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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. 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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’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—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—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.</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 “an -invention in search of an application”.</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—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—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—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.</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’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 -“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.</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—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’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’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’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—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 (10⁻⁸ 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—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">10²² </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">10²⁰ </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">10¹⁸ </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">10¹⁶ </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">10¹⁴ </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">10¹² </td><td class="c">Millimeter waves </td><td class="l">0.1</td></tr> -<tr><td class="r">10¹⁰ </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">10⁸ </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">10⁶ </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 = 10⁴ </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’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 “round and round”, 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 “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 <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—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 “fixed orbits” 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 10¹⁴ and -10¹⁵ 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’t there.</p> -<p>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 <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’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—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—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 “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.</p> -<p>This kind of process is called <i>spontaneous emission</i>, -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.</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—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’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’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’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 “interference”, 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 10⁻¹¹ -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 “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.</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> -(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!</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’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’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 “shot” 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—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">—–— </td><td class="c">—•—</td></tr> -<tr><td colspan="2" class="l">Absorption ~~→</td></tr> -<tr><td class="l"> </td><td class="l">Relaxed state </td><td class="c">—•— </td><td class="c">—–—</td></tr> -<tr><td class="l"> </td><td class="l">Excited state </td><td class="c">—•— </td><td class="c">—–—</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">—–— </td><td class="c">—•— </td><td class="l">~~→</td></tr> -<tr><td class="l"> </td><td class="l">Excited state </td><td class="c">—•— </td><td class="c">—–—</td></tr> -<tr><td colspan="2" class="l">Stimulated emission ~~→</td></tr> -<tr><td class="l"> </td><td class="l">Relaxed state </td><td class="c">—–— </td><td class="c">—•— </td><td class="l">~~→</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="c"> </td><td class="c"> </td><td class="l">~~→</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 “clock”.<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 “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.</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 -“aser” 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—<i>l</i>ight -<i>a</i>mplification by <i>s</i>timulated <i>e</i>mission of <i>r</i>adiation—were -called “optical masers”.</p> -<p>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.</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’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—or -was it hope?—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 “grown”, as shown in -<a href="#fig17">Figure 17</a>. The single large crystal is formed as it is -pulled slowly from the “melt”, 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’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 -“grown” from a melt at a -temperature of 3400°F.</i></p> -</div> -<div class="pb" id="Page_32">32</div> -<h2 id="c9"><span class="small">LASING—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—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 “welding” a -detached retina. The laser is particularly useful here because -laser light can penetrate transparent objects such -as the eye’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. “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 -<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 -¹/₃₂ inch—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 “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 (<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—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’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—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.</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 “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.</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 “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.</p> -<p>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 <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° 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°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.</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—some commercial and some still experimental:</p> -<div class="pb" id="Page_41">41</div> -<ul><li>Earthquake prediction.</li> -<li>Measurement of “tides” in the earth’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 2½ -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—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 “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.</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 “purest” 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’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—blinding flashes of energy—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’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.</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—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 <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 “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.</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 “o” (<a href="#fig30">Figure 30</a>). 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.)</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—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 “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.</p> -<p>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.</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 “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.</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 “prism”.</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 “refuel” 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’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.</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’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 1¼ 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’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—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’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’s Spectrum, F. M. Johnson, <i>Electronics</i>, 39: 82 (April 18, 1966).</dt> -<dt>The Amateur Scientist—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—Homemade Laser, C. L. Stong, <i>Scientific American</i>, 213: 108 (December 1965).</dt> -<dt>The Amateur Scientist—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’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’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—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—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 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.) -</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’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’s constant) × frequency. Planck’s constant -is the energy of 1 quantum of radiation, and equals 6.62556 × 10⁻²⁷ -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’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’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 “Atom Smasher”. -</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 “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.</p> -<p>Published as part of the AEC’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. 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