are to discharge thoughtfully their
-responsibilities as citizens and if they are to realize fully the
-myriad benefits that nuclear energy offers them.</p>
-<p>The United States Atomic Energy Commission provides
-this booklet to help you achieve such understanding.</p>
-<p class="jr1"><img class="inline" src="images/ejb.jpg" alt="Edward J. Brunenkant" width="300" height="103" />
-<br />Edward J. Brunenkant, Director
-<br />Division of Technical Information</p>
-<dl class="undent"><dt>UNITED STATES ATOMIC ENERGY COMMISSION</dt>
-<dt>Dr. Glenn T. Seaborg, Chairman</dt>
-<dt>James T. Ramey</dt>
-<dt>Wilfrid E. Johnson</dt>
-<dt>Dr. Clarence E. Larson</dt></dl>
-<div class="pb" id="Page_ii">ii</div>
-<div class="img">
-<img src="images/laser.jpg" id="ncfig1" alt="LASERS" width="600" height="120" />
-</div>
-<p><span class="lr"><span class="ss">by Hal Hellman</span></span></p>
-<h2 id="toc" class="center">CONTENTS</h2>
-<dl class="toc">
-<dt><a href="#c2">INTRODUCTION</a> 1</dt>
-<dt><a href="#c3">THE ELECTROMAGNETIC SPECTRUM</a> 5</dt>
-<dt><a href="#c4">RADIO WAVES</a> 9</dt>
-<dt><a href="#c5">LIGHT AND THE ATOM</a> 14</dt>
-<dt><a href="#c6">WHAT&rsquo;S SO SPECIAL ABOUT COHERENT LIGHT?</a> 19</dt>
-<dt><a href="#c7">CONTROLLED EMISSION</a> 25</dt>
-<dt><a href="#c8">A LASER IS BORN</a> 29</dt>
-<dt><a href="#c9">LASING&mdash;A NEW WORD</a> 32</dt>
-<dt><a href="#c10">SOME INTERESTING APPLICATIONS</a> 34</dt>
-<dt><a href="#c11">A MULTITUDE OF LASERS</a> 42</dt>
-<dt><a href="#c12">COMMUNICATIONS</a> 48</dt>
-<dt><a href="#c13">A LASER IN YOUR FUTURE?</a> 52</dt>
-<dt><a href="#c14">SUGGESTED REFERENCES</a> 53</dt>
-</dl>
-<p class="tbcenter"><span class="ss">United States Atomic Energy Commission
-<br />Division of Technical Information</span></p>
-<p class="center smaller">Library of Congress Catalog Card Number: 68-60742
-<br />1968; 1969(rev.)</p>
-<div class="pb" id="Page_iii">iii</div>
-<div class="img" id="imgx1">
-<img src="images/p01.jpg" alt="" width="1000" height="1099" />
-<p class="pcap"><i>Nothing about lasers is more astonishing than their ability to produce
-holograms, under arrangements such as shown above. Two
-laser beams (of different colors) emerge from the curtain (rear).
-They are optically combined (left center) and the combined beam is
-then divided by prisms, mirrors and lenses so that part of it shines
-on the figurines (foreground) and part on the square holographic
-plate (right center). When the plate is developed (like an ordinary
-photographic film), it will seem to have only a dull gray surface
-until it is viewed with spatially coherent light (such as from a laser
-or a beam through a pinhole) shining through it. Then an amazing,
-multi-colored, three-dimensional image of the figurines will be
-visible. (See <a href="#Page_19">page 19</a> and <a href="#fig13">Figure 13</a>.)</i></p>
-</div>
-<div class="pb" id="Page_1">1</div>
-<div class="img">
-<img src="images/laser.jpg" id="ncfig2" alt="LASERS" width="600" height="120" />
-</div>
-<p><span class="lr"><span class="ss">By HAL HELLMAN</span></span></p>
-<h2 id="c2"><span class="small">INTRODUCTION</span></h2>
-<p>The transistor burst upon the electronic scene in the
-1950s. Almost overnight the size of new models of radios,
-television sets, and a host of other electronic devices
-shrank like deflating balloons. Suddenly the hard-of-hearing
-could carry their sound amplifiers in their ears. Teenagers
-could listen to favorite music wherever they went. Everywhere
-we turned the transistor was making its mark. There
-was even a proposal before Congress to require that every
-home have a transistor radio in case of emergency.</p>
-<p>The next development to fire the imagination of scientists
-and engineers was the laser&mdash;an instrument that
-produces an enormously intense pencil-thin beam of light.
-Most of us have heard so much about this invention it
-seems hard to believe that the first one was built only a
-few years ago. We were told that the laser was going to
-have an even greater effect on our lives than the transistor.
-It was going to replace everything from dentists&rsquo;
-drills to electric wires. The whole world, it seemed,
-eventually would be nothing but a gigantic collection of
-lasers that would do everything anyone wanted. Roads
-would be blazed through jungles at one sweep; our country
-would be safe once and for all from intercontinental ballistic
-missiles; cancer would be licked; computers would
-be small enough to carry in a purse; and so on and on.</p>
-<div class="pb" id="Page_2">2</div>
-<p>Yet for the first couple of years the laser seemed able
-to do nothing but blaze holes in razor blades for TV commercials.
-Somehow the device never seemed to emerge
-from the laboratory, prompting one cynic to call it &ldquo;an
-invention in search of an application&rdquo;.</p>
-<p>Many of the wild claims came from misunderstandings
-on the part of the press, others from exaggerations by
-a few manufacturers who wanted free publicity. But with
-even less exotic devices than lasers, the road from the laboratory
-to the marketplace may often be long and hard.
-Price, efficiency, reliability, convenience&mdash;these are all
-factors that must be considered. It soon became clear that
-with something as new as the laser, much improvement
-was necessary before it could be used in science and
-medicine, and even more before it could be used in industry.</p>
-<p>It now seems, however, that the turning point has been
-reached. We have seen laser equipment put on the market
-for performing delicate surgery on the eye, spot-welding
-tiny electronic circuits (<a href="#fig1">Figure 1</a>), and controlling machine
-tools with amazing accuracy (<a href="#fig2">Figure 2</a>).</p>
-<div class="img" id="fig1">
-<img src="images/p02.jpg" alt="" width="800" height="867" />
-<p class="pcap"><span class="ss">Figure 1</span> <i>A commercial laser microwelder. A microscope is
-needed for accurate placement of beam energy.</i></p>
-</div>
-<div class="pb" id="Page_3">3</div>
-<p>The pace is quickening. At least a dozen manufacturers
-have announced that they are designing laser technology
-into their products. These are not laboratory experiments
-but practical products for measurement and testing, and
-for industrial, military, medical, and space uses. The
-Army, for example, has announced that it will purchase its
-first equipment for use in the field: a portable, highly accurate
-range finder for artillery observation.</p>
-<p>Still, this hardly accounts for the $100,000,000 spent in
-one recent year on laser research and development by
-some 500 laboratories in the United States. The U. S.
-Government alone has spent about $25,000,000 on laser
-research in a single year. Dozens, and perhaps hundreds,
-of other applications are on the fire&mdash;simmering or boiling
-as the case may be. Some require particular technical
-innovations such as greater power or higher efficiency.
-Others are entirely new applications. One of the most exciting
-of these is holography (pronounced ho LOG ra phy).</p>
-<p>Holography involves a completely different approach to
-photography. In addition to more immediate applications in
-microscopy, information storage and retrieval, and interferometry,
-it promises such bonuses as 3-dimensional
-color movies and TV someday.</p>
-<p>You have to see the holographic process in operation to
-believe it. One moment you are looking at what appears to
-be an underexposed or lightly smudged photographic plate.
-Then suddenly a true-to-life image of the original object
-springs into being behind the negative&mdash;apparently suspended
-in midair! Not only is the full effect of &ldquo;roundness&rdquo;
-and depth there, but you can also see anything lying
-behind the object&rsquo;s image by moving your head, exactly
-as if the original scene containing the object were really
-there.</p>
-<p>Still another important field of application is that of
-communications. Perhaps because it is less spectacular
-than burning holes in razor blades, we haven&rsquo;t heard as
-much about it. Yet there are probably more physicists and
-engineers working on adapting the laser for use in communications
-than on any other single laser project.</p>
-<p>The reason for this is the fact that existing communications
-facilities are becoming overloaded. Space on transoceanic
-<span class="pb" id="Page_4">4</span>
-telephone lines is already at a premium, with
-waiting periods sometimes running into hours. Radio
-&ldquo;ham&rdquo; operators have been threatened with loss of some
-of their best operating frequencies to meet the demand of
-emerging nations of Africa for new channels. Television
-programs must compete for space on cross-country networks
-with telephone, telegraph, and transmission of data.
-The increasing use of computers in science, business, and
-industry will strain our facilities still further. Communication
-satellites will help, but they will not give us the whole
-answer; and much development work remains to be done
-on satellites.</p>
-<div class="img" id="fig2">
-<img src="images/p03.jpg" alt="" width="1000" height="619" />
-<p class="pcap"><span class="ss">Figure 2</span> <i>Precision control of a machine tool by laser light.</i></p>
-</div>
-<p>Why the interest in the laser for communications? In a
-recent experiment all seven of the New York TV channels
-were transmitted over a single laser beam. In terms of
-telephone conversations, one laser system could theoretically
-carry 800,000,000 conversations&mdash;four for each
-person in the United States.</p>
-<p>In this booklet we shall learn what there is about the
-laser that gives it so much promise. We shall investigate
-what it is, how it works, and the different kinds of lasers
-there are. We begin by discussing some of the more familiar
-kinds of radiation, such as radio and microwaves,
-light and X rays.</p>
-<div class="pb" id="Page_5">5</div>
-<h2 id="c3"><span class="small">THE ELECTROMAGNETIC SPECTRUM</span></h2>
-<p>Some 85% of what man learns comes to him through his
-vision in response to the medium of light. Yet, ironically,
-it wasn&rsquo;t until the end of the 17th century that he first began
-to get an inkling of what light really is. It took the great
-scientific genius Isaac Newton to show that so-called white
-light is really a combination of all the colors of the rainbow.
-A few years later the Dutch astronomer Christiaan
-Huygens introduced the idea that light is a wave motion, a
-concept finally validated in 1803 when the British physician
-Thomas Young ingeniously demonstrated interference effects
-in waves. Thus it was finally realized that the only
-difference between the various colors of light was one of
-wavelength.</p>
-<p>For light was indeed found to be a wave phenomenon, no
-different in principle from the water waves you have seen
-a thousand times. If you stand at the seashore, you can
-easily count the number of waves that approach the shore
-in a minute. Divide that number by 60 and you have the
-frequency of the wave motion in the familiar unit, cycles-per-second
-(cps).<a class="fn" id="fr_1" href="#fn_1">[1]</a></p>
-<p>You would have to count pretty quickly to do this for
-light, however. Light waves vibrate or oscillate at the rate
-of some 400 million million times a second. That&rsquo;s the
-vibration rate of waves of red light; violet results from
-vibrations that are just about twice that fast.</p>
-<p>With frequencies of this magnitude, discussion and
-handling of data and dimensions are cumbersome and
-rather awkward. Fortunately there is another approach.
-Let&rsquo;s look again at our ocean waves. We see that there is
-a regularity about them (before they begin to break on the
-shore). The distance from one crest to the next is significant
-and is called the <i>wavelength</i>. Water waves are measured
-in feet, and in comparable units light waves are
-recorded in ten-millionths of an inch&mdash;again a very cumbersome
-number. Scientists therefore use the metric
-<span class="pb" id="Page_6">6</span>
-system<a class="fn" id="fr_2" href="#fn_2">[2]</a>
-and have standardized a unit called the angstrom<a class="fn" id="fr_3" href="#fn_3">[3]</a>,
-which is equal to the one-hundred-millionth part of a
-centimeter (10&#8315;&#8312; cm). Thus we find, as shown in <a href="#fig3">Figure 3</a>,
-that the visible light range runs from the violet at about
-4000 angstroms to red at about 7000 angstroms.</p>
-<div class="img" id="fig3">
-<img src="images/p04.jpg" alt="" width="800" height="241" />
-<p class="pcap"><span class="ss">Figure 3</span> <i>The visible light spectrum ranges between approximately
-4000 and 7000 angstroms.</i></p>
-</div>
-<table class="center">
-<tr class="th"><th> </th><th class="ss">Wavelength (Angstroms)</th></tr>
-<tr><td class="l">Violet </td><td class="c">4000-4300</td></tr>
-<tr><td class="l">Blue </td><td class="c">4300-5000</td></tr>
-<tr><td class="l">Green </td><td class="c">5000-5600</td></tr>
-<tr><td class="l">Yellow </td><td class="c">5600-5800</td></tr>
-<tr><td class="l">Orange </td><td class="c">5800-6100</td></tr>
-<tr><td class="l">Red </td><td class="c">6100-7000</td></tr>
-</table>
-<p>At roughly the same time that the wavelength of light
-was being determined, the German-British astronomer
-William Herschel performed an interesting experiment.
-He held a thermometer in turn in the various colors of
-light that had been spread out by an optical prism. As he
-moved the thermometer from the violet to the red, the
-temperature reading rose&mdash;and it continued to rise as he
-moved the instrument <i>beyond</i> the red area, where no prismatic
-light could be seen.</p>
-<p>Thus Herschel discovered infrared rays (the kind of heat
-we get from the sun) adjoining the visible red light, and
-at the same time found that they were merely a continuation
-of the visible spectrum. Shortly thereafter, ultraviolet
-rays were found on the other end of the visible light band.</p>
-<p>One of the most fascinating movements in science has
-been the constant expansion since then of both ends of the
-radiating-wave spectrum. The result has come to be called
-the <i>electromagnetic spectrum</i>, which, as we see in <a href="#fig4">Figure 4</a>,
-encompasses a wide variety of apparently different kinds of
-radiation. Above the visible band (the higher frequencies),
-we find ultraviolet light, X rays, gamma rays, and some
-cosmic rays; below it are infrared radiation, microwaves,
-<span class="pb" id="Page_7">7</span>
-and radio waves. Only a small proportion of the total
-spectrum is occupied by the visible band. Another point of
-interest is the inverse relationship between wavelength and
-frequency. As one goes up the other goes down.<a class="fn" id="fr_4" href="#fn_4">[4]</a></p>
-<div class="img" id="fig4">
-<img src="images/p04a.jpg" alt="" width="800" height="848" />
-<p class="pcap"><span class="ss">Figure 4</span> <i>Visible light region spans a tiny portion of the total electromagnetic
-spectrum.</i></p>
-</div>
-<table class="center">
-<tr class="th"><th>Frequency (cps) </th><th> </th><th>Wavelength</th></tr>
-<tr class="th"><th> </th><th> </th><th>Angstroms</th></tr>
-<tr><td class="r"> </td><td class="c">Cosmic rays</td></tr>
-<tr><td class="r">10&sup2;&sup2; </td><td class="c"> </td><td class="l">0.0001</td></tr>
-<tr><td class="r"> </td><td class="c"> </td><td class="l">0.001</td></tr>
-<tr><td class="r">10&sup2;&#8304; </td><td class="c">Gamma rays </td><td class="l">0.01</td></tr>
-<tr><td class="r"> </td><td class="c"> </td><td class="l">0.1</td></tr>
-<tr><td class="r">10&sup1;&#8312; </td><td class="c">X rays </td><td class="l">1</td></tr>
-<tr><td class="r"> </td><td class="c"> </td><td class="l">10</td></tr>
-<tr><td class="r">10&sup1;&#8310; </td><td class="c">Ultraviolet radiation </td><td class="l">100</td></tr>
-<tr><td class="r"> </td><td class="c"> </td><td class="l">1,000</td></tr>
-<tr><td class="r"> </td><td class="c">Visible light</td></tr>
-<tr><td class="r">10&sup1;&#8308; </td><td class="c"> </td><td class="l">10,000</td></tr>
-<tr><td class="r"> </td><td class="c">Infrared radiation </td><td class="l">100,000</td></tr>
-<tr class="th"><th> </th><th> </th><th>Angstroms</th></tr>
-<tr><td class="r"> </td><td class="c"> </td><td class="l">0.01</td></tr>
-<tr><td class="r">10&sup1;&sup2; </td><td class="c">Millimeter waves </td><td class="l">0.1</td></tr>
-<tr><td class="r">10&sup1;&#8304; </td><td class="c">Microwaves, radar </td><td class="l">1</td></tr>
-<tr><td class="r"> </td><td class="c"> </td><td class="l">10</td></tr>
-<tr><td class="r">10&#8312; </td><td class="c">TV and FM radio </td><td class="l">100</td></tr>
-<tr><td class="r"> </td><td class="c">Short wave </td><td class="l">1,000</td></tr>
-<tr><td class="r">10&#8310; </td><td class="c">AM radio </td><td class="l">10,000</td></tr>
-<tr><td class="r"> </td><td class="c">Low frequency communications </td><td class="l">100,000</td></tr>
-<tr><td class="r">10,000 = 10&#8308; </td><td class="c"> </td><td class="l">1,000,000</td></tr>
-</table>
-<p>These many kinds of rays and waves vary tremendously
-in the ways they interact with matter. But they are all
-part of a single family. The only difference among them,
-as with the colors of the rainbow, lies in their wavelengths.
-In a few cases, as we shall see later, the mode of
-generation is also different.</p>
-<p>The band of radiation stretching from the infrared to
-cosmic rays has been, up to now, largely the concern of
-<span class="pb" id="Page_8">8</span>
-physical scientists. Because of their high frequencies, these
-radiations are generally handled, when calculations or
-measurements must be made, in terms of wavelength.
-Radio and microwaves<a class="fn" id="fr_5" href="#fn_5">[5]</a>, on the other hand, have been more
-in the domain of communications engineers and are more
-likely to be discussed in terms of frequency. Thus it is
-that your radio is marked off in kilocycles, or thousands
-of cycles per second, while light is described as radiation
-in the 4000 to 7000 angstrom band.</p>
-<p>The relative newness of the various radiations has kept
-scientists busy learning about them and, as information
-and experience have accumulated, putting them to work.</p>
-<div class="pb" id="Page_9">9</div>
-<h2 id="c4"><span class="small">RADIO WAVES</span></h2>
-<p>One of the first of the newly discovered electromagnetic
-radiations to be put to work was the radio wave, which is
-characterized by long wavelength and low frequency.<a class="fn" id="fr_6" href="#fn_6">[6]</a> The
-low frequency makes it relatively easy to produce a wave
-having virtually all its power concentrated at one frequency.</p>
-<p>The advantage of this capability becomes obvious after
-a moment&rsquo;s thought. Think for example of a group of people
-lost in a forest. If they hear sounds of a search party off
-in the distance, all likely will begin to shout in various
-ways for help. Not a very efficient process, is it? But
-suppose all the energy going into the production of this
-noise could be concentrated in a single shout or whistle.
-Clearly, their chances of being found would be much
-improved.</p>
-<div class="img" id="fig5">
-<img src="images/p05.jpg" alt="" width="600" height="276" />
-<p class="pcap"><b>Figure 5</b> <i>(a) Temporally coherent radiation. (b) Temporally incoherent
-radiation.</i></p>
-</div>
-<p>The single frequency capability of radio waves has been
-given the name <i>temporal coherence</i> (or coherence in time)
-and is illustrated in <a href="#fig5">Figure 5</a>. Part <i>a</i> shows a single sine
-wave, the common way of representing electromagnetic
-radiation, and particularly <i>temporally coherent radiation</i>. In
-<i>b</i> we see what <i>temporally incoherent radiation</i> (such as the
-mixed sounds of the stranded party) would look like.</p>
-<p>It was on Christmas Eve 1906 that music and speech came
-out of a radio receiver for the first time. Today the sight
-<span class="pb" id="Page_10">10</span>
-of someone walking, riding, or studying with an earpiece
-plugged into a transistor radio is common. But the early
-radio enthusiasts <i>had</i> to wear earphones because it takes
-considerable power to activate a loudspeaker and the
-received signal was quite weak. Some means of increasing,
-or amplifying, the signal was needed if the process was to
-advance beyond this primitive stage.<a class="fn" id="fr_7" href="#fn_7">[7]</a></p>
-<p>The use of vacuum tube or electron tube amplifiers is
-so widespread that it is unnecessary to explain their operations
-here in any detail. It is important that the principle
-of amplification be understood, however. The input or information
-wave causes the grid to act as a sort of faucet
-as shown in <a href="#fig6">Figure 6</a>. That is, it controls the flow of electrons
-(the current in the circuit) from cathode to anode.
-A weak signal can therefore cause a similar, but much
-stronger, signal to appear in the circuit. The larger signal
-is subsequently used to power a loudspeaker in the radio
-set.</p>
-<div class="img" id="fig6">
-<img src="images/p06.jpg" alt="" width="1000" height="499" />
-<p class="pcap"><span class="ss">Figure 6</span> <i>Amplification by a three-element vacuum tube.</i></p>
-</div>
-<dl class="undent pcap"><dt>Power source</dt>
-<dt>Cathode</dt>
-<dt>Grid</dt>
-<dd>Input wave</dd>
-<dt>Anode</dt>
-<dt>Output wave</dt></dl>
-<p>The amplification principle can be applied in another
-equally important way. Once a signal gets started in the
-circuit, part of it can be <i>fed back into the input</i> of the circuit.
-<span class="pb" id="Page_11">11</span>
-Thus the signal is made to go &ldquo;round and round&rdquo;, continuously
-regenerating itself. The device has become an
-<i>oscillator</i>, that is, a frequency generator that produces a
-steady and temporally coherent wave. The frequency of the
-wave can be rigidly controlled by suitable circuitry.</p>
-<p>The oscillator plays a vital part in radio transmission,
-for a transmitter beams energy continuously, not just when
-sound is being carried. The oscillator generates what is
-called a &ldquo;carrier wave&rdquo;. Information, such as speech or
-music, is carried in the form of audio (detectable-by-ear)
-frequencies, which ride &ldquo;piggyback&rdquo; on the carrier wave.
-In other words, the carrier wave is <i>modulated</i>, or varied,
-in such a way that it can carry meaningful information. The
-familiar expressions AM and FM, for example, stand for
-Amplitude Modulation and Frequency Modulation&mdash;two different
-ways of impressing information on the carrier wave.
-<a href="#fig7">Figure 7</a> shows a basic and an amplitude- (or height-)
-modulated wave.</p>
-<div class="img" id="fig7">
-<img src="images/p06b.jpg" alt="" width="800" height="499" />
-<p class="pcap"><span class="ss">Figure 7</span> <i>(a) Unmodulated radio wave.</i> <i>(b) Amplitude-modulated
-wave carries information.</i></p>
-</div>
-<p>The electron tube made its giant contribution to radio,
-television, and other electronic devices by making it possible
-to generate, detect, and amplify radio waves.</p>
-<p>Because radio waves are easily controlled, something
-useful can be done with them. Suppose we set up five radio
-transmitters, all beaming at the same frequency. The waves
-might look like those shown in <a href="#fig8">Figure 8</a>. Although the waves
-<span class="pb" id="Page_12">12</span>
-are temporally (or time) coherent, they are out of step, and
-not <i>spatially coherent</i>. But since good control is possible
-in radio circuits, we can force each antenna to radiate in
-<i>phase</i> (that is, in step) with the others, thus producing fully
-coherent radiation (<a href="#fig8">Figure 8</a>).</p>
-<div class="img" id="fig8">
-<img src="images/p07.jpg" alt="" width="643" height="800" />
-<p class="pcap"><span class="ss">Figure 8</span> <i>(a) Spatially incoherent radiation.</i> <i>(b) Spatially coherent
-radiation.</i></p>
-</div>
-<p>Such a process can increase the radiation <i>power</i> to an
-almost unlimited degree. But it does nothing to solve the
-problem of the limited total carrying capacity of the radio
-spectrum.</p>
-<p>The most obvious and best way out of this difficulty is
-to raise the operating frequencies into the higher frequency
-bands. There are two reasons for this. First, it is clear
-that the wider the frequency band (the number of frequencies
-available) with which we work, the greater the number of
-communication channels that can be created.</p>
-<div class="pb" id="Page_13">13</div>
-<p>But second, and more important, the higher the frequency
-of the wave, the greater is its information-carrying
-capacity. In almost the same way that a large truck can
-carry a bigger load than a small one, the greater number
-of cycles per second in a high frequency wave permits it
-to carry more information than a low frequency wave.</p>
-<p>However, high frequencies must be generated in different
-ways than low frequency waves are; they require
-special equipment to handle them. Radio waves are transmitted
-by causing masses of free electrons to oscillate or
-swing back and forth in the transmitting antenna. (Any time
-electrons are made to change their speed or direction
-they radiate electromagnetic energy.)</p>
-<p>Each kind of oscillator has some limit to the frequencies
-at which it can operate. The three-element electron tube
-has been successfully developed to oscillate at frequencies
-up to, but not including, the vibration rate of the microwave
-region. Here ordinary tubes have trouble for the unexpected
-reason that free electrons are just too slow in
-their reactions to oscillate as rapidly as required in
-microwave transmission.</p>
-<p>To overcome this obstacle, two new types of electron
-tubes were developed: the klystron in 1938 and the traveling-wave
-tube some 10 years later. These lifted operation well
-up into the microwave region; it was the klystron that
-made wartime radar possible. Today many communication
-links depend heavily upon microwave frequencies.</p>
-<p>At this point in our story we have a situation where low
-temporally coherent radio waves and microwaves can be
-generated, but nothing of higher frequency. Communications
-engineers have gazed wistfully, but almost hopelessly,
-at light waves, whose frequencies are millions of times
-higher than radio waves. Thus, just by way of example,
-some 15 million separate TV channels could operate in the
-frequency range between red and orange in the visible band.</p>
-<p>What, then, is the problem?</p>
-<p>Why is light so much more difficult to handle?</p>
-<div class="pb" id="Page_14">14</div>
-<h2 id="c5"><span class="small">LIGHT AND THE ATOM</span></h2>
-<p>Since light waves have such high frequencies, a different
-mode of generation comes into play. We can no
-longer count on the controlled movement of free electrons
-<i>outside</i> atoms and molecules. Rather, light and all
-the radiations in the higher frequencies are generated by
-the movement of electrons <i>inside</i> atoms and molecules.</p>
-<p>Let us review momentarily the modern, albeit highly
-simplified, conception of an atom. Remember that no one
-has yet seen one. We describe the atom on the basis of
-how it acts, as well as how it reacts to things scientists
-do to it.</p>
-<p>For the present purpose, the best model we have of the
-atom is that of a miniature solar system, with a nucleus
-or heavy part at the center and a cloud of electrons dashing
-around the nucleus in fixed orbits.</p>
-<p>The term &ldquo;fixed orbits&rdquo; is used advisedly.</p>
-<p>Our planet moves in a certain orbit around the sun. If
-we attached a large enough rocket to the earth we theoretically
-<i>could</i> move it closer to or farther away from the
-sun. In the atom, we have learned, this cannot be done. An
-electron can only exist in one of a certain number of fixed
-orbits; different kinds of atoms have different numbers
-of orbits.</p>
-<p>We might think in terms of an elevator that can only
-stop at the various floors of an apartment building. Each
-upper floor is like an orbit of the electron. But you get
-nothing for nothing in the world of physics, and just as it
-takes energy to raise an elevator to a higher floor, it takes
-energy to move an electron to an outer orbit.</p>
-<p>Hence the atom is said to be raised to higher <i>energy</i> levels
-when an electron is nudged to an outer orbit. The energy
-input can be of many different kinds. Examples are heat,
-pressure, electrical current, chemical energy, and various
-forms of electromagnetic radiation. If too much energy is
-put into the elevator it goes flying out the roof. If too much
-energy is put into the atom, one or more of its electrons
-will go flying out of the atom. This is called <i>ionization</i>, and
-the atom, now minus one of its negative electrons and therefore
-positively charged, is called a positive <i>ion</i>.</p>
-<div class="pb" id="Page_15">15</div>
-<p>But if the <i>right</i> amount of energy is put into the atom,
-one of its electrons will merely be raised to a higher
-energy level. Shown in <a href="#fig9">Figure 9</a>, for instance, are the
-ground state (Circle No. 1) and two possible higher energy
-levels. As you can see there are three possible transitions.</p>
-<div class="img" id="fig9">
-<img src="images/p08.jpg" alt="" width="600" height="610" />
-<p class="pcap"><b>Figure 9</b> <i>Schematic representation of the electron orbits and energy
-levels of an atom. Each circle represents a separate possible
-orbit and each arrow a possible energy level difference.</i></p>
-</div>
-<p>The higher energy levels are abnormal, or excited,
-states, however, and the electron will shortly fall back to
-its normal (ground state) orbit (assuming some other
-electron has not fallen into it first). In order for the electron
-to do this (go back to its normal orbit), it must give
-off the energy it has acquired. This it does in the form of
-electromagnetic radiation.</p>
-<p>The energy difference between the two levels will determine
-what kind of radiation is emitted, for there is a
-direct correlation between energy and frequency.<a class="fn" id="fr_8" href="#fn_8">[8]</a> If the
-energy difference between the two levels is such that the
-frequency of emitted radiation is roughly between 10&sup1;&#8308; and
-10&sup1;&#8309; cycles per second, we see the radiation as light. When
-<span class="pb" id="Page_16">16</span>
-more energy is added, the radiation emerges as ultraviolet
-or X rays. In other words the higher the energy difference,
-the higher the frequency, and vice versa. Thus it is that
-cosmic rays, with the highest frequencies known to man,
-can pass right through us as if we weren&rsquo;t there.</p>
-<p>This simple picture of energy levels and associated
-frequencies doesn&rsquo;t quite hold for ordinary white light,
-however. Such light is generally produced by a process
-called incandescence, which results from the heating of a
-material until it glows. True, the atoms of the incandescent
-material are being raised to higher energy levels by
-chemical energy (as in fire), electricity (light bulb), or
-nuclear energy (the sun). In a hot solid, however, the explanation
-becomes more complicated. Many different electronic
-configurations are possible and the differences in
-energy among the various levels (which can be many more
-than the three shown in <a href="#fig9">Figure 9</a>) vary only slightly from
-one another. The result is a wide band of radiation.</p>
-<p>Thus, while the incandescent electric bulb is a great
-advance over fire, it is still a very inefficient source of
-light. Because it depends upon incandescence, a considerable
-portion of the electrical input goes into the production
-of unwanted heat, for the bulb&rsquo;s filament radiates in the
-infrared as well as the visible region.</p>
-<p>For providing illumination, the fluorescent tube is far
-more efficient than the incandescent lamp: a 40-watt fluorescent
-tube gives as much light as a 150-watt incandescent
-light. This is because its radiation is more controlled,
-operating more in accord with our description of electronic
-energy levels. Hence more of its output is in the
-desired visual region of the spectrum.</p>
-<p>In certain types of lighting, particular energy level
-changes may predominate, leading to the characteristic
-colors of neon tubes and vapor lamps. Although the resulting
-radiation bandwidth is narrow enough in these
-devices to appear as a definite color instead of the broad
-spectrum we know as white, it is still quite broad. In other
-words, the radiation is still frequency incoherent&mdash;and it
-is still spatially incoherent.</p>
-<p>To understand this, let us return for a moment to the
-group of radio antennas we showed in <a href="#fig8">Figure 8</a>. All of
-<span class="pb" id="Page_17">17</span>
-them, you will recall, could be made to radiate in phase.
-In the production of light, however, each antenna is replaced
-by a single atom!</p>
-<p>This creates two problems. First, because the energy
-stored in the atom is quite small, it comes out not as a
-continuous wave but as a tiny packet of radiation&mdash;a <i>photon</i>.<a class="fn" id="fr_9" href="#fn_9">[9]</a>
-It has an effect more like the hack of an ax than the
-buzz of a power saw.</p>
-<p>Second, atoms are notoriously &ldquo;individualistic&rdquo;. When a
-batch of atoms in a material has been raised to higher
-energy levels there is no way to know in what order, or in
-what direction, they will release their energy.</p>
-<p>This kind of process is called <i>spontaneous emission</i>,
-since each atom &ldquo;makes up its own mind&rdquo;. All we know is
-that within a certain period of time&mdash;a short period, to be
-sure&mdash;a certain percentage of these higher energy atoms
-will release their photons.</p>
-<div class="img" id="fig10">
-<img src="images/p09.jpg" alt="" width="500" height="439" />
-<p class="pcap"><span class="ss">Figure 10</span> <i>Ordinary light
-is a jumble of frequencies,
-directions, and phases.</i></p>
-</div>
-<p>What we have, then, is incoherent
-radiation&mdash;a jumble
-of frequencies (or colors), directions,
-and phases. Such
-light, symbolized in <a href="#fig10">Figure 10</a>,
-works well enough in lighting
-up this page, but is almost
-worthless as a carrier of information
-(and in other ways,
-as we shall see shortly). About
-the best that can be done with
-it is to turn it on and off in a
-sort of visual Morse code,
-which is exactly what is done
-on the blinker communication systems sometimes used for
-ship-to-ship communication.</p>
-<p>In other words, ordinary light cannot be modulated as
-radio waves can.</p>
-<p>It is of interest to note, however, that ordinary white
-light <i>can</i> be made coherent, to some extent, but at a very
-<span class="pb" id="Page_18">18</span>
-high cost in the intensity of the light. For example, we
-might first pass the light through a series of filters, each
-of which would subtract some portion of the spectrum,
-until only the desired wavelength came through. As can be
-seen in <a href="#fig11">Figure 11</a>, only a small fraction of the original
-light would be left.</p>
-<div class="img" id="fig11">
-<img src="images/p10.jpg" alt="" width="800" height="237" />
-<p class="pcap"><span class="ss">Figure 11</span> <i>Obtaining coherent radiation the hard way. Filters and
-pinhole block all but a small amount of the original radiation.</i></p>
-</div>
-<dl class="undent pcap"><dd>Incoherent</dd>
-<dt>Filters</dt>
-<dd>Coherent in time</dd>
-<dt>Pinhole</dt>
-<dd>Coherent in time and space</dd></dl>
-<p>We would then have monochromatic (one color) light,
-which is temporally coherent radiation, but it would still
-be spatially incoherent. In our diagram, we show three
-monochromatic waves. If we then passed this light through
-a tiny pinhole as shown, most of these few remaining waves
-would be blocked; the ones that got through would be pretty
-much in step. (In a similar manner, a true point source of
-light would produce spatially coherent radiation; but, as in
-the process described here, there wouldn&rsquo;t be very much of
-it.)</p>
-<p>We have, finally, obtained coherent light.</p>
-<p>The important thing about the laser is that, by its very
-nature, it produces coherent light automatically.</p>
-<p>Now....</p>
-<div class="pb" id="Page_19">19</div>
-<h2 id="c6"><span class="small">WHAT&rsquo;S SO SPECIAL ABOUT COHERENT LIGHT?</span></h2>
-<p>So desirable are the qualities of coherent light that the
-complicated filtering process described above has actually
-been used. For example, one British experimenter, Dennis
-Gabor, used it in the 1940s in an attempt to make a better
-microscope. But so great was the effort, and so meager
-the resulting light, that this project was abandoned.</p>
-<p>In the course of Dr. Gabor&rsquo;s experiments, however, he
-did manage to make a special kind of picture, using coherent
-light, which he called a <i>hologram</i>. He derived the
-name from two Greek words meaning a <i>whole picture</i>. We
-shall see why in a moment.</p>
-<p>Ordinary black and white photographs merely record
-darks and lights, or the intensity of the illumination,
-thereby providing a scale of grays, nothing more. But
-because waves of coherent light consistently maintain their
-relative spacing, they can be used to record additional information,
-namely the distance from objects.</p>
-<p>For example, if we shine a beam of coherent (laser)
-light between two objects we can, knowing the light wavelength,
-determine the distance between them to a high
-degree of accuracy. The basic idea is diagramed in <a href="#fig12">Figure 12</a>.
-It can be seen that the number of waves times the
-wavelength gives the precise distance (to within 1 wavelength
-of light) from the laser source to each object. But
-this would be a difficult process to implement.</p>
-<p>A better way, and one that is already in operation, is
-to use conventional methods to measure the approximate
-distance and use the laser beam for precise or fine measurement.
-In the device shown in <a href="#fig2">Figure 2</a>, the beam is
-split into two parts. One part is kept in the instrument
-itself to act as a reference. The other is aimed at a reflector,
-which sends it back to a detector in the main
-device, where it is automatically compared with the reference
-beam. If the two beams are in phase (that is, if
-their crests are superimposed), the waves combine and
-produce a high intensity beam at the detector. As the reflector
-moves closer to or farther away from the laser
-source the beam intensity decreases and then increases
-<span class="pb" id="Page_20">20</span>
-again as the wave crests move in and out of phase. The
-instrument counts the changes and displays the distance
-the reflector moves, as a function of the wavelengths, on
-the control cabinet meters.</p>
-<div class="img" id="fig12">
-<img src="images/p11.jpg" alt="" width="800" height="349" />
-<p class="pcap"><span class="ss">Figure 12</span> <i>Principle of distance measurement using coherent light.
-Wavelength times number of waves gives precise distance between
-laser and object.</i></p>
-</div>
-<dl class="undent pcap"><dt>Distance to be measured</dt>
-<dt>Laser</dt>
-<dt>Object No. 2</dt>
-<dt>1 Wavelength</dt>
-<dt>Object No. 1</dt></dl>
-<p>Since the word for the interaction of the waves in a
-system like this is &ldquo;interference&rdquo;, the measurement process
-is called <i>interferometry</i> (pronounced in ter fer OM e
-try). Although not new, it can now be applied for the first
-time in machine tool applications, providing the accuracy
-needed in this age of space technology and microminiaturization.
-Measurements with a laser interferometer can be
-made with an accuracy of 0.5 part per million at distances
-up to 200 inches. Such precision was previously unheard
-of in a machine shop environment, having been limited
-to laboratory measurements, and only at a range of a few
-inches. Under similar laboratory conditions, measurements
-by laser interferometry now detect movements of 10&#8315;&sup1;&sup1;
-centimeter, a distance approaching the dimensions of an
-atomic nucleus.</p>
-<p>Now let us suppose we expand the laser beam as shown on
-<a href="#Page_22">page 22</a>, and, with the aid of a mirror, direct part of it (the
-reference beam) at a photographic plate. The remaining
-portion of the diverging beam is used to illuminate the
-object to be photographed. Some of this light (the object
-beam) is reflected toward the plate and carries with it
-information about the object, as indicated by the wavy line.
-<span class="pb" id="Page_21">21</span>
-In the region where these two beams intersect, interference
-occurs, and a sample of this interference is recorded within
-the photographic emulsion. Where two crests meet a
-dark spot is recorded; where the waves are out of phase
-the processed plate is clear. The result is a hologram, a
-complex pattern of &ldquo;fringes&rdquo;, characteristic of the contour
-and light and dark areas of the object, as well as its distance
-from the plate. These fringes have the ability to
-diffract light rays. When light from a laser, or a point
-source of white light, is directed at the hologram from the
-same direction as the reference beam, part of the light is
-&ldquo;bent&rdquo; so that it appears to come from the place once occupied
-by the object. The result is a remarkably realistic
-3-dimensional image.</p>
-<p>There, in a nutshell, is the incredible new technique of
-holography. The extreme order of laser light is illustrated
-by the regularity of the dots on the cover of this booklet.</p>
-<p>This strange kind of light provides us with yet other
-advantages. Indeed, one of the most important is the fact
-that the energy of the laser is not being sprayed out in all
-directions. All of it is concentrated in the narrow beam
-that emerges from the device. And it <i>stays</i> narrow. Laser
-light has already been shone on the moon, the beam
-spreading out to only a few miles in traveling there from
-earth. The best optical searchlight beam would spread
-wider than the moon itself, thus dissipating its energy.</p>
-<p>It is for this reason, as well as its temporal coherence,
-that laser light is being considered for communications.
-A narrow beam is particularly important for space communications
-because of the long distances involved.</p>
-<p>But it is also possible to focus laser light as no light
-has ever been focused before. At close range a laser beam
-can be focused down to a circle just a few wavelengths
-across, concentrating its energy and making it possible
-to drill holes only 0.0002 inch in diameter. The photo on
-<a href="#Page_52">page 52</a> shows the exquisite control that can be exercised.</p>
-<p>Let us see what this focusability means in terms of
-power. Consider, by way of analogy, a dainty 100-pound
-lady in a pair of spike-heeled shoes. As she takes a step,
-her weight will be concentrated on one of those heels. If
-the area of the heel is, say, one quarter of a square inch
-<span class="pb" id="Page_22">22</span>
-(&frac12; &times; &frac12; inch), the pressure exerted on the poor tile or
-carpet rises to 400 pounds per square inch (4 &times; 100) and if
-the heel is only &frac14; inch on a side, the pressure will be
-1600 pounds per square inch!</p>
-<div class="img">
-<img src="images/p12.jpg" id="ncfig3" alt="Making and Viewing a Hologram" width="800" height="915" />
-</div>
-<dl class="undent pcap"><dt>MAKING A HOLOGRAM</dt>
-<dd>Object</dd>
-<dd>Object beam</dd>
-<dd>Holographic plate</dd>
-<dd>Mirror</dd>
-<dd>Reference beam</dd>
-<dd>Laser</dd>
-<dt>VIEWING A HOLOGRAM</dt>
-<dd>Hologram</dd>
-<dd>Image</dd>
-<dd>Eye</dd>
-<dd>Coherent light source</dd></dl>
-<p>What we are getting at, of course, is the fact that the
-coherence of the laser beam permits it to be concentrated
-into a tiny area. Thus whatever total energy is being sent
-out by the laser can be concentrated to the point where its
-effective energy is tremendous. The sun emits some
-6500 watts per square centimeter. Laser beams have
-already reached 500 <i>million</i> watts per square centimeter.</p>
-<p>But the power of the laser does not derive solely from
-its ability to be focused. Even an unfocused beam is several
-times more powerful than the sun&rsquo;s output (per square
-centimeter).</p>
-<div class="pb" id="Page_23">23</div>
-<div class="img" id="fig13">
-<img src="images/p12c.jpg" alt="" width="800" height="826" />
-<p class="pcap"><span class="ss">Figure 13</span> <i>The typical hologram,
-looks like a geometric
-design, but it contains more information
-than would an ordinary
-photograph. The <a href="#ncfig4">images below</a>, made from a hologram,
-show the detail, apparent solidity,
-and parallax effect of the reconstructed
-light waves. The parallax
-effect is the ability to see around
-the objects just as one could if
-they were really there. (See <a href="#imgx1">frontispiece</a>.)</i></p>
-</div>
-<div class="img">
-<img src="images/p12c1.jpg" id="ncfig4" alt="Model tank" width="1000" height="643" />
-</div>
-<div class="img">
-<img src="images/p12d.jpg" id="ncfig5" alt="Tank, from another angle" width="1000" height="650" />
-</div>
-<div class="pb" id="Page_24">24</div>
-<p>The crucial difference between the sun&rsquo;s light or any
-ordinary kind of light and laser light lies in the extent to
-which the emission of energy can be controlled. In the
-production of ordinary light the atoms, as we know, emit
-spontaneously, or in an uncontrolled fashion. But if the
-atoms could be forced to take in the proper amount of
-energy, store it, and release it when we wanted them to,
-we would have <i>stimulated</i>, rather than spontaneous, emission.</p>
-<p>This, however, is practically the same as the amplification
-principle we discussed earlier. In that case, a small
-radio signal is jacked up into a large one by stimulating
-an available power source to release its energy at the
-same wavelength and in step with the smaller signal.</p>
-<p>The question is, how can we do this with light?</p>
-<div class="pb" id="Page_25">25</div>
-<h2 id="c7"><span class="small">CONTROLLED EMISSION</span></h2>
-<p>The laser and its parent, the maser, can be traced back
-half a century to its theoretical beginnings. The great
-physicist Albert Einstein is most widely known for his
-work in relativity. But he did early and important work
-on that other gigantic 20th century scientific achievement,
-the quantum theory.<a class="fn" id="fr_10" href="#fn_10">[10]</a> In one of his papers, published first
-in Zurich, Switzerland, in 1916, Einstein showed that controlled
-emission of light energy could be obtained from an
-atom under certain conditions. When an atom or molecule
-has somehow had its energy level raised, the release of
-this stored energy could be stimulated by subjecting the
-atom or molecule to a small &ldquo;shot&rdquo; of electromagnetic
-radiation of the proper frequency.</p>
-<p>Einstein wrote that when such a photon of energy caused
-an electron to drop from a higher to a lower orbit, the
-electron would emit another photon of the same frequency
-and in the same direction as the one that hit it.<a class="fn" id="fr_11" href="#fn_11">[11]</a> In other
-words, the energy of the emitted photon would be added
-to that of the photon that stimulated the emission in the
-first place. Here, <i>potentially</i>, was light amplification. The
-three major factors, absorption of energy, spontaneous
-emission, and stimulated emission are diagrammed in
-<a href="#fig14">Figure 14</a>.</p>
-<p>There the matter lay for more than 30 years.</p>
-<p>In 1951 Charles H. Townes, then on the Columbia University
-faculty, was interested in ways of extending to still
-higher frequencies the range of microwaves available for
-use in communications and in other scientific applications.
-Townes and other scientists who were interested in the
-problem were to meet in Washington, D. C., on the 26th of
-April. The night before the meeting he slept in a small
-Washington hotel; but he awoke at 5:30&mdash;pondering, pondering
-the high frequency problem.</p>
-<div class="pb" id="Page_26">26</div>
-<p>He dressed and took a walk, then sat on a park bench
-and savored the beauty of azaleas in bloom. But all the
-while his mind was running over the various aspects of
-the problem.</p>
-<div class="img" id="fig14">
-<img src="images/p13.jpg" alt="" width="800" height="495" />
-<p class="pcap"><span class="ss">Figure 14</span> <i>An atom can release absorbed energy spontaneously or
-it can be stimulated to do so.</i></p>
-</div>
-<table class="center">
-<tr class="th"><th> </th><th> </th><th>Before </th><th>After</th></tr>
-<tr><td class="l"> </td><td class="l">Excited state </td><td class="c">&mdash;&ndash;&mdash; </td><td class="c">&mdash;&#8226;&mdash;</td></tr>
-<tr><td colspan="2" class="l">Absorption ~~&rarr;</td></tr>
-<tr><td class="l"> </td><td class="l">Relaxed state </td><td class="c">&mdash;&#8226;&mdash; </td><td class="c">&mdash;&ndash;&mdash;</td></tr>
-<tr><td class="l"> </td><td class="l">Excited state </td><td class="c">&mdash;&#8226;&mdash; </td><td class="c">&mdash;&ndash;&mdash;</td></tr>
-<tr><td colspan="2" class="l">Spontaneous emission</td></tr>
-<tr><td class="l"> </td><td class="l">Relaxed state </td><td class="c">&mdash;&ndash;&mdash; </td><td class="c">&mdash;&#8226;&mdash; </td><td class="l">~~&rarr;</td></tr>
-<tr><td class="l"> </td><td class="l">Excited state </td><td class="c">&mdash;&#8226;&mdash; </td><td class="c">&mdash;&ndash;&mdash;</td></tr>
-<tr><td colspan="2" class="l">Stimulated emission ~~&rarr;</td></tr>
-<tr><td class="l"> </td><td class="l">Relaxed state </td><td class="c">&mdash;&ndash;&mdash; </td><td class="c">&mdash;&#8226;&mdash; </td><td class="l">~~&rarr;</td></tr>
-<tr><td class="l"> </td><td class="l"> </td><td class="c"> </td><td class="c"> </td><td class="l">~~&rarr;</td></tr>
-</table>
-<p>Suddenly the answer came to him.</p>
-<p>Normally more of the molecules in any substance are
-in low-energy states than in high ones. He would change
-the natural balance and create a situation with an abnormally
-large number of high-energy molecules. Then he
-would stimulate them to emit their energy by nudging them
-with microwaves. Here was amplification.</p>
-<p>He could even take some of the emitted radiation and
-feed it back into the device to stimulate additional molecules,
-thereby creating an oscillator. This <i>feedback</i> arrangement,
-he knew, could be carried out in a cavity,
-which would resonate (just like an organ pipe) at the proper
-frequency. The resonator would be a box whose dimensions
-were comparable with the wavelength of the radiation,
-that is, a few centimeters on a side.</p>
-<p>On the back of an envelope he figured out some of the
-basic requirements. Three years, and many experiments,
-later the maser (<i>m</i>icrowave <i>a</i>mplification by <i>s</i>timulated
-<span class="pb" id="Page_27">27</span>
-<i>e</i>mission of <i>r</i>adiation) was a reality. The original maser
-was a small metal box into which excited ammonia molecules
-were added. When microwaves were beamed into the
-excited ammonia the box emitted a pure, strong beam of
-high frequency microwaves, far more temporally coherent
-than any that had ever been achieved before. The output of
-an ammonia maser is stable to one part in 100 billion,
-making it an extremely accurate atomic &ldquo;clock&rdquo;.<a class="fn" id="fr_12" href="#fn_12">[12]</a> Moreover,
-the amplifying properties of masers have been found
-to be very useful for magnifying faint radio signals from
-space, and for satellite communications.</p>
-<p>Ammonia gas was chosen for the first maser because
-molecules of ammonia have two individual energy states
-that are separated by a gap corresponding in frequency to
-23,870 megacycles (23,870 million cycles) per second.
-Ammonia molecules also react to a nonuniform electric
-field in ways that depend on their energy level: low-level
-molecules can be attracted and high-level ones repelled by
-the same field. Thus it is possible to separate the low-energy
-molecules from the high, and to get the excited
-molecules into the cavity without too much trouble.</p>
-<p>This procedure for getting the majority of atoms or
-molecules in a container into a higher energy state, is
-called <i>population inversion</i> and is basic to the operation of
-both masers and lasers.</p>
-<p>It should be noted that two Russians, N. G. Basov and
-A. M. Prokhorov, were working along similar lines independently
-of Townes. In 1952 they presented a paper at an
-All-Union (U.S.S.R.) Conference, in which they discussed
-the possibility of constructing a &ldquo;molecular generator&rdquo;,
-that is, a maser. Their proposal, first published in 1954,
-was in many respects similar to Townes&rsquo;s. In 1955, Basov
-and Prokhorov discussed, in a short note, a new way to
-obtain the active atomic systems for a maser, a method
-that turned out to be of great importance.</p>
-<div class="pb" id="Page_28">28</div>
-<p>Thus on October 29, 1964, the Nobel Prize in Physics
-was awarded, not only to Townes, but to Basov and Prokhorov
-as well. The award was for fundamental work in
-the field of quantum electronics, which has led to the
-construction of oscillators and amplifiers based on the
-&ldquo;aser&rdquo; principle.</p>
-<div class="pb" id="Page_29">29</div>
-<h2 id="c8"><span class="small">A LASER IS BORN</span></h2>
-<p>Following the maser development, there was much
-speculation about the possibility of extending the principle
-to the optical region. Indeed the first lasers&mdash;<i>l</i>ight
-<i>a</i>mplification by <i>s</i>timulated <i>e</i>mission of <i>r</i>adiation&mdash;were
-called &ldquo;optical masers&rdquo;.</p>
-<p>The difficulty, of course, was that optical wavelengths
-are so tiny&mdash;about &sup1;/&#8321;&#8320;,&#8320;&#8320;&#8320; that of microwaves. The
-maser principle depended upon a physical resonator, a
-box a few centimeters (or even millimeters) in length. But
-at millimeter wavelengths, such resonators are already
-so small that they are hard to make accurately. Making a
-box &sup1;/&#8321;,&#8320;&#8320;&#8320; that size was out of the question. Another
-approach was necessary.</p>
-<p>In 1958 A. L. Schawlow of Bell Telephone Laboratories
-and Dr. Townes outlined the theory and proposed a structure
-for an optical maser. They suggested that resonance
-could be obtained by making the waves travel back and
-forth along a relatively long, thin column of amplifying
-substance that had parallel reflectors at the ends.</p>
-<p>After their theory of the optical maser had been published,
-the race to build the first actual device began in
-earnest. The winner, in 1960, was Dr. T. H. Maiman, then
-with Hughes Aircraft Company. (He is now president of
-Maiman Associates.) The active substance he used was a
-single crystal of ruby, with the ends ground flat and
-silvered.</p>
-<p>Ruby is an aluminum oxide in which a small fraction of
-the aluminum atoms in the molecular structure, or lattice,
-have been replaced with chromium atoms. These atoms
-absorb green and blue light and hence impart a red color
-to the ruby. The chromium atoms can be boosted from
-their ground state into excited states when they absorb the
-green or blue light. This process, by which population
-inversion is achieved, has been given the name pumping.<a class="fn" id="fr_13" href="#fn_13">[13]</a></p>
-<div class="pb" id="Page_30">30</div>
-<p>Pumping in a crystal laser is generally achieved by
-placing the ruby rod within a spiral flash lamp (<a href="#fig15">Figure 15</a>)
-that operates like those used in high-speed (stroboscopic)
-photography. When the lamp is flashed, a bright beam of
-red light emerges from the ruby, shining out through one
-end, which has been only partially silvered.</p>
-<div class="img" id="fig15">
-<img src="images/p14.jpg" alt="" width="800" height="428" />
-<p class="pcap"><span class="ss">Figure 15</span> <i>A ruby
-laser system.</i></p>
-</div>
-<dl class="undent pcap"><dt>Ruby</dt>
-<dt>Flash lamp</dt>
-<dt>Partially silvered end</dt>
-<dt>Laser output</dt>
-<dt>Power</dt>
-<dt>Cooling</dt></dl>
-<p>The duration of this flash of red light is quite brief,
-lasting only some 300 millionths of a second, but it is very
-intense. In the early lasers, such a flash reached a peak
-power of some 10,000 watts.</p>
-<p>When Maiman&rsquo;s device was successfully built and operating,
-a public relations expert was called in to help
-introduce this revolutionary device to the world. He took
-one look at the laser and decided that it was too small and
-insignificant looking and would not photograph well. Looking
-around the lab, he spotted a larger laser and decided
-that that one was better.</p>
-<p>Dr. Maiman informed him in his best scientific manner
-that laser action had not been achieved with that one. But
-the world of promotion won out, and Dr. Maiman allowed
-the larger device to be photographed on the assumption&mdash;or
-was it hope?&mdash;that he would be able to get it to operate
-in the future. (He did.)</p>
-<p>The device shown in <a href="#fig16">Figure 16</a> is the true first laser.
-The all-important crystal rod is seen at the center. These
-crystals, incidentally, must be quite free of extraneous
-material; hence they are artificially &ldquo;grown&rdquo;, as shown in
-<a href="#fig17">Figure 17</a>. The single large crystal is formed as it is
-pulled slowly from the &ldquo;melt&rdquo;, after which it is ground to
-size and polished.</p>
-<div class="pb" id="Page_31">31</div>
-<div class="img" id="fig16">
-<img src="images/p14a.jpg" alt="" width="1000" height="761" />
-<p class="pcap"><span class="ss">Figure 16</span> <i>Dr. Maiman&rsquo;s
-first laser. Output was
-10,000 watts.</i></p>
-</div>
-<div class="img" id="fig17">
-<img src="images/p14c.jpg" alt="" width="500" height="801" />
-<p class="pcap"><span class="ss">Figure 17</span> <i>An exotic crystal
-of the garnet family is
-&ldquo;grown&rdquo; from a melt at a
-temperature of 3400&deg;F.</i></p>
-</div>
-<div class="pb" id="Page_32">32</div>
-<h2 id="c9"><span class="small">LASING&mdash;A NEW WORD</span></h2>
-<p>Now we can begin to put together the various processes
-and equipment we have been discussing separately. Perhaps
-the best way to do this is to look again at the word
-<i>laser</i> and recall its meaning: <i>l</i>ight <i>a</i>mplification by <i>s</i>timulated
-<i>e</i>mission of <i>r</i>adiation. Our objective is to create a
-powerful, narrow, coherent beam of light. Let us see how
-to do this.</p>
-<p>In <a href="#fig18">Figure 18</a> we imagine a laser crystal containing many
-atoms in the ground state (white dots) and a few in the
-excited state (black dots). Pumping light (wavy arrows in <i>a</i>)
-raises most of the atoms to the excited state, creating the
-required population inversion.</p>
-<div class="img" id="fig18">
-<img src="images/p15.jpg" alt="" width="799" height="668" />
-<p class="pcap"><span class="ss">Figure 18</span> <i>Sequence of operations in a solid crystal laser. (a)
-Pumping light raises many atoms to excited state. (b) Lasing begins
-when a photon is spontaneously emitted along the axis of the
-crystal. This stimulates other atoms in its path to emit. (c) The
-resulting wave is reflected back and forth many times between the
-ends of the crystal and builds in intensity until finally it flashes out
-of the partially silvered end.</i></p>
-</div>
-<dl class="undent pcap"><dt>(a)</dt>
-<dd>Ruby crystal</dd>
-<dd>Pumping light</dd>
-<dd>Atom in ground state</dd>
-<dd>Excited atom</dd>
-<dd>Partial reflecting mirror</dd>
-<dd>Full reflecting mirror</dd>
-<dt>(b)</dt>
-<dd>Excited atom emits photon parallel to axis</dd>
-<dt>(c)</dt></dl>
-<div class="pb" id="Page_33">33</div>
-<p><i>Lasing</i> begins when an excited atom spontaneously emits
-a photon parallel to the axis of the crystal (<i>b</i>). (Photons
-emitted in other directions merely pass out of the crystal.)
-The photon stimulates another atom in its path to contribute
-a second photon, in step, and in the same direction.</p>
-<p>This process continues as the photons are reflected back
-and forth between the ends of the crystal. (We might think
-of lone soldiers falling into step with a column of marching
-men.) The beam builds up until, when amplification is great
-enough (<i>c</i>), it flashes out through the partially silvered
-mirror at the right&mdash;a narrow, parallel, concentrated,
-coherent beam of light, ready for....</p>
-<div class="pb" id="Page_34">34</div>
-<h2 id="c10"><span class="small">SOME INTERESTING APPLICATIONS</span></h2>
-<p>Application of lasers can be divided into two broad
-categories: (1) commercial, industrial, military, and medical
-uses, and (2) scientific research. In the first case,
-lasers are used to do something that has been done in
-another way up to now (but not as well). Sometimes a
-laser solves a particular problem. For example, one of the
-first applications was in eye surgery, for &ldquo;welding&rdquo; a
-detached retina. The laser is particularly useful here because
-laser light can penetrate transparent objects such
-as the eye&rsquo;s lens (<a href="#fig19">Figure 19</a>), eliminating the need to
-make a cut into the eye.</p>
-<div class="img" id="fig19">
-<img src="images/p16.jpg" alt="" width="800" height="349" />
-<p class="pcap"><span class="ss">Figure 19</span> <i>Diagram of human eye showing laser beam focused on
-retina.</i></p>
-</div>
-<dl class="undent pcap"><dt>Cornea</dt>
-<dt>Lens</dt>
-<dt>Optic Nerve</dt>
-<dt>Beam angle</dt>
-<dt>Fovea centralis</dt>
-<dt>Iris</dt>
-<dt>Image</dt>
-<dt>Retina</dt></dl>
-<p>Surgeons have long wanted a better technique for treating
-extremely small areas of tissue. A laser beam, focused
-into a small spot, performs perfectly as a lilliputian surgical
-knife. An additional advantage is that the beam, being
-of such high intensity, can also sterilize or cauterize tissue
-as it cuts.</p>
-<p>The narrowness of the laser beam has made it ideal for
-applications requiring accurate alignment. Perhaps the
-ultimate here is the 2-mile-long linear accelerator built
-by Stanford University for the United States Atomic Energy
-Commission. &ldquo;Arrow-straight&rdquo; would not have been nearly
-good enough to assure expected performance. A laser beam
-was the only technique that could accomplish the incredible
-task of keeping the &#8542; inch bore of the accelerator straight
-along its 2-mile length. A remote monitoring system,
-based on the same laser beam, tells operators when a
-<span class="pb" id="Page_35">35</span>
-section of the accelerator has shifted out of line (due for
-example to small earth movements) by more than about
-&sup1;/&#8323;&#8322; inch&mdash;and identifies the section.<a class="fn" id="fr_14" href="#fn_14">[14]</a></p>
-<p><a href="#fig20">Figure 20</a> shows the 2-mile-long &ldquo;klystron gallery&rdquo; that
-generates the power for kicking the high-energy particles
-down the tube. The gallery parallels the accelerator housing
-and lies 25 feet beneath it (<a href="#fig21">Figure 21</a>). The large tube
-houses the optical alignment system and supports the
-smaller accelerator tube above. Target patterns dropped
-into the large tube at selected points produce an interference
-pattern at the far end of the tube similar to the one
-in <a href="#fig13">Figure 13</a>. Precise alignment of the tube is achieved by
-aiming the laser at the center dot of the pattern. Then the
-section that is out of line is physically moved until the dot
-appears in the proper place at the other end of the tube. It
-is the extreme coherence of the laser beam that makes
-this technique possible.</p>
-<p>Having heard that laser light has bored through steel
-and is being used in microwelding, some have asked
-whether the laser will ever be used to weld bridge members
-and other structural girders. This is missing the whole
-point of the laser: It would be like washing your floor with
-a toothbrush (even one with extra stiff bristles)! There
-would be no advantage to using lasers for large-scale
-welding; present equipment for this operation is quite
-satisfactory and far less wasteful of input power. The
-sensible approach is to use lasers where existing processes
-leave something to be desired.</p>
-<p>Until the advent of the laser, for example, there was no
-good way to weld wires 0.001 inch in diameter. Nor was
-there a good way to bore the tiny hole in a diamond that is
-used as a die for drawing such fine wire. It used to take 2
-days to drill a single diamond. With laser light the operation
-takes 2 minutes&mdash;and there is no problem with rapid
-wear of a cutting tool.</p>
-<p>So much for the first category of application. In the
-second category, namely use of the laser as a scientific
-tool, we enter a more theoretical domain. Here we use
-<span class="pb" id="Page_36">36</span>
-coherent light as an extension of ourselves, to probe into
-and to look at the world around us.</p>
-<div class="img" id="fig20">
-<img src="images/p17.jpg" alt="" width="1000" height="780" />
-<p class="pcap"><span class="ss">Figure 20</span> <i>A laser beam was used (and continues to be used) for
-precise alignment of Stanford University&rsquo;s 2-mile-long linear accelerator.
-This view shows the aboveground portion during construction.</i></p>
-</div>
-<p>Much experimental science is a matter of cooling, heating,
-grinding, squeezing, or otherwise abusing matter to
-see how it will react. With each new tool&mdash;ultrafast centrifuges,
-high- and low-pressure and extreme-temperature
-chambers, intense magnetic fields, atomic accelerators and
-so on&mdash;more has been learned about this still-puzzling
-world.</p>
-<p>Since coherent light is something new, we can do things
-to matter that have not been done before, and see how it
-reacts. The laser is being used to investigate many problem
-areas in biology, chemistry, and physics. For example,
-sound waves of extremely high frequency can be
-generated in matter by subjecting it to laser light. These
-intense vibrations may have profound effects on materials.</p>
-<div class="pb" id="Page_37">37</div>
-<div class="img" id="fig21">
-<img src="images/p17a.jpg" alt="" width="1000" height="706" />
-<p class="pcap"><span class="ss">Figure 21</span> <i>Subterranean view of Stanford accelerator housing.
-Alignment optics (laser systems) are housed in the large tube,
-which also acts as support for the smaller accelerator tube above
-it.</i></p>
-</div>
-<div class="img" id="fig22">
-<img src="images/p17c.jpg" alt="" width="572" height="801" />
-<p class="pcap"><span class="ss">Figure 22</span> <i>Laser beam
-spot as observed at the end
-of the accelerator.</i></p>
-</div>
-<div class="pb" id="Page_38">38</div>
-<p>In the chemical field the sharp beam and monochromatic
-energy of the laser hold great promise in the exploration
-of molecular structure and the nature of chemical reactions.
-Chemical reactions usually are set off by heat,
-agitation, electricity, or other broadly applied means.
-None of these energizers allow the fine control that the
-laser beam does. Its extremely fine beam can be focused
-to a tiny spot, thus allowing chemical activity to be pinpointed.
-But there is a second advantage: The monochromaticity
-of coherent light also makes it possible to
-control the energy (in addition to the intensity) of the beam
-accurately by simply varying the wavelength. Thus it may
-be possible, for instance, to cause a reaction in one group
-of molecules and not in another.</p>
-<p>One application in chemistry that holds great promise
-is the use of laser energy for causing specific chemical
-reactions such as those involved in the making of plastics.
-Bell Telephone Laboratory scientists have changed the
-styrene monomer (a &ldquo;raw&rdquo; plastic material) to its final
-state, polystyrene, in this way. The success of these and
-similar experiments elsewhere opens for exploration a
-vast area of molecular phenomena.</p>
-<p>In another scientific application, the laser is being used
-more and more as a teaching tool. Coherence is a concept
-that formerly had to be demonstrated by diagrams, formulas,
-and inference from experiments. The laser makes
-it possible to see coherence &ldquo;in action&rdquo;, along with many
-of the physical effects that result from it. Such phenomena
-as diffraction, interference, the so-called Airy disc patterns,
-and spatial harmonics, always difficult to demonstrate
-to students in the abstract, can now be seen quite
-concretely.</p>
-<p>Other interesting things can also be seen more plainly
-now. At the Los Alamos Scientific Laboratory, laser light
-is being used to &ldquo;look&rdquo; at plasmas; the result of one such
-look is shown in <a href="#fig23">Figure 23</a>. Plasmas are ionized gaseous
-mixtures. Their study lies at the heart of a constant search
-by atomic scientists for a self-sustained, controlled fusion
-reaction that can be used to provide useful thermonuclear
-power. This kind of reaction provides the almost unlimited
-energy in the sun and other stars. It is more efficient and
-releases less radioactivity than the other principal nuclear
-<span class="pb" id="Page_39">39</span>
-process, fission, which is used in atomic-electric
-power plants.<a class="fn" id="fr_15" href="#fn_15">[15]</a></p>
-<div class="img" id="fig23">
-<img src="images/p18.jpg" alt="" width="800" height="795" />
-<p class="pcap"><span class="ss">Figure 23</span> <i>Shadowgraph of deuterium discharge taken in laser
-light. Turbulence of the plasma is clearly seen.</i></p>
-</div>
-<p>Westinghouse Electric Corporation scientists, on the
-other hand, have used the concentrated energy of the laser,
-not to look at, but to <i>produce</i> a plasma (<a href="#fig24">Figure 24</a>). They
-blasted an aluminum target the size of a pinhead with a
-laser beam, thereby vaporizing it and creating a plasma.
-The calculated temperature in the electrically charged
-gas was 3,000,000&deg; centigrade. This is pretty hot, but still
-not hot enough for a thermonuclear reaction.</p>
-<div class="pb" id="Page_40">40</div>
-<div class="img" id="fig24">
-<img src="images/p19.jpg" alt="" width="1000" height="746" />
-<p class="pcap"><span class="ss">Figure 24</span> <i>Plasma heating by laser light.</i></p>
-</div>
-<dl class="undent pcap"><dt>Diamagnetic loop</dt>
-<dt>Laser beam</dt>
-<dt>Vacuum chamber</dt>
-<dt>Magnetic field</dt>
-<dt>Magnetic coils</dt>
-<dt>Electrostatic probe</dt>
-<dt>Plasma</dt>
-<dt>Lens</dt>
-<dt>Mirror</dt>
-<dt>To vacuum pump</dt>
-<dt>Camera</dt></dl>
-<p>The temperature of a plasma necessary to sustain a
-thermonuclear reaction is so high (above 10,000,000&deg;C)
-that any material is vaporized instantly on coming into
-contact with it. The only means developed so far to contain
-the plasma is an intense magnetic field, or &ldquo;magnetic
-bottle&rdquo;; containment has been accomplished for only a few
-thousandths of a second at most. The objective of the
-Westinghouse research, which was supported by the Atomic
-Energy Commission, was to study in detail the interaction
-of the plasma with a magnetic field.</p>
-<p>We do not have room to describe more applications in
-detail, but it may be interesting to list a few other uses of
-lasers&mdash;some commercial and some still experimental:</p>
-<div class="pb" id="Page_41">41</div>
-<ul><li>Earthquake prediction.</li>
-<li>Measurement of &ldquo;tides&rdquo; in the earth&rsquo;s crust under the sea.</li>
-<li>Laser gyroscopes.</li>
-<li>Highly accurate velocity measurement (useful in certain assembly line and continuous manufacturing processes).</li>
-<li>Scanner for analyzing photographs of bubble chamber tracks and astronomical phenomena.</li>
-<li>Computer output and storage systems; perhaps even complete optical data processing systems.</li>
-<li>Lightning-fast printing devices.</li>
-<li>High-speed photography (<a href="#fig25">Figure 25</a>).</li>
-<li>Missile tracking and accurate alignment of antennas.</li>
-<li>Automatic flaw spotter for big radio antennas.</li>
-<li>Aircraft landing aid for poor weather conditions.</li>
-<li>Fast, painless dental drill.</li>
-<li>Cancer research.</li></ul>
-<div class="img" id="fig25">
-<img src="images/p19a.jpg" alt="" width="800" height="661" />
-<p class="pcap"><span class="ss">Figure 25</span> <i>Twenty-two caliber bullet and its shock wave are photographed
-from the image produced by a doubly exposed laser hologram.
-The original hologram was exposed twice by a ruby laser
-within half a thousandth of a second as the bullet sped past at 2&frac12;
-times the speed of sound.</i></p>
-</div>
-<div class="pb" id="Page_42">42</div>
-<h2 id="c11"><span class="small">A MULTITUDE OF LASERS</span></h2>
-<p>It is almost self-evident that no single device, even one as
-incredible as the laser, could accomplish all the feats mentioned
-in the preceding paragraphs. After all, some of these
-applications require high power but not extremely high monochromaticity,
-while in others the reverse may be true. Yet,
-by its very nature, any laser produces a beam with one, or
-at the most a few, wavelengths, and many different materials
-would be needed to provide the many different
-wavelengths required for all the tasks listed.</p>
-<p>Also, the first laser was a pulsed device. Light energy
-was pumped in and a bullet of energy emerged from it.
-Then the whole process had to be repeated. Pulsed operation
-is fine for spot-welding and for applications such as
-radar-type rangefinding, where pulses of energy are normally
-used anyway. With lasers smaller objects can be
-detected than when using the usual microwaves. But a
-pulsed process is not useful for communications. In other
-words, pulsing is good for certain applications but not for
-others.</p>
-<p>And of course solid crystals are difficult to manufacture.
-Hence, it was natural for laser pioneers to look hopefully
-at gases. Gas lasers would be easier to make&mdash;simply fill
-a glass tube with the proper gas and seal it.</p>
-<p>But other advantages would accrue. For one thing the
-relatively sparse population of emitting atoms in a gas
-provides an almost ideally homogeneous medium. That is,
-the emitting atoms (corresponding to chromium in the ruby
-crystal) are not &ldquo;contaminated&rdquo; by the lattice or host atoms.
-Since only active atoms need be used, the frequency coherence
-of a gas laser would probably be even better than
-that of the crystal laser, they reasoned.</p>
-<p>It was less than a year after the development of the ruby
-laser that Ali Javan of Bell Telephone Laboratories proposed
-a gas laser employing a mixture of helium and neon
-gases. This was an ingeniously contrived partnership
-whereby one gas did the energizing and the other did the
-amplifying. Gas lasers now utilize many different gases
-for different wavelength outputs and powers and provide
-the &ldquo;purest&rdquo; light of all. An additional advantage is that
-<span class="pb" id="Page_43">43</span>
-the optical pumping light could be dispensed with. An input
-of radio waves of the proper frequency did the job very
-nicely.</p>
-<p>But most significant of all, Javan&rsquo;s gas laser provided
-the first continuous output. This is commonly referred to
-as CW (continuous wave) operation. The distinction between
-pulsed and CW operation is like the difference between
-baking one loaf of bread at a time and putting the
-ingredients in one end of a baking machine and having a
-continuous loaf emerge at the other.</p>
-<p>When a non-expert thinks of a laser, he is apt to think of
-power&mdash;blinding flashes of energy&mdash;as illustrated in <a href="#fig26">Figure 26</a>.
-As we know, this is only a small part of the capability
-of the laser. Nevertheless, since lasers are often
-specified in terms of power output it may be well to discuss
-this aspect.</p>
-<p>The two units generally used are <i>joules</i> and <i>watts</i>. You
-are familiar with a watt and have an idea of its magnitude:
-think, for example, of a 15-watt or a 150-watt bulb. A watt
-is a unit of <i>power</i>; it is the rate at which (electrical) work
-is being done.</p>
-<div class="img" id="fig26">
-<img src="images/p20.jpg" alt="" width="800" height="564" />
-<p class="pcap"><span class="ss">Figure 26</span> <i>High power is demonstrated as a laser beam blasts
-through metal chain.</i></p>
-</div>
-<div class="pb" id="Page_44">44</div>
-<p>The joule is a unit of <i>energy</i> and can be thought of as the
-total capacity to do work. One joule is equivalent to 1 watt-second,
-or 1 watt applied for 1 second. But it can also
-mean a 10-watt burst of laser light lasting 0.1 second, or
-a billion watts lasting a billionth of a second.</p>
-<p>In general, the crystal (ruby) lasers are the most powerful,
-although other recently introduced materials, such
-as liquids (see <a href="#fig27">Figure 27</a>) and specially prepared glass,
-are providing competition. With proper auxiliary equipment,
-bursts of several <i>billion</i> watts have been achieved;
-but the burst lasts only about 100 millionths of a second.
-For certain uses, that&rsquo;s just what is wanted: a highly concentrated
-burst of energy that does its work without giving
-the material being &ldquo;shot&rdquo; a chance to heat up and spread
-the energy, perhaps damaging adjacent areas.</p>
-<div class="img" id="fig27">
-<img src="images/p21.jpg" alt="" width="548" height="800" />
-<p class="pcap"><span class="ss">Figure 27</span> <i>Active substance
-for a modern liquid laser is
-made in an uncomplicated
-10-minute procedure. Bluish
-powder of the rare earth,
-neodymium, is dissolved in
-a solution of selenium oxychloride
-and sealed in a glass
-tube.</i></p>
-</div>
-<div class="pb" id="Page_45">45</div>
-<p>Since the joule gives a measure of the total energy in
-a laser burst it is not applicable to CW output. Power in
-this area began low&mdash;in the milliwatt (one thousandth of a
-watt) region&mdash;but has been creeping up steadily. A recent
-gas laser utilizing carbon dioxide has already reached
-550 watts of continuous infrared radiation. This is the
-giant 44-footer shown in <a href="#fig28">Figure 28</a>. An advantage of gas
-(and liquid) lasers is that they can be made just about as
-large as one wishes. By way of comparison, the smallest
-gas laser in use is shown in <a href="#fig29">Figure 29</a>.</p>
-<div class="img" id="fig28">
-<img src="images/p21a.jpg" alt="" width="709" height="1001" />
-<p class="pcap"><span class="ss">Figure 28</span> <i>A giant 44-foot gas
-laser produces 550 watts of continuous
-power and is expected to
-reach 1000 watts. Glowing of the
-tube comes from gas discharge,
-not from laser light, which is in
-the infrared region and cannot be
-seen.</i></p>
-</div>
-<p>One of the least satisfactory aspects of the laser has
-been its notoriously low efficiency. For a while the best
-that could be accomplished was about 1%. That is, a hundred
-watts of light had to be put in to get 1 watt of coherent
-light out. In gas lasers the efficiency was even
-lower, ranging from 0.01% to 0.1%.</p>
-<p>In gas lasers this was no great problem since high power
-was not the objective. But with the high-power solid lasers,
-pumping power could be a major undertaking. A high-power
-<span class="pb" id="Page_46">46</span>
-laser pump built by Westinghouse Research Laboratories
-handles 70,000 joules. In more familiar terms,
-the peak power input while the pump is on is about
-100,000,000 watts. For a brief instant this is roughly equal
-to all the electrical power needs of a city of 100,000 people.</p>
-<p>Two relatively new developments have changed the efficiency
-levels. One, the carbon dioxide gas laser, is quite
-efficient, with the figure having passed 15%. The second is
-the injection, or semiconductor laser, in which efficiencies
-of more than 40% have been obtained. Unless unforeseen
-difficulties arise this figure is expected to continue to rise
-to a theoretical maximum of close to 100%.</p>
-<div class="img" id="fig29">
-<img src="images/p22.jpg" alt="" width="797" height="782" />
-<p class="pcap"><span class="ss">Figure 29</span> <i>A miniature gas
-laser produces continuous
-output in visible red region.</i></p>
-</div>
-<p>The semiconductor laser is to solid and gas lasers what
-the transistor was to the vacuum tube; all the functions of
-the laser have been packed into a tiny semiconductor crystal.
-In this case, electrons and &ldquo;holes&rdquo; (vacancies in the
-crystal structure that act like positive charges) accomplish
-the job done by excited atoms in the other types. That
-is, when they are stimulated they fall from upper energy
-states to lower ones, and emit coherent radiation in the
-process. Aside from this the principle of operation is the
-same.</p>
-<div class="pb" id="Page_47">47</div>
-<p>The device itself, however, is vastly different. For one
-thing it is about the size of this letter &ldquo;o&rdquo; (<a href="#fig30">Figure 30</a>). For
-another, it is self-contained; since it can convert electric
-current directly into laser light&mdash;the first time this has
-been possible&mdash;an external pumping source is not required.
-This makes it possible to modulate the beam by
-simply modulating the current. (A different approach has
-been to modulate a magnetic field around the device. This,
-it turns out, can also be done with some newer solid crystal
-lasers.)</p>
-<p>An additional advantage offered by the semiconductor
-laser is simplicity. There are no gases or liquids to deal
-with, no glassware to break, and no mirrors to align.
-Although it will not deliver high power, it can already
-deliver enough CW power for certain communications
-purposes. Its simplicity, efficiency, and light weight make
-it ideal for use in space.</p>
-<div class="img" id="fig30">
-<img src="images/p22a.jpg" alt="" width="800" height="647" />
-<p class="pcap"><span class="ss">Figure 30</span> <i>A tiny injection laser works in infrared region. The
-beam is visible because photo was taken with infrared film. The
-laser itself is a tiny crystal of gallium arsenide inside the metal
-mount being held between the fingers.</i></p>
-</div>
-<div class="pb" id="Page_48">48</div>
-<h2 id="c12"><span class="small">COMMUNICATIONS</span></h2>
-<p>Future deep space missions are expected to require extremely
-high data transmission rates (on the order of a
-million bits<a class="fn" id="fr_16" href="#fn_16">[16]</a> per second) to relay the huge quantities of
-scientific and engineering information gathered by the
-spacecraft. Higher data rates are necessary to increase
-both the total capacity and the speed of transmission. By
-comparison, the Mariner-4 spacecraft that sent back TV
-pictures of Mars had a data rate of only eight bits per
-second&mdash;a hundred thousand times too small for future
-missions. The use of lasers would mean that results could
-be transmitted to earth in seconds instead of the 8 hours it
-took for the photos to be sent from Mariner-4.</p>
-<p>One of the problems to be solved in using lasers for
-deep space communication, oddly enough, is that of pointing
-accuracy. Since the beam of laser energy is narrow, it
-would be possible for the radiation to miss the earth altogether
-and be lost entirely unless the laser were pointed
-at the receiver with extreme precision. Aiming a gun at a
-target 50 yards away is one thing; aiming a laser from an
-unmanned spacecraft 100 million miles away is quite another.
-It is believed, however, that present techniques can
-cope with the problem.</p>
-<p>Another peculiarity of laser communication is that it will
-probably be accomplished faster and more readily in space
-than here on earth. Powerful though laser light may be, it
-is light and is therefore impeded to some extent by our
-atmosphere even under good conditions. Data transmissions
-of 20 and 30 miles have already been accomplished in good
-weather with lasers.</p>
-<p>But if you have ever tried to force a searchlight beam
-or shine automobile headlights through heavy fog, rain, or
-snow, you will appreciate the magnitude of the problem
-under these conditions. The use of infrared frequencies
-helps to some extent, since infrared is somewhat more
-penetrating, but the poor-weather problem is a serious one.</p>
-<div class="pb" id="Page_49">49</div>
-<p>A possible solution is the use of &ldquo;light pipes&rdquo;, similar
-to the wave guides already in use for certain microwave
-applications over short distances. But as often happens,
-new developments create new needs; how, for example,
-can we get the laser beam to stay centered in the pipe and
-follow curves? A series of closely spaced lenses, about
-1000 per mile, probably would accomplish this, but too
-much light would be lost by scattering from the many lens
-surfaces.</p>
-<p>Scientists are experimenting with a new kind of &ldquo;lens&rdquo;,
-one that uses variations in the density of gases to focus
-and guide the beam automatically. Since there are no surfaces
-in the path of the light beam, and since the gas is
-transparent and free of turbulence, the laser beam is not
-appreciably weakened or scattered as it travels through
-the pipe.</p>
-<div class="img" id="fig31">
-<img src="images/p23.jpg" alt="" width="800" height="516" />
-<p class="pcap"><span class="ss">Figure 31</span> <i>Laser light beam being guided
-through a &ldquo;light pipe&rdquo; by
-a gas &ldquo;lens&rdquo;. Heating coil (lower left) or mixture of gases (lower
-right) are two possible ways of maintaining proper density gradient
-in the gas.</i></p>
-</div>
-<p><a href="#fig31">Figure 31</a> shows how the gas focusing principle might
-be used to guide a beam through a curving pipe. The shading
-represents the density of the gas. Several means have
-been developed to keep the gas denser in the center than
-<span class="pb" id="Page_50">50</span>
-around the outside. When the pipe curves, the light beam
-starts moving off the axis of the pipe. The gas then acts
-like a prism, deflecting the light beam in the direction of
-the curvature of the &ldquo;prism&rdquo;.</p>
-<p>In communication between distant space and earth, a
-light pipe might be a little cumbersome; hence it may prove
-necessary to set up an intermediate orbiting relay station
-that will, particularly in cases of poor weather, intercept
-the incoming laser beam and convert it to radio frequencies
-that can penetrate our atmosphere with greater reliability.</p>
-<p>Powering space-borne lasers will, of course, be a problem.
-Indeed one of the major unsolved problems in production
-of spacecraft and long-term satellites is the provision
-of an adequate supply of power. Fuel cells and solar cells
-have helped but do not give the whole answer.<a class="fn" id="fr_17" href="#fn_17">[17]</a></p>
-<p>One other approach has already been developed: a sun-pumped
-laser. Sunlight focused onto the side of the laser
-(see <a href="#fig32">Figure 32</a>) provides the pumping power, enabling the
-device to put out 1 watt of continuous infrared radiation,
-enough for special space applications. Descendents of this
-device could produce visible light if this is deemed desirable.</p>
-<p>Another approach, using <i>chemical lasers</i>, is even more
-intriguing and may have greater consequences. Chemical
-lasers will derive their energy from their internal chemistry
-rather than from the outside. A mixture of two chemicals
-may be all that is needed to initiate laser action
-aboard a spacecraft or satellite. (Chemical lasers also
-offer the promise of even greater concentrations of power
-than have been achieved heretofore, which may make them
-useful in plasma research.)</p>
-<p>With all these possibilities, it may still be that spacecraft
-will need more power than is available on board. The
-narrow beam of the laser offers one more fascinating
-possibility, especially in the case of satellites relatively
-near earth. The light of a laser might actually be used to
-beam energy to a receiver, either for immediate use or
-<span class="pb" id="Page_51">51</span>
-storage. It would then become possible to &ldquo;refuel&rdquo; satellites
-at will, giving them much greater capabilities.</p>
-<p>If available laser power is great enough, laser beams
-might even be used to push satellites back into their proper
-orbits when they begin to wander off course, as they almost
-invariably do after a while.</p>
-<div class="img" id="fig32">
-<img src="images/p24.jpg" alt="" width="644" height="777" />
-<p class="pcap"><span class="ss">Figure 32</span> <i>Artist&rsquo;s rendering of sun-pumped laser as it would operate
-in space. The sun&rsquo;s rays are collected by a parabolic reflector
-and are focused on the laser&rsquo;s surface by two cylindrical mirrors.</i></p>
-</div>
-<dl class="undent pcap"><dt>Sun</dt>
-<dt>Parabolic Collector</dt>
-<dt>Hyperbolic-cylindric secondary mirror</dt>
-<dt>Semi-circular-cylindric tertiary mirror</dt>
-<dt>Laser beam</dt></dl>
-<div class="pb" id="Page_52">52</div>
-<h2 id="c13"><span class="small">A LASER IN YOUR FUTURE?</span></h2>
-<p>Atomic energy, only a scientific
-dream a few short years
-ago, is now providing needed
-power in many parts of the
-world. In the same way, the
-laser, also an atomic phenomenon,
-has made its way
-out of the laboratory and into
-the fields of medicine, commerce,
-and industry. If it
-hasn&rsquo;t touched your life as
-yet, you need only be patient.
-It will.</p>
-<p>Indeed the most exciting
-probability of all is that lasers
-undoubtedly will change our
-lives in ways we cannot even
-conceive of now.</p>
-<div class="img" id="fig33">
-<img src="images/p25.jpg" alt="" width="198" height="798" />
-<p class="pcap"><span class="ss">Figure 33</span> <i>Tiny hole drilled in
-paper clip demonstrates remarkable
-capability of laser beam. Paper
-clip is 1&frac14; inches long. Hole
-(top) was drilled by the laser microwelder
-shown in <a href="#fig1">Figure 1</a>.</i></p>
-</div>
-<div class="pb" id="Page_53">53</div>
-<h2 id="c14"><span class="small">SUGGESTED REFERENCES</span></h2>
-<h3 id="c15">Books</h3>
-<dl class="undent"><dt><i>ABC&rsquo;s of Masers and Lasers</i>, Allan H. Lytel, Howard W. Sams and Company, Inc., Publishers, Indianapolis, Indiana 46206, 1966, 96 pp., $2.25.</dt>
-<dt><i>The Laser: Light That Never Was Before</i>, Ben Patrusky, Dodd, Mead and Company, New York 10016, 1966, 128 pp., $3.50.</dt>
-<dt><i>Masers and Lasers</i>, Manfred Brotherton, McGraw-Hill Book Company, New York 10036, 1964, 224 pp., $8.50.</dt>
-<dt><i>Masers and Lasers</i>, H. Arthur Klein, J. B. Lippincott Company, Philadelphia, Pennsylvania 19105, 1963, 184 pp., $3.95.</dt>
-<dt><i>The Story of the Laser</i>, John M. Carroll, E. P. Dutton and Company, Inc., New York 10003, 1964, 181 pp., $3.95.</dt>
-<dt><i>Quantum Electronics: The Fundamentals of Transistors and Lasers</i>, John R. Pierce, Doubleday and Company, Inc., New York 10017, 1966, 138 pp., $1.25.</dt>
-<dt><i>Lasers and Their Applications</i>, Kurt R. Stehling, The World Publishing Company, Cleveland, Ohio 44102, 1966, 192 pp., $6.00.</dt>
-<dt><i>Understanding Lasers and Masers</i>, Stanley Leinwoll, Hayden Book Companies, New York 10011, 1964, 96 pp., $1.95.</dt>
-<dt><i>Atomic Light: Lasers</i>, Richard B. Nehrich, Jr., Glenn I. Voran, and Norman F. Dessel, Sterling Publishing Company, Inc., New York 10016, 1967, 136 pp., $3.95.</dt></dl>
-<h3 id="c16">Articles&mdash;General and Historical</h3>
-<dl class="undent"><dt>Advances in Optical Masers, A. L. Schawlow, <i>Scientific American</i>, 209: 34 (July 1963).</dt>
-<dt>The Evolution of the Physicist&rsquo;s Picture of Matter, P. A. M. Dirac, <i>Scientific American</i>, 208: 45 (May 1963).</dt>
-<dt>Filling in the Blanks in the Laser&rsquo;s Spectrum, F. M. Johnson, <i>Electronics</i>, 39: 82 (April 18, 1966).</dt>
-<dt>The Amateur Scientist&mdash;How a persevering amateur can build a gas laser in the home, C. L. Stong, <i>Scientific American</i>, 211: 227 (September 1964).</dt>
-<dt>The Amateur Scientist&mdash;Homemade Laser, C. L. Stong, <i>Scientific American</i>, 213: 108 (December 1965).</dt>
-<dt>The Amateur Scientist&mdash;How to make holograms and experiment with them or with ready-made holograms, C. L. Stong, <i>Scientific American</i>, 216: 122 (February 1967).</dt>
-<dt>The Maser, James P. Gordon, <i>Scientific American</i>, 199: 42 (December 1958).</dt>
-<dt>The Quantum Theory: Early Years to 1923, Karl Darrow, <i>Scientific American</i>, 186: 47 (March 1952).</dt>
-<dt>Laser&rsquo;s Bright Magic, T. Meloy, <i>National Geographic Magazine</i>, 130: 858 (December 1966).</dt>
-<dt>Infrared and Optical Masers (original paper), A. L. Schawlow and C. H. Townes, <i>Physical Review</i>, 112: 1940 (December 15, 1958).</dt>
-<dt>Laser Market Enters Era of Practicality, W. Mathews, <i>Electronic News</i>, 11: 1 (April 18, 1966).</dt>
-<dt class="pb" id="Page_54">54</dt>
-<dt>Lasers, A. K. Levine, <i>American Scientist</i>, 51: 14 (March 1963).</dt>
-<dt>Lasers, A. L. Schawlow, <i>Science</i>, 149: 13 (July 2, 1965).</dt>
-<dt>Lasers and Coherent Light, A. L. Schawlow, <i>Physics Today</i>, 17: 28 (January 1964).</dt>
-<dt>The Laser&rsquo;s Dazzling Future, L. Lessing, <i>Fortune</i>, 67: 138 (June 1963).</dt>
-<dt>Optical Masers, A. L. Schawlow, <i>Scientific American</i>, 204: 52 (June 1961).</dt>
-<dt>Optical Pumping, A. L. Bloom, <i>Scientific American</i>, 202: 72 (October 1960).</dt>
-<dt>Research on Maser-Laser Principle Wins Nobel Prize in Physics, J. P. Gordon, <i>Science</i>, 146: 897 (November 13, 1964).</dt>
-<dt>Resource Letter MOP-1 on Masers (Microwave through Optical) and on Optical Pumping, H. W. Moos, <i>American Journal of Physics</i>, 32: 589 (August 1964), extensive bibliography. Available from American Institute of Physics, 335 East 45th Street, New York 10017. Enclose stamped return envelope.</dt>
-<dt>Advances in Holography, K. S. Pennington, <i>Scientific American</i>, 218: 40 (February 1968).</dt>
-<dt>Applications of Laser Light, D. R. Herriott, <i>Scientific American</i>, 219: 140 (September 1968).</dt>
-<dt>Holography for the Sophomore Laboratory, R. H. Webb, <i>American Journal of Physics</i>, 36: 62 (January 1968).</dt>
-<dt>Laser Light, A. L. Schawlow, <i>Scientific American</i>, 219: 120 (September 1968).</dt>
-<dt>The Modulation of Laser Light, D. F. Nelson, <i>Scientific American</i>, 218: 17 (June 1968).</dt></dl>
-<h3 id="c17">Articles&mdash;Special Subjects</h3>
-<dl class="undent"><dt>Biological Effects of High Peak Power Radiation, S. Fine et al., <i>Life Sciences</i>, 3: 209 (1964).</dt>
-<dt>The Interaction of Light with Light, J. A. Giordmaine, <i>Scientific American</i>, 210: 38 (April 1964).</dt>
-<dt>Chemical Lasers, George C. Pimental, <i>Scientific American</i>, 214: 32 (April 1966).</dt>
-<dt>Color Laser Stores Data, J. Eberhart, <i>Science News</i>, 90: 51 (July 23, 1966).</dt>
-<dt>Communication by Laser, Stewart E. Miller, <i>Scientific American</i>, 214: 19 (January 1966).</dt>
-<dt>Guidelines for Selecting Laser Materials, R. H. Hoskins, <i>Electronic Design</i>, 13: <i>M</i>29 (July 19, 1965).</dt>
-<dt>Holography: The Picture Looks Good, J. Blum, <i>Electronics</i>, 39: 139 (April 18, 1966).</dt>
-<dt>How Dangerous Are Lasers?, L. H. Dulberger, <i>Electronics</i>, 35: 27 (January 26, 1962).</dt>
-<dt>Injection Lasers, R. W. Keyes, <i>Industrial Research</i>, 6: 46 (October 1964).</dt>
-<dt>Laser Potential in Deep-Space Link Grows, B. Miller, <i>Aviation Week and Space Technology</i>, 84: 71 (January 31, 1966).</dt>
-<dt>Laser Retinal Photocoagulator, N. S. Kapany et al., <i>Applied Optics</i>, 4: 517 (May 1965).</dt>
-<dt class="pb" id="Page_55">55</dt>
-<dt>Laser Welding in Electronic Circuit Fabrication, J. P. Epperson, <i>Electrical Design News</i> (EDN), 10: 8 (October 1965).</dt>
-<dt>The Light That Slices Inch into Millionths, (use of interferometry in industry), <i>Steel</i>, 158: 38 (February 28, 1966).</dt>
-<dt>The Optical Heterodyne&mdash;Key to Advanced Space Signaling, S. Jacobs, <i>Electronics</i>, 36: 29 (July 12, 1963).</dt>
-<dt>Photography by Laser, E. N. Leith and J. Upatnieks, <i>Scientific American</i>, 212: 24 (June 1965).</dt>
-<dt>Liquid Lasers, Alexander Lempicki and Harold Samelson, <i>Scientific American</i>, 216: 81 (June 1967).</dt>
-<dt>Plasma Experiments with a 570-kJ Theta-Pinch, F. C. Yahoda, et al., <i>Journal of Applied Physics</i>, 35: 2351 (August 1964).</dt>
-<dt>A Sun-Pumped CW One-Watt Laser, C. G. Young, <i>Applied Optics</i>, 5: 993 (June 1966).</dt>
-<dt>3-D Image Made at Home, <i>Science News</i>, 90: 185 (10 September 1966).</dt>
-<dt>Scanning with Lasers, Robert A. Myers, <i>International Science and Technology</i>, 65: 40 (May 1967).</dt></dl>
-<h3 id="c18">Booklets</h3>
-<dl class="undent"><dt><i>Applications of Lasers to Information Handling</i>, The Perkin-Elmer Corporation, Norwalk, Connecticut 06852, 1966, 32 pp., free. Reprint of five talks given by company personnel.</dt>
-<dt><i>Laser Interferometer</i>, Airborne Instruments Laboratory, Division of Cutler-Hammer, Inc., Deer Park, Long Island, New York 11729, 1965, 20 pp., free. Collection of article reprints.</dt>
-<dt><i>Laser: The New Light</i>, Bell Telephone Laboratories, Murray Hill, New Jersey 07971, 19 pp., free. Full color, nontechnical brochure presents some background, principles, and applications of the laser.</dt></dl>
-<div class="pb" id="Page_56">56</div>
-<div class="img" id="fig34">
-<img src="images/p26.jpg" alt="" width="636" height="999" />
-<p class="pcap"><i>Argon laser, which emits high-power blue-green beam continuously,
-has application in signal processing, communications, and spectroscopy.
-This unit is being beamed through prisms that separate
-its several discrete wavelengths of light, displayed on card at left
-foreground.</i></p>
-</div>
-<div class="pb" id="Page_57">57</div>
-<h2 id="c19"><span class="small">FOOTNOTES</span></h2>
-<div class="fnblock"><div class="fndef"><a class="fn"  id="fn_1" href="#fr_1">[1]</a>Sometimes referred to as <i>hertz</i> (abbreviated Hz), for the 19th
-Century German physicist Heinrich Hertz; 1000 Hz = 1000 cps.
-</div><div class="fndef"><a class="fn"  id="fn_2" href="#fr_2">[2]</a>Devised in France and officially adopted there in 1799, the
-metric system uses the meter as the basic unit of length and has
-been proposed for all measurements in this country.
-</div><div class="fndef"><a class="fn"  id="fn_3" href="#fr_3">[3]</a>Named
-for the Swedish physicist Anders J. Angstrom.
-</div><div class="fndef"><a class="fn"  id="fn_4" href="#fr_4">[4]</a>The wavelength,
-indicated by the Greek letter &lambda; (lambda) is
-related to frequency (f) in the proportion &lambda; (in meters) =
-300,000,000/f. (The number 300,000,000 is the velocity of light in
-meters per second.)
-</div><div class="fndef"><a class="fn"  id="fn_5" href="#fr_5">[5]</a>Microwaves are radio waves with frequencies above 1000
-megacycles per second.
-</div><div class="fndef"><a class="fn"  id="fn_6" href="#fr_6">[6]</a>Ten
-to 30,000,000 kilocycles per second; this is low in the
-electromagnetic spectrum, but not low in terms of the radio
-spectrum, which has a low-frequency classification of its own.
-</div><div class="fndef"><a class="fn"  id="fn_7" href="#fr_7">[7]</a>Primitive as early
-radios were by today&rsquo;s standards, they
-brought a new era to communication at the time. Unmodulated
-CW (continuous wave) transmissions and crystal receivers were
-used to summon rescuers in the <i>Titanic</i> disaster of 1912, for example.
-</div><div class="fndef"><a class="fn"  id="fn_8" href="#fr_8">[8]</a>Energy = h (Planck&rsquo;s constant) &times; frequency. Planck&rsquo;s constant
-is the energy of 1 quantum of radiation, and equals 6.62556 &times; 10&#8315;&sup2;&#8311;
-erg-sec.
-</div><div class="fndef"><a class="fn"  id="fn_9" href="#fr_9">[9]</a>Each photon carries 1 <i>quantum</i> of radiation energy, which is a
-unit equal to the product of the radiation frequency and Planck&rsquo;s
-constant (see footnote <a href="#Page_15">page 15</a>).
-</div><div class="fndef"><a class="fn"  id="fn_10" href="#fr_10">[10]</a>Einstein was awarded the Nobel Prize in 1921 for his 1905
-explanation of the photoelectric effect (in terms of quanta of
-energy) and <i>not</i> for his relativity theory.
-</div><div class="fndef"><a class="fn"  id="fn_11" href="#fr_11">[11]</a>Einstein&rsquo;s theoretical explanation applies in the case of stimulation
-of a single atom. In practical stimulation, directionality is
-enhanced by stimulating many atoms in phase.
-</div><div class="fndef"><a class="fn"  id="fn_12" href="#fr_12">[12]</a>An atomic clock is a device that uses the extremely fast vibrations
-of molecules or atomic nuclei to measure time. These
-vibrations remain constant with time, consequently short intervals
-can be measured with much higher precision than by mechanical
-or electrical clocks.
-</div><div class="fndef"><a class="fn"  id="fn_13" href="#fr_13">[13]</a>The 1966 Nobel Prize in Physics was awarded to Prof. Alfred
-Kastler of the University of Paris for his research on optical
-pumping and studies on the energy levels of atoms.
-</div><div class="fndef"><a class="fn"  id="fn_14" href="#fr_14">[14]</a>See <i>Accelerators</i>, a companion booklet in this series, for a full
-account of the Stanford &ldquo;Atom Smasher&rdquo;.
-</div><div class="fndef"><a class="fn"  id="fn_15" href="#fr_15">[15]</a>For descriptions of fission and fusion processes, see <i>Controlled
-Nuclear Fusion</i>, <i>Nuclear Reactors</i>, and <i>Nuclear Power
-Plants</i>, other booklets in this series.
-</div><div class="fndef"><a class="fn"  id="fn_16" href="#fr_16">[16]</a>A bit is a digit, or unit of information, in the binary (base-of-two)
-system used in electronic data transmission systems.
-</div><div class="fndef"><a class="fn"  id="fn_17" href="#fr_17">[17]</a>See <i>SNAP</i>, <i>Nuclear Space Reactors</i> and <i>Power from Radioisotopes</i>,
-other booklets in this series, for descriptions of nuclear
-sources of power for space.
-</div>
-</div>
-<div class="pb" id="Page_58">58</div>
-<p class="tb">This booklet is one of the &ldquo;Understanding the Atom&rdquo;
-Series. Comments are invited on this booklet and others
-in the series; please send them to the Division of Technical
-Information, U. S. Atomic Energy Commission, Washington,
-D. C. 20545.</p>
-<p>Published as part of the AEC&rsquo;s educational assistance
-program, the series includes these titles:</p>
-<div class="verse">
-<p class="t0"><i>Accelerators</i></p>
-<p class="t0"><i>Animals in Atomic Research</i></p>
-<p class="t0"><i>Atomic Fuel</i></p>
-<p class="t0"><i>Atomic Power Safety</i></p>
-<p class="t0"><i>Atoms at the Science Fair</i></p>
-<p class="t0"><i>Atoms in Agriculture</i></p>
-<p class="t0"><i>Atoms, Nature, and Man</i></p>
-<p class="t0"><i>Books on Atomic Energy for Adults and Children</i></p>
-<p class="t0"><i>Careers in Atomic Energy</i></p>
-<p class="t0"><i>Computers</i></p>
-<p class="t0"><i>Controlled Nuclear Fusion</i></p>
-<p class="t0"><i>Cryogenics, The Uncommon Cold</i></p>
-<p class="t0"><i>Direct Conversion of Energy</i></p>
-<p class="t0"><i>Fallout From Nuclear Tests</i></p>
-<p class="t0"><i>Food Preservation by Irradiation</i></p>
-<p class="t0"><i>Genetic Effects of Radiation</i></p>
-<p class="t0"><i>Index to the UAS Series</i></p>
-<p class="t0"><i>Lasers</i></p>
-<p class="t0"><i>Microstructure of Matter</i></p>
-<p class="t0"><i>Neutron Activation Analysis</i></p>
-<p class="t0"><i>Nondestructive Testing</i></p>
-<p class="t0"><i>Nuclear Clocks</i></p>
-<p class="t0"><i>Nuclear Energy for Desalting</i></p>
-<p class="t0"><i>Nuclear Power and Merchant Shipping</i></p>
-<p class="t0"><i>Nuclear Power Plants</i></p>
-<p class="t0"><i>Nuclear Propulsion for Space</i></p>
-<p class="t0"><i>Nuclear Reactors</i></p>
-<p class="t0"><i>Nuclear Terms, A Brief Glossary</i></p>
-<p class="t0"><i>Our Atomic World</i></p>
-<p class="t0"><i>Plowshare</i></p>
-<p class="t0"><i>Plutonium</i></p>
-<p class="t0"><i>Power from Radioisotopes</i></p>
-<p class="t0"><i>Power Reactors in Small Packages</i></p>
-<p class="t0"><i>Radioactive Wastes</i></p>
-<p class="t0"><i>Radioisotopes and Life Processes</i></p>
-<p class="t0"><i>Radioisotopes in Industry</i></p>
-<p class="t0"><i>Radioisotopes in Medicine</i></p>
-<p class="t0"><i>Rare Earths</i></p>
-<p class="t0"><i>Research Reactors</i></p>
-<p class="t0"><i>SNAP, Nuclear Space Reactors</i></p>
-<p class="t0"><i>Sources of Nuclear Fuel</i></p>
-<p class="t0"><i>Space Radiation</i></p>
-<p class="t0"><i>Spectroscopy</i></p>
-<p class="t0"><i>Synthetic Transuranium Elements</i></p>
-<p class="t0"><i>The Atom and the Ocean</i></p>
-<p class="t0"><i>The Chemistry of the Noble Gases</i></p>
-<p class="t0"><i>The Elusive Neutrino</i></p>
-<p class="t0"><i>The First Reactor</i></p>
-<p class="t0"><i>The Natural Radiation Environment</i></p>
-<p class="t0"><i>Whole Body Counters</i></p>
-<p class="t0"><i>Your Body and Radiation</i></p>
-</div>
-<p>A single copy of any one booklet, or of no more than three
-different booklets, may be obtained free by writing to:</p>
-<p class="center"><span class="smaller ss">USAEC, P. O. BOX 62, OAK RIDGE, TENNESSEE</span> <span class="hst"><span class="smaller ss">37830</span></span></p>
-<p>Complete sets of the series are available to school and
-public librarians, and to teachers who can make them
-available for reference or for use by groups. Requests
-should be made on school or library letterheads and indicate
-the proposed use.</p>
-<p>Students and teachers who need other material on specific
-aspects of nuclear science, or references to other
-reading material, may also write to the Oak Ridge address.
-Requests should state the topic of interest exactly, and the
-use intended.</p>
-<p>In all requests, include &ldquo;Zip Code&rdquo; in return address.</p>
-<p class="tbcenter">Printed in the United States of America
-<br />USAEC Division of Technical Information Extension, Oak Ridge, Tennessee</p>
-<h2>Transcriber&rsquo;s Notes</h2>
-<ul>
-<li>Silently corrected a few typos.</li>
-<li>Retained publication information from the printed edition: this eBook is public-domain in the country of publication.</li>
-<li>In the text versions only, text in italics is delimited by _underscores_.</li>
-</ul>
-<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK LASERS ***</div>
-<div style='text-align:left'>
-
-<div style='display:block; margin:1em 0'>
-Updated editions will replace the previous one&#8212;the old editions will
-be renamed.
-</div>
-
-<div style='display:block; margin:1em 0'>
-Creating the works from print editions not protected by U.S. copyright
-law means that no one owns a United States copyright in these works,
-so the Foundation (and you!) can copy and distribute it in the United
-States without permission and without paying copyright
-royalties. Special rules, set forth in the General Terms of Use part
-of this license, apply to copying and distributing Project
-Gutenberg&#8482; electronic works to protect the PROJECT GUTENBERG&#8482;
-concept and trademark. Project Gutenberg is a registered trademark,
-and may not be used if you charge for an eBook, except by following
-the terms of the trademark license, including paying royalties for use
-of the Project Gutenberg trademark. If you do not charge anything for
-copies of this eBook, complying with the trademark license is very
-easy. You may use this eBook for nearly any purpose such as creation
-of derivative works, reports, performances and research. Project
-Gutenberg eBooks may be modified and printed and given away--you may
-do practically ANYTHING in the United States with eBooks not protected
-by U.S. copyright law. Redistribution is subject to the trademark
-license, especially commercial redistribution.
-</div>
-
-<div style='margin:0.83em 0; font-size:1.1em; text-align:center'>START: FULL LICENSE<br />
-<span style='font-size:smaller'>THE FULL PROJECT GUTENBERG LICENSE<br />
-PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK</span>
-</div>
-
-<div style='display:block; margin:1em 0'>
-To protect the Project Gutenberg&#8482; mission of promoting the free
-distribution of electronic works, by using or distributing this work
-(or any other work associated in any way with the phrase &#8220;Project
-Gutenberg&#8221;), you agree to comply with all the terms of the Full
-Project Gutenberg&#8482; License available with this file or online at
-www.gutenberg.org/license.
-</div>
-
-<div style='display:block; font-size:1.1em; margin:1em 0; font-weight:bold'>
-Section 1. General Terms of Use and Redistributing Project Gutenberg&#8482; electronic works
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.A. By reading or using any part of this Project Gutenberg&#8482;
-electronic work, you indicate that you have read, understand, agree to
-and accept all the terms of this license and intellectual property
-(trademark/copyright) agreement. If you do not agree to abide by all
-the terms of this agreement, you must cease using and return or
-destroy all copies of Project Gutenberg&#8482; electronic works in your
-possession. If you paid a fee for obtaining a copy of or access to a
-Project Gutenberg&#8482; electronic work and you do not agree to be bound
-by the terms of this agreement, you may obtain a refund from the person
-or entity to whom you paid the fee as set forth in paragraph 1.E.8.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.B. &#8220;Project Gutenberg&#8221; is a registered trademark. It may only be
-used on or associated in any way with an electronic work by people who
-agree to be bound by the terms of this agreement. There are a few
-things that you can do with most Project Gutenberg&#8482; electronic works
-even without complying with the full terms of this agreement. See
-paragraph 1.C below. There are a lot of things you can do with Project
-Gutenberg&#8482; electronic works if you follow the terms of this
-agreement and help preserve free future access to Project Gutenberg&#8482;
-electronic works. See paragraph 1.E below.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.C. The Project Gutenberg Literary Archive Foundation (&#8220;the
-Foundation&#8221; or PGLAF), owns a compilation copyright in the collection
-of Project Gutenberg&#8482; electronic works. Nearly all the individual
-works in the collection are in the public domain in the United
-States. If an individual work is unprotected by copyright law in the
-United States and you are located in the United States, we do not
-claim a right to prevent you from copying, distributing, performing,
-displaying or creating derivative works based on the work as long as
-all references to Project Gutenberg are removed. Of course, we hope
-that you will support the Project Gutenberg&#8482; mission of promoting
-free access to electronic works by freely sharing Project Gutenberg&#8482;
-works in compliance with the terms of this agreement for keeping the
-Project Gutenberg&#8482; name associated with the work. You can easily
-comply with the terms of this agreement by keeping this work in the
-same format with its attached full Project Gutenberg&#8482; License when
-you share it without charge with others.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.D. The copyright laws of the place where you are located also govern
-what you can do with this work. Copyright laws in most countries are
-in a constant state of change. If you are outside the United States,
-check the laws of your country in addition to the terms of this
-agreement before downloading, copying, displaying, performing,
-distributing or creating derivative works based on this work or any
-other Project Gutenberg&#8482; work. The Foundation makes no
-representations concerning the copyright status of any work in any
-country other than the United States.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.E. Unless you have removed all references to Project Gutenberg:
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.E.1. The following sentence, with active links to, or other
-immediate access to, the full Project Gutenberg&#8482; License must appear
-prominently whenever any copy of a Project Gutenberg&#8482; work (any work
-on which the phrase &#8220;Project Gutenberg&#8221; appears, or with which the
-phrase &#8220;Project Gutenberg&#8221; is associated) is accessed, displayed,
-performed, viewed, copied or distributed:
-</div>
-
-<blockquote>
-  <div style='display:block; margin:1em 0'>
-    This eBook is for the use of anyone anywhere in the United States and most
-    other parts of the world at no cost and with almost no restrictions
-    whatsoever. You may copy it, give it away or re-use it under the terms
-    of the Project Gutenberg License included with this eBook or online
-    at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you
-    are not located in the United States, you will have to check the laws
-    of the country where you are located before using this eBook.
-  </div>
-</blockquote>
-
-<div style='display:block; margin:1em 0'>
-1.E.2. If an individual Project Gutenberg&#8482; electronic work is
-derived from texts not protected by U.S. copyright law (does not
-contain a notice indicating that it is posted with permission of the
-copyright holder), the work can be copied and distributed to anyone in
-the United States without paying any fees or charges. If you are
-redistributing or providing access to a work with the phrase &#8220;Project
-Gutenberg&#8221; associated with or appearing on the work, you must comply
-either with the requirements of paragraphs 1.E.1 through 1.E.7 or
-obtain permission for the use of the work and the Project Gutenberg&#8482;
-trademark as set forth in paragraphs 1.E.8 or 1.E.9.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.E.3. If an individual Project Gutenberg&#8482; electronic work is posted
-with the permission of the copyright holder, your use and distribution
-must comply with both paragraphs 1.E.1 through 1.E.7 and any
-additional terms imposed by the copyright holder. Additional terms
-will be linked to the Project Gutenberg&#8482; License for all works
-posted with the permission of the copyright holder found at the
-beginning of this work.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.E.4. Do not unlink or detach or remove the full Project Gutenberg&#8482;
-License terms from this work, or any files containing a part of this
-work or any other work associated with Project Gutenberg&#8482;.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.E.5. Do not copy, display, perform, distribute or redistribute this
-electronic work, or any part of this electronic work, without
-prominently displaying the sentence set forth in paragraph 1.E.1 with
-active links or immediate access to the full terms of the Project
-Gutenberg&#8482; License.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.E.6. You may convert to and distribute this work in any binary,
-compressed, marked up, nonproprietary or proprietary form, including
-any word processing or hypertext form. However, if you provide access
-to or distribute copies of a Project Gutenberg&#8482; work in a format
-other than &#8220;Plain Vanilla ASCII&#8221; or other format used in the official
-version posted on the official Project Gutenberg&#8482; website
-(www.gutenberg.org), you must, at no additional cost, fee or expense
-to the user, provide a copy, a means of exporting a copy, or a means
-of obtaining a copy upon request, of the work in its original &#8220;Plain
-Vanilla ASCII&#8221; or other form. Any alternate format must include the
-full Project Gutenberg&#8482; License as specified in paragraph 1.E.1.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.E.7. Do not charge a fee for access to, viewing, displaying,
-performing, copying or distributing any Project Gutenberg&#8482; works
-unless you comply with paragraph 1.E.8 or 1.E.9.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.E.8. You may charge a reasonable fee for copies of or providing
-access to or distributing Project Gutenberg&#8482; electronic works
-provided that:
-</div>
-
-<div style='margin-left:0.7em;'>
-    <div style='text-indent:-0.7em'>
-        &bull; You pay a royalty fee of 20% of the gross profits you derive from
-        the use of Project Gutenberg&#8482; works calculated using the method
-        you already use to calculate your applicable taxes. The fee is owed
-        to the owner of the Project Gutenberg&#8482; trademark, but he has
-        agreed to donate royalties under this paragraph to the Project
-        Gutenberg Literary Archive Foundation. Royalty payments must be paid
-        within 60 days following each date on which you prepare (or are
-        legally required to prepare) your periodic tax returns. Royalty
-        payments should be clearly marked as such and sent to the Project
-        Gutenberg Literary Archive Foundation at the address specified in
-        Section 4, &#8220;Information about donations to the Project Gutenberg
-        Literary Archive Foundation.&#8221;
-    </div>
-
-    <div style='text-indent:-0.7em'>
-        &bull; You provide a full refund of any money paid by a user who notifies
-        you in writing (or by e-mail) within 30 days of receipt that s/he
-        does not agree to the terms of the full Project Gutenberg&#8482;
-        License. You must require such a user to return or destroy all
-        copies of the works possessed in a physical medium and discontinue
-        all use of and all access to other copies of Project Gutenberg&#8482;
-        works.
-    </div>
-
-    <div style='text-indent:-0.7em'>
-        &bull; You provide, in accordance with paragraph 1.F.3, a full refund of
-        any money paid for a work or a replacement copy, if a defect in the
-        electronic work is discovered and reported to you within 90 days of
-        receipt of the work.
-    </div>
-
-    <div style='text-indent:-0.7em'>
-        &bull; You comply with all other terms of this agreement for free
-        distribution of Project Gutenberg&#8482; works.
-    </div>
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.E.9. If you wish to charge a fee or distribute a Project
-Gutenberg&#8482; electronic work or group of works on different terms than
-are set forth in this agreement, you must obtain permission in writing
-from the Project Gutenberg Literary Archive Foundation, the manager of
-the Project Gutenberg&#8482; trademark. Contact the Foundation as set
-forth in Section 3 below.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.F.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.F.1. Project Gutenberg volunteers and employees expend considerable
-effort to identify, do copyright research on, transcribe and proofread
-works not protected by U.S. copyright law in creating the Project
-Gutenberg&#8482; collection. Despite these efforts, Project Gutenberg&#8482;
-electronic works, and the medium on which they may be stored, may
-contain &#8220;Defects,&#8221; such as, but not limited to, incomplete, inaccurate
-or corrupt data, transcription errors, a copyright or other
-intellectual property infringement, a defective or damaged disk or
-other medium, a computer virus, or computer codes that damage or
-cannot be read by your equipment.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the &#8220;Right
-of Replacement or Refund&#8221; described in paragraph 1.F.3, the Project
-Gutenberg Literary Archive Foundation, the owner of the Project
-Gutenberg&#8482; trademark, and any other party distributing a Project
-Gutenberg&#8482; electronic work under this agreement, disclaim all
-liability to you for damages, costs and expenses, including legal
-fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
-LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
-PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE
-TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
-LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
-INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
-DAMAGE.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
-defect in this electronic work within 90 days of receiving it, you can
-receive a refund of the money (if any) you paid for it by sending a
-written explanation to the person you received the work from. If you
-received the work on a physical medium, you must return the medium
-with your written explanation. The person or entity that provided you
-with the defective work may elect to provide a replacement copy in
-lieu of a refund. If you received the work electronically, the person
-or entity providing it to you may choose to give you a second
-opportunity to receive the work electronically in lieu of a refund. If
-the second copy is also defective, you may demand a refund in writing
-without further opportunities to fix the problem.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.F.4. Except for the limited right of replacement or refund set forth
-in paragraph 1.F.3, this work is provided to you &#8216;AS-IS&#8217;, WITH NO
-OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT
-LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.F.5. Some states do not allow disclaimers of certain implied
-warranties or the exclusion or limitation of certain types of
-damages. If any disclaimer or limitation set forth in this agreement
-violates the law of the state applicable to this agreement, the
-agreement shall be interpreted to make the maximum disclaimer or
-limitation permitted by the applicable state law. The invalidity or
-unenforceability of any provision of this agreement shall not void the
-remaining provisions.
-</div>
-
-<div style='display:block; margin:1em 0'>
-1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the
-trademark owner, any agent or employee of the Foundation, anyone
-providing copies of Project Gutenberg&#8482; electronic works in
-accordance with this agreement, and any volunteers associated with the
-production, promotion and distribution of Project Gutenberg&#8482;
-electronic works, harmless from all liability, costs and expenses,
-including legal fees, that arise directly or indirectly from any of
-the following which you do or cause to occur: (a) distribution of this
-or any Project Gutenberg&#8482; work, (b) alteration, modification, or
-additions or deletions to any Project Gutenberg&#8482; work, and (c) any
-Defect you cause.
-</div>
-
-<div style='display:block; font-size:1.1em; margin:1em 0; font-weight:bold'>
-Section 2. Information about the Mission of Project Gutenberg&#8482;
-</div>
-
-<div style='display:block; margin:1em 0'>
-Project Gutenberg&#8482; is synonymous with the free distribution of
-electronic works in formats readable by the widest variety of
-computers including obsolete, old, middle-aged and new computers. It
-exists because of the efforts of hundreds of volunteers and donations
-from people in all walks of life.
-</div>
-
-<div style='display:block; margin:1em 0'>
-Volunteers and financial support to provide volunteers with the
-assistance they need are critical to reaching Project Gutenberg&#8482;&#8217;s
-goals and ensuring that the Project Gutenberg&#8482; collection will
-remain freely available for generations to come. In 2001, the Project
-Gutenberg Literary Archive Foundation was created to provide a secure
-and permanent future for Project Gutenberg&#8482; and future
-generations. To learn more about the Project Gutenberg Literary
-Archive Foundation and how your efforts and donations can help, see
-Sections 3 and 4 and the Foundation information page at www.gutenberg.org.
-</div>
-
-<div style='display:block; font-size:1.1em; margin:1em 0; font-weight:bold'>
-Section 3. Information about the Project Gutenberg Literary Archive Foundation
-</div>
-
-<div style='display:block; margin:1em 0'>
-The Project Gutenberg Literary Archive Foundation is a non-profit
-501(c)(3) educational corporation organized under the laws of the
-state of Mississippi and granted tax exempt status by the Internal
-Revenue Service. The Foundation&#8217;s EIN or federal tax identification
-number is 64-6221541. Contributions to the Project Gutenberg Literary
-Archive Foundation are tax deductible to the full extent permitted by
-U.S. federal laws and your state&#8217;s laws.
-</div>
-
-<div style='display:block; margin:1em 0'>
-The Foundation&#8217;s business office is located at 809 North 1500 West,
-Salt Lake City, UT 84116, (801) 596-1887. Email contact links and up
-to date contact information can be found at the Foundation&#8217;s website
-and official page at www.gutenberg.org/contact
-</div>
-
-<div style='display:block; font-size:1.1em; margin:1em 0; font-weight:bold'>
-Section 4. Information about Donations to the Project Gutenberg Literary Archive Foundation
-</div>
-
-<div style='display:block; margin:1em 0'>
-Project Gutenberg&#8482; depends upon and cannot survive without widespread
-public support and donations to carry out its mission of
-increasing the number of public domain and licensed works that can be
-freely distributed in machine-readable form accessible by the widest
-array of equipment including outdated equipment. Many small donations
-($1 to $5,000) are particularly important to maintaining tax exempt
-status with the IRS.
-</div>
-
-<div style='display:block; margin:1em 0'>
-The Foundation is committed to complying with the laws regulating
-charities and charitable donations in all 50 states of the United
-States. Compliance requirements are not uniform and it takes a
-considerable effort, much paperwork and many fees to meet and keep up
-with these requirements. We do not solicit donations in locations
-where we have not received written confirmation of compliance. To SEND
-DONATIONS or determine the status of compliance for any particular state
-visit <a href="https://www.gutenberg.org/donate/">www.gutenberg.org/donate</a>.
-</div>
-
-<div style='display:block; margin:1em 0'>
-While we cannot and do not solicit contributions from states where we
-have not met the solicitation requirements, we know of no prohibition
-against accepting unsolicited donations from donors in such states who
-approach us with offers to donate.
-</div>
-
-<div style='display:block; margin:1em 0'>
-International donations are gratefully accepted, but we cannot make
-any statements concerning tax treatment of donations received from
-outside the United States. U.S. laws alone swamp our small staff.
-</div>
-
-<div style='display:block; margin:1em 0'>
-Please check the Project Gutenberg web pages for current donation
-methods and addresses. Donations are accepted in a number of other
-ways including checks, online payments and credit card donations. To
-donate, please visit: www.gutenberg.org/donate
-</div>
-
-<div style='display:block; font-size:1.1em; margin:1em 0; font-weight:bold'>
-Section 5. General Information About Project Gutenberg&#8482; electronic works
-</div>
-
-<div style='display:block; margin:1em 0'>
-Professor Michael S. Hart was the originator of the Project
-Gutenberg&#8482; concept of a library of electronic works that could be
-freely shared with anyone. For forty years, he produced and
-distributed Project Gutenberg&#8482; eBooks with only a loose network of
-volunteer support.
-</div>
-
-<div style='display:block; margin:1em 0'>
-Project Gutenberg&#8482; eBooks are often created from several printed
-editions, all of which are confirmed as not protected by copyright in
-the U.S. unless a copyright notice is included. Thus, we do not
-necessarily keep eBooks in compliance with any particular paper
-edition.
-</div>
-
-<div style='display:block; margin:1em 0'>
-Most people start at our website which has the main PG search
-facility: <a href="https://www.gutenberg.org">www.gutenberg.org</a>.
-</div>
-
-<div style='display:block; margin:1em 0'>
-This website includes information about Project Gutenberg&#8482;,
-including how to make donations to the Project Gutenberg Literary
-Archive Foundation, how to help produce our new eBooks, and how to
-subscribe to our email newsletter to hear about new eBooks.
-</div>
-
-</div>
-</body>
-</html>
diff --git a/old/65512-h/images/cover.jpg b/old/65512-h/images/cover.jpg
deleted file mode 100644
index c60f09b..0000000
--- a/old/65512-h/images/cover.jpg
+++ /dev/null
diff --git a/old/65512-h/images/ejb.jpg b/old/65512-h/images/ejb.jpg
deleted file mode 100644
index 49ed817..0000000
--- a/old/65512-h/images/ejb.jpg
+++ /dev/null
diff --git a/old/65512-h/images/laser.jpg b/old/65512-h/images/laser.jpg
deleted file mode 100644
index f09b947..0000000
--- a/old/65512-h/images/laser.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p01.jpg b/old/65512-h/images/p01.jpg
deleted file mode 100644
index f95c8c5..0000000
--- a/old/65512-h/images/p01.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p02.jpg b/old/65512-h/images/p02.jpg
deleted file mode 100644
index 54818f7..0000000
--- a/old/65512-h/images/p02.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p03.jpg b/old/65512-h/images/p03.jpg
deleted file mode 100644
index bf2c4c6..0000000
--- a/old/65512-h/images/p03.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p04.jpg b/old/65512-h/images/p04.jpg
deleted file mode 100644
index 7c43370..0000000
--- a/old/65512-h/images/p04.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p04a.jpg b/old/65512-h/images/p04a.jpg
deleted file mode 100644
index 8f198c2..0000000
--- a/old/65512-h/images/p04a.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p05.jpg b/old/65512-h/images/p05.jpg
deleted file mode 100644
index bf274de..0000000
--- a/old/65512-h/images/p05.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p06.jpg b/old/65512-h/images/p06.jpg
deleted file mode 100644
index 113502a..0000000
--- a/old/65512-h/images/p06.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p06b.jpg b/old/65512-h/images/p06b.jpg
deleted file mode 100644
index 38914ec..0000000
--- a/old/65512-h/images/p06b.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p07.jpg b/old/65512-h/images/p07.jpg
deleted file mode 100644
index a59e77d..0000000
--- a/old/65512-h/images/p07.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p08.jpg b/old/65512-h/images/p08.jpg
deleted file mode 100644
index dd9827d..0000000
--- a/old/65512-h/images/p08.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p09.jpg b/old/65512-h/images/p09.jpg
deleted file mode 100644
index 3303cc4..0000000
--- a/old/65512-h/images/p09.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p10.jpg b/old/65512-h/images/p10.jpg
deleted file mode 100644
index 8f3c5fe..0000000
--- a/old/65512-h/images/p10.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p11.jpg b/old/65512-h/images/p11.jpg
deleted file mode 100644
index f1dfddd..0000000
--- a/old/65512-h/images/p11.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p12.jpg b/old/65512-h/images/p12.jpg
deleted file mode 100644
index d92bf88..0000000
--- a/old/65512-h/images/p12.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p12c.jpg b/old/65512-h/images/p12c.jpg
deleted file mode 100644
index f856fe2..0000000
--- a/old/65512-h/images/p12c.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p12c1.jpg b/old/65512-h/images/p12c1.jpg
deleted file mode 100644
index 61f8f79..0000000
--- a/old/65512-h/images/p12c1.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p12d.jpg b/old/65512-h/images/p12d.jpg
deleted file mode 100644
index 9a7d237..0000000
--- a/old/65512-h/images/p12d.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p13.jpg b/old/65512-h/images/p13.jpg
deleted file mode 100644
index 878f096..0000000
--- a/old/65512-h/images/p13.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p14.jpg b/old/65512-h/images/p14.jpg
deleted file mode 100644
index 4a6e7a2..0000000
--- a/old/65512-h/images/p14.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p14a.jpg b/old/65512-h/images/p14a.jpg
deleted file mode 100644
index cd14c83..0000000
--- a/old/65512-h/images/p14a.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p14c.jpg b/old/65512-h/images/p14c.jpg
deleted file mode 100644
index e08e4b6..0000000
--- a/old/65512-h/images/p14c.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p15.jpg b/old/65512-h/images/p15.jpg
deleted file mode 100644
index 293c3ce..0000000
--- a/old/65512-h/images/p15.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p16.jpg b/old/65512-h/images/p16.jpg
deleted file mode 100644
index 015f465..0000000
--- a/old/65512-h/images/p16.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p17.jpg b/old/65512-h/images/p17.jpg
deleted file mode 100644
index 3b9b627..0000000
--- a/old/65512-h/images/p17.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p17a.jpg b/old/65512-h/images/p17a.jpg
deleted file mode 100644
index c95bff7..0000000
--- a/old/65512-h/images/p17a.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p17c.jpg b/old/65512-h/images/p17c.jpg
deleted file mode 100644
index 3accb4c..0000000
--- a/old/65512-h/images/p17c.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p18.jpg b/old/65512-h/images/p18.jpg
deleted file mode 100644
index c394812..0000000
--- a/old/65512-h/images/p18.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p19.jpg b/old/65512-h/images/p19.jpg
deleted file mode 100644
index b2dfb01..0000000
--- a/old/65512-h/images/p19.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p19a.jpg b/old/65512-h/images/p19a.jpg
deleted file mode 100644
index a4c0a55..0000000
--- a/old/65512-h/images/p19a.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p20.jpg b/old/65512-h/images/p20.jpg
deleted file mode 100644
index 2349bc1..0000000
--- a/old/65512-h/images/p20.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p21.jpg b/old/65512-h/images/p21.jpg
deleted file mode 100644
index 63085ed..0000000
--- a/old/65512-h/images/p21.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p21a.jpg b/old/65512-h/images/p21a.jpg
deleted file mode 100644
index 4382473..0000000
--- a/old/65512-h/images/p21a.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p22.jpg b/old/65512-h/images/p22.jpg
deleted file mode 100644
index 99e55a6..0000000
--- a/old/65512-h/images/p22.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p22a.jpg b/old/65512-h/images/p22a.jpg
deleted file mode 100644
index 3e359e3..0000000
--- a/old/65512-h/images/p22a.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p23.jpg b/old/65512-h/images/p23.jpg
deleted file mode 100644
index 4866df8..0000000
--- a/old/65512-h/images/p23.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p24.jpg b/old/65512-h/images/p24.jpg
deleted file mode 100644
index 7a420aa..0000000
--- a/old/65512-h/images/p24.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p25.jpg b/old/65512-h/images/p25.jpg
deleted file mode 100644
index 97d5fc1..0000000
--- a/old/65512-h/images/p25.jpg
+++ /dev/null
diff --git a/old/65512-h/images/p26.jpg b/old/65512-h/images/p26.jpg
deleted file mode 100644
index 606ec4d..0000000
--- a/old/65512-h/images/p26.jpg
+++ /dev/null
diff --git a/old/65512-h/images/spine.jpg b/old/65512-h/images/spine.jpg
deleted file mode 100644
index b673ea6..0000000
--- a/old/65512-h/images/spine.jpg
+++ /dev/null