The Project Gutenberg eBook ofLasers

The Project Gutenberg eBook ofLasersThis 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 atwww.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: LasersAuthor: Hal HellmanRelease date: June 4, 2021 [eBook #65512]Most recently updated: October 18, 2024Language: EnglishCredits: Stephen Hutcheson and the Online Distributed Proofreading Team at https://www.pgdp.net*** START OF THE PROJECT GUTENBERG EBOOK LASERS ***

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 atwww.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: LasersAuthor: Hal HellmanRelease date: June 4, 2021 [eBook #65512]Most recently updated: October 18, 2024Language: EnglishCredits: Stephen Hutcheson and the Online Distributed Proofreading Team at https://www.pgdp.net

Title: Lasers

Author: Hal Hellman

Release date: June 4, 2021 [eBook #65512]Most recently updated: October 18, 2024

Language: English

Credits: Stephen Hutcheson and the Online Distributed Proofreading Team at https://www.pgdp.net

*** START OF THE PROJECT GUTENBERG EBOOK LASERS ***

Lasersby Hal HellmanU.S. ATOMIC ENERGY COMMISSIONDivision of Technical InformationUnderstanding the Atom SeriesATOMIC ENERGY COMMISSIONUNITED STATES OF AMERICA

by Hal Hellman

U.S. ATOMIC ENERGY COMMISSIONDivision of Technical InformationUnderstanding the Atom Series

ATOMIC ENERGY COMMISSIONUNITED STATES OF AMERICA

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. BrunenkantEdward J. Brunenkant, DirectorDivision of Technical Information

LASERS

by Hal Hellman

United States Atomic Energy CommissionDivision of Technical Information

Library of Congress Catalog Card Number: 68-607421968; 1969(rev.)

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. (Seepage 19andFigure 13.)

LASERS

By HAL HELLMAN

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).

Figure 1A 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 transoceanictelephone 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.

Figure 2Precision 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.

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 thewavelength. 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 metricsystem[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 inFigure 3, that the visible light range runs from the violet at about 4000 angstroms to red at about 7000 angstroms.

Figure 3The visible light spectrum ranges between approximately 4000 and 7000 angstroms.

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 instrumentbeyondthe 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 theelectromagnetic spectrum, which, as we see inFigure 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]

Figure 4Visible light region spans a tiny portion of the total electromagnetic spectrum.

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 ofphysical 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.

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.

Figure 5(a) Temporally coherent radiation. (b) Temporally incoherent radiation.

The single frequency capability of radio waves has been given the nametemporal coherence(or coherence in time) and is illustrated inFigure 5. Partashows a single sine wave, the common way of representing electromagnetic radiation, and particularlytemporally coherent radiation. Inbwe see whattemporally 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 sightof someone walking, riding, or studying with an earpiece plugged into a transistor radio is common. But the early radio enthusiastshadto 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 inFigure 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.

Figure 6Amplification by a three-element vacuum tube.

The amplification principle can be applied in another equally important way. Once a signal gets started in the circuit, part of it can befed back into the inputof the circuit.Thus the signal is made to go “round and round”, continuously regenerating itself. The device has become anoscillator, 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 ismodulated, 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 7shows a basic and an amplitude- (or height-) modulated wave.

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 inFigure 8. Although the wavesare temporally (or time) coherent, they are out of step, and notspatially coherent. But since good control is possible in radio circuits, we can force each antenna to radiate inphase(that is, in step) with the others, thus producing fully coherent radiation (Figure 8).

Figure 8(a) Spatially incoherent radiation.(b) Spatially coherent radiation.

Such a process can increase the radiationpowerto 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?

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 electronsoutsideatoms and molecules. Rather, light and all the radiations in the higher frequencies are generated by the movement of electronsinsideatoms 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 theoreticallycouldmove 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 higherenergylevels 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 calledionization, and the atom, now minus one of its negative electrons and therefore positively charged, is called a positiveion.

But if therightamount of energy is put into the atom, one of its electrons will merely be raised to a higher energy level. Shown inFigure 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.

Figure 9Schematic 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. Whenmore 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 inFigure 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 inFigure 8. All ofthem, 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—aphoton.[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 calledspontaneous 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.

Figure 10Ordinary 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 inFigure 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 lightcanbe made coherent, to some extent, but at a veryhigh 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 inFigure 11, only a small fraction of the original light would be left.

Figure 11Obtaining coherent radiation the hard way. Filters and pinhole block all but a small amount of the original radiation.

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....

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 ahologram. He derived the name from two Greek words meaning awhole 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 inFigure 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 inFigure 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 increasesagain 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.

Figure 12Principle of distance measurement using coherent light. Wavelength times number of waves gives precise distance between laser and object.

Since the word for the interaction of the waves in a system like this is “interference”, the measurement process is calledinterferometry(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 onpage 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 itstaysnarrow. 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 onpage 52shows 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!

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