,lasers by ,hal ,hellman ,u4,s4 ,,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 man1 woman1 and child in the ,united ,states today4 ,in the years ahead it will affect increasingly all the peoples of the earth4 ,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 them4 ,the ,united ,states ,atomic ,energy ,commission provides this booklet to help you achieve such understanding4 ,edward ,j4 ,brunenkant ,edward ,j4 ,brunenkant1 ,director ,division of ,technical ,information ,,united ,,states ,,atomic ,,energy ,,commission ,dr4 ,glenn ,t4 ,seaborg1 ,chairman ,james ,t4 ,ramey ,wilfrid ,e4 ,johnson ,dr4 ,clarence ,e4 ,larson ,illustration3 ,,lasers by ,hal ,hellman ,,contents ,,introduction #a ,,the ,,electromagnetic ,,spectrum #e ,,radio ,,waves #i ,,light ,,and ,,the ,,atom #ad ,,what',s ,,so ,,special ,,about ,,coherent ,,light8 #ai ,,controlled ,,emission #be ,a ,,laser ,,is ,,born #bi ,,lasing--,a ,,new ,,word #cb ,,some ,,interesting ,,applications #cd ,a ,,multitude ,,of ,,lasers #db ,,communications #dh ,a ,,laser ,,in ,,your ,,future8 #eb ,,suggested ,,references #ec ,united ,states ,atomic ,energy ,commission ,division of ,technical ,information ,library of ,congress ,catalog ,card ,number3 #fh-fjgdb #aifh2 #aifi7rev47 ,illustration3 ,nothing about lasers is more astonishing than their ability to produce holograms1 under arrangements such as shown above4 ,two laser beams 7of different colors7 emerge from the curtain 7rear74 ,they are optically combined 7left center7 and the combined beam is then divided by prisms1 mirrors and lenses so that part of it shines on the figurines 7foreground7 and part on the square holographic plate 7right center74 ,when the plate is developed 7like an ordinary photographic film71 it will seem to have only a dull gray surface until it is viewed with spatially coherent light 7such as from a laser or a beam through a pinhole7 shining through it4 ,then an amazing1 multi-colored1 three-dimensional image of the figurines will be visible4 7,see page #ai and ,figure #ac47 ,illustration3 ,,lasers ,by ,,hal ,,hellman ,,introduction ,the transistor burst upon the electronic scene in the #aiejs4 ,almost overnight the size of new models of radios1 television sets1 and a host of other electronic devices shrank like deflating balloons4 ,suddenly the hard-of-hearing could carry their sound amplifiers in their ears4 ,teenagers could listen to favorite music wherever they went4 ,everywhere we turned the transistor was making its mark4 ,there was even a proposal before ,congress to require that every home have a transistor radio in case of emergency4 ,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 light4 ,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 ago4 ,we were told that the laser was going to have an even greater effect on our lives than the transistor4 ,it was going to replace everything from dentists' drills to electric wires4 ,the whole world1 it seemed1 eventually would be nothing but a gigantic collection of lasers that would do everything anyone wanted4 ,roads would be blazed through jungles at one sweep2 our country would be safe once and for all from intercontinental ballistic missiles2 cancer would be licked2 computers would be small enough to carry in a purse2 and so on and on4 ,yet for the first couple of years the laser seemed able to do nothing but blaze holes in razor blades for ,,tv commercials4 ,somehow the device never seemed to emerge from the laboratory1 prompting one cynic to call it 8an invention in search of an application04 ,many of the wild claims came from misunderstandings on the part of the press1 others from exaggerations by a few manufacturers who wanted free publicity4 ,but with even less exotic devices than lasers1 the road from the laboratory to the marketplace may often be long and hard4 ,price1 efficiency1 reliability1 convenience--these are all factors that must be considered4 ,it soon became clear that with something as new as the laser1 much improvement was necessary before it could be used in science and medicine1 and even more before it could be used in industry4 ,it now seems1 however1 that the turning point has been reached4 ,we have seen laser equipment put on the market for performing delicate surgery on the eye1 spot-welding tiny electronic circuits 7,figure #a71 and controlling machine tools with amazing accuracy 7,figure #b74 ,illustration3 ,figure #a ,a commercial laser microwelder4 ,a microscope is needed for accurate placement of beam energy4 ,the pace is quickening4 ,at least a dozen manufacturers have announced that they are designing laser technology into their products4 ,these are not laboratory experiments but practical products for measurement and testing1 and for industrial1 military1 medical1 and space uses4 ,the ,army1 for example1 has announced that it will purchase its first equipment for use in the field3 a portable1 highly accurate range finder for artillery observation4 ,still1 this hardly accounts for the #ajj1jjj1jjj spent in one recent year on laser research and development by some #ejj laboratories in the ,united ,states4 ,the ,u4 ,s4 ,government alone has spent about #be1jjj1jjj on laser research in a single year4 ,dozens1 and perhaps hundreds1 of other applications are on the fire--simmering or boiling as the case may be4 ,some require particular technical innovations such as greater power or higher efficiency4 ,others are entirely new applications4 ,one of the most exciting of these is holography 7pronounced ho ,,log ra phy74 ,holography involves a completely different approach to photography4 ,in addition to more immediate applications in microscopy1 information storage and retrieval1 and interferometry1 it promises such bonuses as #c-;dimensional color movies and ,,tv someday4 ,you have to see the holographic process in operation to believe it4 ,one moment you are looking at what appears to be an underexposed or lightly smudged photographic plate4 ,then suddenly a true-to-life image of the original object springs into being behind the negative--apparently suspended in midair6 ,not only is the full effect of 8roundness0 and depth there1 but you can also see anything lying behind the object's image by moving your head1 exactly as if the original scene containing the object were really there4 ,still another important field of application is that of communications4 ,perhaps because it is less spectacular than burning holes in razor blades1 we haven't heard as much about it4 ,yet there are probably more physicists and engineers working on adapting the laser for use in communications than on any other single laser project4 ,the reason for this is the fact that existing communications facilities are becoming overloaded4 ,space on transoceanic telephone lines is already at a premium1 with waiting periods sometimes running into hours4 ,radio 8ham0 operators have been threatened with loss of some of their best operating frequencies to meet the demand of emerging nations of ,africa for new channels4 ,television programs must compete for space on cross-country networks with telephone1 telegraph1 and transmission of data4 ,the increasing use of computers in science1 business1 and industry will strain our facilities still further4 ,communication satellites will help1 but they will not give us the whole answer2 and much development work remains to be done on satellites4 ,illustration3 ,figure #b ,precision control of a machine tool by laser light4 ,why the interest in the laser for communications8 ,in a recent experiment all seven of the ,new ,york ,,tv channels were transmitted over a single laser beam4 ,in terms of telephone conversations1 one laser system could theoretically carry #hjj1jjj1jjj conversations--four for each person in the ,united ,states4 ,in this booklet we shall learn what there is about the laser that gives it so much promise4 ,we shall investigate what it is1 how it works1 and the different kinds of lasers there are4 ,we begin by discussing some of the more familiar kinds of radiation1 such as radio and microwaves1 light and ,x rays4 ,,the ,,electromagnetic ,,spectrum ,some #he of what man learns comes to him through his vision in response to the medium of light4 ,yet1 ironically1 it wasn't until the end of the #agth century that he first began to get an inkling of what light really is4 ,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 rainbow4 ,a few years later the ,dutch astronomer ,christiaan ,huygens introduced the idea that light is a wave motion1 a concept finally validated in #ahjc when the ,british physician ,thomas ,young ingeniously demonstrated interference effects in waves4 ,thus it was finally realized that the only difference between the various colors of light was one of wavelength4 ,for light was indeed found to be a wave phenomenon1 no different in principle from the water waves you have seen a thousand times4 ,if you stand at the seashore1 you can easily count the number of waves that approach the shore in a minute4 ,divide that number by #fj and you have the frequency of the wave motion in the familiar unit1 cycles-per-second 7cps74#a ,you would have to count pretty quickly to do this for light1 however4 ,light waves vibrate or oscillate at the rate of some #djj million million times a second4 ,that's the vibration rate of waves of red light2 violet results from vibrations that are just about twice that fast4 ,with frequencies of this magnitude1 discussion and handling of data and dimensions are cumbersome and rather awkward4 ,fortunately there is another approach4 ,let's look again at our ocean waves4 ,we see that there is a regularity about them 7before they begin to break on the shore74 ,the distance from one crest to the next is significant and is called the wavelength4 ,water waves are measured in feet1 and in comparable units light waves are recorded in ten-millionths of an inch--again a very cumbersome number4 ,scientists therefore use the metric system#b and have standardized a unit called the angstrom#c1 which is equal to the one-hundred-millionth part of a centimeter 7#aj cm74 ,thus we find1 as shown in ,figure #c1 that the visible light range runs from the violet at about #djjj angstroms to red at about #gjjj angstroms4 ,illustration3 ,figure #c ,the visible light spectrum ranges between approximately #djjj and #gjjj angstroms4 ,wavelength 7,angstroms7 ,violet #djjj-dcjj ,blue #dcjj-ejjj ,green #ejjj-efjj ,yellow #efjj-ehjj ,orange #ehjj-fajj ,red #fajj-gjjj ,at roughly the same time that the wavelength of light was being determined1 the ,german-,british astronomer ,william ,herschel performed an interesting experiment4 ,he held a thermometer in turn in the various colors of light that had been spread out by an optical prism4 ,as he moved the thermometer from the violet to the red1 the temperature reading rose--and it continued to rise as he moved the instrument beyond the red area1 where no prismatic light could be seen4 ,thus ,herschel discovered infrared rays 7the kind of heat we get from the sun7 adjoining the visible red light1 and at the same time found that they were merely a continuation of the visible spectrum4 ,shortly thereafter1 ultraviolet rays were found on the other end of the visible light band4 ,one of the most fascinating movements in science has been the constant expansion since then of both ends of the radiating-wave spectrum4 ,the result has come to be called the electromagnetic spectrum1 which1 as we see in ,figure #d1 encompasses a wide variety of apparently different kinds of radiation4 ,above the visible band 7the higher frequencies71 we find ultraviolet light1 ,x rays1 gamma rays1 and some cosmic rays2 below it are infrared radiation1 microwaves1 and radio waves4 ,only a small proportion of the total spectrum is occupied by the visible band4 ,another point of interest is the inverse relationship between wavelength and frequency4 ,as one goes up the other goes down4#d ,illustration3 ,figure #d ,visible light region spans a tiny portion of the total electromagnetic spectrum4 ,frequency 7cps7 ,wavelength ,angstroms ,cosmic rays #aj #j4jjja #j4jja #aj ,gamma rays #j4ja #j4a #aj ,x rays #a #aj #aj ,ultraviolet radiation #ajj #a1jjj ,visible light #aj #aj1jjj ,infrared radiation #ajj1jjj ,angstroms #j4ja #aj ,millimeter waves #j4a #aj ,microwaves1 radar #a #aj #aj ,,tv and ,,fm radio #ajj ,short wave #a1jjj #aj ,,am radio #aj1jjj ,low frequency communications #ajj1jjj #aj1jjj #aj #a1jjj1jjj ,these many kinds of rays and waves vary tremendously in the ways they interact with matter4 ,but they are all part of a single family4 ,the only difference among them1 as with the colors of the rainbow1 lies in their wavelengths4 ,in a few cases1 as we shall see later1 the mode of generation is also different4 ,the band of radiation stretching from the infrared to cosmic rays has been1 up to now1 largely the concern of physical scientists4 ,because of their high frequencies1 these radiations are generally handled1 when calculations or measurements must be made1 in terms of wavelength4 ,radio and microwaves#e1 on the other hand1 have been more in the domain of communications engineers and are more likely to be discussed in terms of frequency4 ,thus it is that your radio is marked off in kilocycles1 or thousands of cycles per second1 while light is described as radiation in the #djjj to #gjjj angstrom band4 ,the relative newness of the various radiations has kept scientists busy learning about them and1 as information and experience have accumulated1 putting them to work4 ,,radio ,,waves ,one of the first of the newly discovered electromagnetic radiations to be put to work was the radio wave1 which is characterized by long wavelength and low frequency4#f ,the low frequency makes it relatively easy to produce a wave having virtually all its power concentrated at one frequency4 ,the advantage of this capability becomes obvious after a moment's thought4 ,think for example of a group of people lost in a forest4 ,if they hear sounds of a search party off in the distance1 all likely will begin to shout in various ways for help4 ,not a very efficient process1 is it8 ,but suppose all the energy going into the production of this noise could be concentrated in a single shout or whistle4 ,clearly1 their chances of being found would be much improved4 ,illustration3 ,figure #e 7a7 ,temporally coherent radiation4 7b7 ,temporally incoherent radiation4 ,the single frequency capability of radio waves has been given the name temporal coherence 7or coherence in time7 and is illustrated in ,figure #e4 ,part a shows a single sine wave1 the common way of representing electromagnetic radiation1 and particularly temporally coherent radiation4 ,in b we see what temporally incoherent radiation 7such as the mixed sounds of the stranded party7 would look like4 ,it was on ,christmas ,eve #aijf that music and speech came out of a radio receiver for the first time4 ,today the sight of someone walking1 riding1 or studying with an earpiece plugged into a transistor radio is common4 ,but the early radio enthusiasts had to wear earphones because it takes considerable power to activate a loudspeaker and the received signal was quite weak4 ,some means of increasing1 or amplifying1 the signal was needed if the process was to advance beyond this primitive stage4#g ,the use of vacuum tube or electron tube amplifiers is so widespread that it is unnecessary to explain their operations here in any detail4 ,it is important that the principle of amplification be understood1 however4 ,the input or information wave causes the grid to act as a sort of faucet as shown in ,figure #f4 ,that is1 it controls the flow of electrons 7the current in the circuit7 from cathode to anode4 ,a weak signal can therefore cause a similar1 but much stronger1 signal to appear in the circuit4 ,the larger signal is subsequently used to power a loudspeaker in the radio set4 ,illustration3 ,figure #f ,amplification by a three-element vacuum tube4 ,power source ,cathode ,grid ,input wave ,anode ,output wave ,the amplification principle can be applied in another equally important way4 ,once a signal gets started in the circuit1 part of it can be fed back into the input of the circuit4 ,thus the signal is made to go 8round and round01 continuously regenerating itself4 ,the device has become an oscillator1 that is1 a frequency generator that produces a steady and temporally coherent wave4 ,the frequency of the wave can be rigidly controlled by suitable circuitry4 ,the oscillator plays a vital part in radio transmission1 for a transmitter beams energy continuously1 not just when sound is being carried4 ,the oscillator generates what is called a 8carrier wave04 ,information1 such as speech or music1 is carried in the form of audio 7detectable-by-ear7 frequencies1 which ride 8piggyback0 on the carrier wave4 ,in other words1 the carrier wave is modulated1 or varied1 in such a way that it can carry meaningful information4 ,the familiar expressions ,,am and ,,fm1 for example1 stand for ,amplitude ,modulation and ,frequency ,modulation--two different ways of impressing information on the carrier wave4 ,figure #g shows a basic and an amplitude- 7or height-7 modulated wave4 ,illustration3 ,figure #g 7a7 ,unmodulated radio wave4 7b7 ,amplitude-modulated wave carries information4 ,the electron tube made its giant contribution to radio1 television1 and other electronic devices by making it possible to generate1 detect1 and amplify radio waves4 ,because radio waves are easily controlled1 something useful can be done with them4 ,suppose we set up five radio transmitters1 all beaming at the same frequency4 ,the waves might look like those shown in ,figure #h4 ,although the waves are temporally 7or time7 coherent1 they are out of step1 and not spatially coherent4 ,but since good control is possible in radio circuits1 we can force each antenna to radiate in phase 7that is1 in step7 with the others1 thus producing fully coherent radiation 7,figure #h74 ,illustration3 ,figure #h 7a7 ,spatially incoherent radiation4 7b7 ,spatially coherent radiation4 ,such a process can increase the radiation power to an almost unlimited degree4 ,but it does nothing to solve the problem of the limited total carrying capacity of the radio spectrum4 ,the most obvious and best way out of this difficulty is to raise the operating frequencies into the higher frequency bands4 ,there are two reasons for this4 ,first1 it is clear that the wider the frequency band 7the number of frequencies available7 with which we work1 the greater the number of communication channels that can be created4 ,but second1 and more important1 the higher the frequency of the wave1 the greater is its information-carrying capacity4 ,in almost the same way that a large truck can carry a bigger load than a small one1 the greater number of cycles per second in a high frequency wave permits it to carry more information than a low frequency wave4 ,however1 high frequencies must be generated in different ways than low frequency waves are2 they require special equipment to handle them4 ,radio waves are transmitted by causing masses of free electrons to oscillate or swing back and forth in the transmitting antenna4 7,any time electrons are made to change their speed or direction they radiate electromagnetic energy47 ,each kind of oscillator has some limit to the frequencies at which it can operate4 ,the three-element electron tube has been successfully developed to oscillate at frequencies up to1 but not including1 the vibration rate of the microwave region4 ,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 transmission4 ,to overcome this obstacle1 two new types of electron tubes were developed3 the klystron in #aich and the traveling-wave tube some #aj years later4 ,these lifted operation well up into the microwave region2 it was the klystron that made wartime radar possible4 ,today many communication links depend heavily upon microwave frequencies4 ,at this point in our story we have a situation where low temporally coherent radio waves and microwaves can be generated1 but nothing of higher frequency4 ,communications engineers have gazed wistfully1 but almost hopelessly1 at light waves1 whose frequencies are millions of times higher than radio waves4 ,thus1 just by way of example1 some #ae million separate ,,tv channels could operate in the frequency range between red and orange in the visible band4 ,what1 then1 is the problem8 ,why is light so much more difficult to handle8 ,,light ,,and ,,the ,,atom ,since light waves have such high frequencies1 a different mode of generation comes into play4 ,we can no longer count on the controlled movement of free electrons outside atoms and molecules4 ,rather1 light and all the radiations in the higher frequencies are generated by the movement of electrons inside atoms and molecules4 ,let us review momentarily the modern1 albeit highly simplified1 conception of an atom4 ,remember that no one has yet seen one4 ,we describe the atom on the basis of how it acts1 as well as how it reacts to things scientists do to it4 ,for the present purpose1 the best model we have of the atom is that of a miniature solar system1 with a nucleus or heavy part at the center and a cloud of electrons dashing around the nucleus in fixed orbits4 ,the term 8fixed orbits0 is used advisedly4 ,our planet moves in a certain orbit around the sun4 ,if we attached a large enough rocket to the earth we theoretically could move it closer to or farther away from the sun4 ,in the atom1 we have learned1 this cannot be done4 ,an electron can only exist in one of a certain number of fixed orbits2 different kinds of atoms have different numbers of orbits4 ,we might think in terms of an elevator that can only stop at the various floors of an apartment building4 ,each upper floor is like an orbit of the electron4 ,but you get nothing for nothing in the world of physics1 and just as it takes energy to raise an elevator to a higher floor1 it takes energy to move an electron to an outer orbit4 ,hence the atom is said to be raised to higher energy levels when an electron is nudged to an outer orbit4 ,the energy input can be of many different kinds4 ,examples are heat1 pressure1 electrical current1 chemical energy1 and various forms of electromagnetic radiation4 ,if too much energy is put into the elevator it goes flying out the roof4 ,if too much energy is put into the atom1 one or more of its electrons will go flying out of the atom4 ,this is called ionization1 and the atom1 now minus one of its negative electrons and therefore positively charged1 is called a positive ion4 ,but if the right amount of energy is put into the atom1 one of its electrons will merely be raised to a higher energy level4 ,shown in ,figure #i1 for instance1 are the ground state 7,circle ,no4 #a7 and two possible higher energy levels4 ,as you can see there are three possible transitions4 ,illustration3 ,figure #i ,schematic representation of the electron orbits and energy levels of an atom4 ,each circle represents a separate possible orbit and each arrow a possible energy level difference4 ,the higher energy levels are abnormal1 or excited1 states1 however1 and the electron will shortly fall back to its normal 7ground state7 orbit 7assuming some other electron has not fallen into it first74 ,in order for the electron to do this 7go back to its normal orbit71 it must give off the energy it has acquired4 ,this it does in the form of electromagnetic radiation4 ,the energy difference between the two levels will determine what kind of radiation is emitted1 for there is a direct correlation between energy and frequency4#h ,if the energy difference between the two levels is such that the frequency of emitted radiation is roughly between #aj and #aj cycles per second1 we see the radiation as light4 ,when more energy is added1 the radiation emerges as ultraviolet or ,x rays4 ,in other words the higher the energy difference1 the higher the frequency1 and vice versa4 ,thus it is that cosmic rays1 with the highest frequencies known to man1 can pass right through us as if we weren't there4 ,this simple picture of energy levels and associated frequencies doesn't quite hold for ordinary white light1 however4 ,such light is generally produced by a process called incandescence1 which results from the heating of a material until it glows4 ,true1 the atoms of the incandescent material are being raised to higher energy levels by chemical energy 7as in fire71 electricity 7light bulb71 or nuclear energy 7the sun74 ,in a hot solid1 however1 the explanation becomes more complicated4 ,many different electronic configurations are possible and the differences in energy among the various levels 7which can be many more than the three shown in ,figure #i7 vary only slightly from one another4 ,the result is a wide band of radiation4 ,thus1 while the incandescent electric bulb is a great advance over fire1 it is still a very inefficient source of light4 ,because it depends upon incandescence1 a considerable portion of the electrical input goes into the production of unwanted heat1 for the bulb's filament radiates in the infrared as well as the visible region4 ,for providing illumination1 the fluorescent tube is far more efficient than the incandescent lamp3 a #dj-watt fluorescent tube gives as much light as a #aej-watt incandescent light4 ,this is because its radiation is more controlled1 operating more in accord with our description of electronic energy levels4 ,hence more of its output is in the desired visual region of the spectrum4 ,in certain types of lighting1 particular energy level changes may predominate1 leading to the characteristic colors of neon tubes and vapor lamps4 ,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 white1 it is still quite broad4 ,in other words1 the radiation is still frequency incoherent--and it is still spatially incoherent4 ,to understand this1 let us return for a moment to the group of radio antennas we showed in ,figure #h4 ,all of them1 you will recall1 could be made to radiate in phase4 ,in the production of light1 however1 each antenna is replaced by a single atom6 ,this creates two problems4 ,first1 because the energy stored in the atom is quite small1 it comes out not as a continuous wave but as a tiny packet of radiation--a photon4#i ,it has an effect more like the hack of an ax than the buzz of a power saw4 ,second1 atoms are notoriously 8individualistic04 ,when a batch of atoms in a material has been raised to higher energy levels there is no way to know in what order1 or in what direction1 they will release their energy4 ,this kind of process is called spontaneous emission1 since each atom 8makes up its own mind04 ,all we know is that within a certain period of time--a short period1 to be sure--a certain percentage of these higher energy atoms will release their photons4 ,illustration3 ,figure #aj ,ordinary light is a jumble of frequencies1 directions1 and phases4 ,what we have1 then1 is incoherent radiation--a jumble of frequencies 7or colors71 directions1 and phases4 ,such light1 symbolized in ,figure #aj1 works well enough in lighting up this page1 but is almost worthless as a carrier of information 7and in other ways1 as we shall see shortly74 ,about the best that can be done with it is to turn it on and off in a sort of visual ,morse code1 which is exactly what is done on the blinker communication systems sometimes used for ship-to-ship communication4 ,in other words1 ordinary light cannot be modulated as radio waves can4 ,it is of interest to note1 however1 that ordinary white light can be made coherent1 to some extent1 but at a very high cost in the intensity of the light4 ,for example1 we might first pass the light through a series of filters1 each of which would subtract some portion of the spectrum1 until only the desired wavelength came through4 ,as can be seen in ,figure #aa1 only a small fraction of the original light would be left4 ,illustration3 ,figure #aa ,obtaining coherent radiation the hard way4 ,filters and pinhole block all but a small amount of the original radiation4 ,incoherent ,filters ,coherent in time ,pinhole ,coherent in time and space ,we would then have monochromatic 7one color7 light1 which is temporally coherent radiation1 but it would still be spatially incoherent4 ,in our diagram1 we show three monochromatic waves4 ,if we then passed this light through a tiny pinhole as shown1 most of these few remaining waves would be blocked2 the ones that got through would be pretty much in step4 7,in a similar manner1 a true point source of light would produce spatially coherent radiation2 but1 as in the process described here1 there wouldn't be very much of it47 ,we have1 finally1 obtained coherent light4 ,the important thing about the laser is that1 by its very nature1 it produces coherent light automatically4 ,now4444 ,,what',s ,,so ,,special ,,about ,,coherent ,,light8 ,so desirable are the qualities of coherent light that the complicated filtering process described above has actually been used4 ,for example1 one ,british experimenter1 ,dennis ,gabor1 used it in the #aidjs in an attempt to make a better microscope4 ,but so great was the effort1 and so meager the resulting light1 that this project was abandoned4 ,in the course of ,dr4 ,gabor's experiments1 however1 he did manage to make a special kind of picture1 using coherent light1 which he called a hologram4 ,he derived the name from two ,greek words meaning a whole picture4 ,we shall see why in a moment4 ,ordinary black and white photographs merely record darks and lights1 or the intensity of the illumination1 thereby providing a scale of grays1 nothing more4 ,but because waves of coherent light consistently maintain their relative spacing1 they can be used to record additional information1 namely the distance from objects4 ,for example1 if we shine a beam of coherent 7laser7 light between two objects we can1 knowing the light wavelength1 determine the distance between them to a high degree of accuracy4 ,the basic idea is diagramed in ,figure #ab4 ,it can be seen that the number of waves times the wavelength gives the precise distance 7to within #a wavelength of light7 from the laser source to each object4 ,but this would be a difficult process to implement4 ,a better way1 and one that is already in operation1 is to use conventional methods to measure the approximate distance and use the laser beam for precise or fine measurement4 ,in the device shown in ,figure #b1 the beam is split into two parts4 ,one part is kept in the instrument itself to act as a reference4 ,the other is aimed at a reflector1 which sends it back to a detector in the main device1 where it is automatically compared with the reference beam4 ,if the two beams are in phase 7that is1 if their crests are superimposed71 the waves combine and produce a high intensity beam at the detector4 ,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 phase4 ,the instrument counts the changes and displays the distance the reflector moves1 as a function of the wavelengths1 on the control cabinet meters4 ,illustration3 ,figure #ab ,principle of distance measurement using coherent light4 ,wavelength times number of waves gives precise distance between laser and object4 ,distance to be measured ,laser ,object ,no4 #b #a ,wavelength ,object ,no4 #a ,since the ,word ,for the ,interaction of the ,waves in a ,system ,like ,this ,is 8,interference01 the ,measurement ,process ,is ,called interferometry 7,pronounced in ,ter ,fer ,om ,e ,try74 ,although ,not ,new1 ,it ,can ,now ,be ,applied ,for the ,first ,time in ,machine ,tool ,applications1 ,providing the ,accuracy ,needed in ,this ,age of ,space ,technology and ,microminiaturization4 ,measurements ,with a ,laser ,interferometer ,can ,be ,made ,with an ,accuracy of #j4e ,part ,per ,million at ,distances ,up to #bjj ,inches4 ,such ,precision ,was ,previously ,unheard of in a ,machine ,shop ,environment1 ,having ,been ,limited to ,laboratory ,measurements1 and ,only at ,a ,range of a ,few ,inches4 ,under ,similar ,laboratory ,conditions1 ,measurements by ,laser ,interferometry ,now ,detect ,movements of #aj ,centimeter1 a ,distance ,approaching the ,dimensions of an ,atomic ,nucleus4 ,now let us suppose we expand the laser beam as shown on page #bb1 and1 with the aid of a mirror1 direct part of it 7the reference beam7 at a photographic plate4 ,the remaining portion of the diverging beam is used to illuminate the object to be photographed4 ,some of this light 7the object beam7 is reflected toward the plate and carries with it information about the object1 as indicated by the wavy line4 ,in the region where these two beams intersect1 interference occurs1 and a sample of this interference is recorded within the photographic emulsion4 ,where two crests meet a dark spot is recorded2 where the waves are out of phase the processed plate is clear4 ,the result is a hologram1 a complex pattern of 8fringes01 characteristic of the contour and light and dark areas of the object1 as well as its distance from the plate4 ,these fringes have the ability to diffract light rays4 ,when light from a laser1 or a point source of white light1 is directed at the hologram from the same direction as the reference beam1 part of the light is 8bent0 so that it appears to come from the place once occupied by the object4 ,the result is a remarkably realistic #c-;dimensional image4 ,there1 in a nutshell1 is the incredible new technique of holography4 ,the extreme order of laser light is illustrated by the regularity of the dots on the cover of this booklet4 ,this strange kind of light provides us with yet other advantages4 ,indeed1 one of the most important is the fact that the energy of the laser is not being sprayed out in all directions4 ,all of it is concentrated in the narrow beam that emerges from the device4 ,and it stays narrow4 ,laser light has already been shone on the moon1 the beam spreading out to only a few miles in traveling there from earth4 ,the best optical searchlight beam would spread wider than the moon itself1 thus dissipating its energy4 ,it is for this reason1 as well as its temporal coherence1 that laser light is being considered for communications4 ,a narrow beam is particularly important for space communications because of the long distances involved4 ,but it is also possible to focus laser light as no light has ever been focused before4 ,at close range a laser beam can be focused down to a circle just a few wavelengths across1 concentrating its energy and making it possible to drill holes only #j4jjjb inch in diameter4 ,the photo on page #eb shows the exquisite control that can be exercised4 ,let us see what this focusability means in terms of power4 ,consider1 by way of analogy1 a dainty #ajj-pound lady in a pair of spike-heeled shoes4 ,as she takes a step1 her weight will be concentrated on one of those heels4 ,if the area of the heel is1 say1 one quarter of a square inch 7 inch71 the pressure exerted on the poor tile or carpet rises to #djj pounds per square inch 7#d #ajj7 and if the heel is only inch on a side1 the pressure will be #afjj pounds per square inch6 ,illustration3 ,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 at1 of course1 is the fact that the coherence of the laser beam permits it to be concentrated into a tiny area4 ,thus whatever total energy is being sent out by the laser can be concentrated to the point where its effective energy is tremendous4 ,the sun emits some #fejj watts per square centimeter4 ,laser beams have already reached #ejj million watts per square centimeter4 ,but the power of the laser does not derive solely from its ability to be focused4 ,even an unfocused beam is several times more powerful than the sun's output 7per square centimeter74 ,illustration3 ,figure #ac ,the typical hologram1 looks like a geometric design1 but it contains more information than would an ordinary photograph4 ,the images below1 made from a hologram1 show the detail1 apparent solidity1 and parallax effect of the reconstructed light waves4 ,the parallax effect is the ability to see around the objects just as one could if they were really there4 7,see frontispiece47 ,illustration3 ,model tank ,illustration3 ,tank1 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 controlled4 ,in the production of ordinary light the atoms1 as we know1 emit spontaneously1 or in an uncontrolled fashion4 ,but if the atoms could be forced to take in the proper amount of energy1 store it1 and release it when we wanted them to1 we would have stimulated1 rather than spontaneous1 emission4 ,this1 however1 is practically the same as the amplification principle we discussed earlier4 ,in that case1 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 signal4 ,the question is1 how can we do this with light8 ,,controlled ,,emission ,the laser and its parent1 the maser1 can be traced back half a century to its theoretical beginnings4 ,the great physicist ,albert ,einstein is most widely known for his work in relativity4 ,but he did early and important work on that other gigantic #bjth century scientific achievement1 the quantum theory4#aj ,in one of his papers1 published first in ,zurich1 ,switzerland1 in #aiaf1 ,einstein showed that controlled emission of light energy could be obtained from an atom under certain conditions4 ,when an atom or molecule has somehow had its energy level raised1 the release of this stored energy could be stimulated by subjecting the atom or molecule to a small 8shot0 of electromagnetic radiation of the proper frequency4 ,einstein wrote that when such a photon of energy caused an electron to drop from a higher to a lower orbit1 the electron would emit another photon of the same frequency and in the same direction as the one that hit it4#aa ,in other words1 the energy of the emitted photon would be added to that of the photon that stimulated the emission in the first place4 ,here1 potentially1 was light amplification4 ,the three major factors1 absorption of energy1 spontaneous emission1 and stimulated emission are diagrammed in ,figure #ad4 ,there the matter lay for more than #cj years4 ,in #aiea ,charles ,h4 ,townes1 then on the ,columbia ,university faculty1 was interested in ways of extending to still higher frequencies the range of microwaves available for use in communications and in other scientific applications4 ,townes and other scientists who were interested in the problem were to meet in ,washington1 ,d4 ,c41 on the #bfth of ,april4 ,the night before the meeting he slept in a small ,washington hotel2 but he awoke at #e3cj--pondering1 pondering the high frequency problem4 ,he dressed and took a walk1 then sat on a park bench and savored the beauty of azaleas in bloom4 ,but all the while his mind was running over the various aspects of the problem4 ,illustration3 ,figure #ad ,an atom can release absorbed energy spontaneously or it can be stimulated to do so4 ,before ,after ,excited state ----- ---- ,absorption ,relaxed state ---- ----- ,excited state ---- ----- ,spontaneous emission ,relaxed state ----- ---- ,excited state ---- ----- ,stimulated emission ,relaxed state ----- ---- ,suddenly the answer came to him4 ,normally more of the molecules in any substance are in low-energy states than in high ones4 ,he would change the natural balance and create a situation with an abnormally large number of high-energy molecules4 ,then he would stimulate them to emit their energy by nudging them with microwaves4 ,here was amplification4 ,he could even take some of the emitted radiation and feed it back into the device to stimulate additional molecules1 thereby creating an oscillator4 ,this feedback arrangement1 he knew1 could be carried out in a cavity1 which would resonate 7just like an organ pipe7 at the proper frequency4 ,the resonator would be a box whose dimensions were comparable with the wavelength of the radiation1 that is1 a few centimeters on a side4 ,on the back of an envelope he figured out some of the basic requirements4 ,three years1 and many experiments1 later the maser 7microwave amplification by stimulated emission of radiation7 was a reality4 ,the original maser was a small metal box into which excited ammonia molecules were added4 ,when microwaves were beamed into the excited ammonia the box emitted a pure1 strong beam of high frequency microwaves1 far more temporally coherent than any that had ever been achieved before4 ,the output of an ammonia maser is stable to one part in #ajj billion1 making it an extremely accurate atomic 8clock04#ab ,moreover1 the amplifying properties of masers have been found to be very useful for magnifying faint radio signals from space1 and for satellite communications4 ,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 #bc1hgj megacycles 7#bc1hgj million cycles7 per second4 ,ammonia molecules also react to a nonuniform electric field in ways that depend on their energy level3 low-level molecules can be attracted and high-level ones repelled by the same field4 ,thus it is possible to separate the low-energy molecules from the high1 and to get the excited molecules into the cavity without too much trouble4 ,this procedure for getting the majority of atoms or molecules in a container into a higher energy state1 is called population inversion and is basic to the operation of both masers and lasers4 ,it should be noted that two ,russians1 ,n4 ,g4 ,basov and ,a4 ,m4 ,prokhorov1 were working along similar lines independently of ,townes4 ,in #aieb they presented a paper at an ,all-,union 7,u4,s4,s4,r47 ,conference1 in which they discussed the possibility of constructing a 8molecular generator01 that is1 a maser4 ,their proposal1 first published in #aied1 was in many respects similar to ,townes's4 ,in #aiee1 ,basov and ,prokhorov discussed1 in a short note1 a new way to obtain the active atomic systems for a maser1 a method that turned out to be of great importance4 ,thus on ,october #bi1 #aifd1 the ,nobel ,prize in ,physics was awarded1 not only to ,townes1 but to ,basov and ,prokhorov as well4 ,the award was for fundamental work in the field of quantum electronics1 which has led to the construction of oscillators and amplifiers based on the 8aser0 principle4 ,a ,,laser ,,is ,,born ,following the maser development1 there was much speculation about the possibility of extending the principle to the optical region4 ,indeed the first lasers--light amplification by stimulated emission of radiation--were called 8optical masers04 ,the difficulty1 of course1 was that optical wavelengths are so tiny--about _/1 that of microwaves4 ,the maser principle depended upon a physical resonator1 a box a few centimeters 7or even millimeters7 in length4 ,but at millimeter wavelengths1 such resonators are already so small that they are hard to make accurately4 ,making a box _/1 that size was out of the question4 ,another approach was necessary4 ,in #aieh ,a4 ,l4 ,schawlow of ,bell ,telephone ,laboratories and ,dr4 ,townes outlined the theory and proposed a structure for an optical maser4 ,they suggested that resonance could be obtained by making the waves travel back and forth along a relatively long1 thin column of amplifying substance that had parallel reflectors at the ends4 ,after their theory of the optical maser had been published1 the race to build the first actual device began in earnest4 ,the winner1 in #aifj1 was ,dr4 ,t4 ,h4 ,maiman1 then with ,hughes ,aircraft ,company4 7,he is now president of ,maiman ,associates47 ,the active substance he used was a single crystal of ruby1 with the ends ground flat and silvered4 ,ruby is an aluminum oxide in which a small fraction of the aluminum atoms in the molecular structure1 or lattice1 have been replaced with chromium atoms4 ,these atoms absorb green and blue light and hence impart a red color to the ruby4 ,the chromium atoms can be boosted from their ground state into excited states when they absorb the green or blue light4 ,this process1 by which population inversion is achieved1 has been given the name pumping4#ac ,pumping in a crystal laser is generally achieved by placing the ruby rod within a spiral flash lamp 7,figure #ae7 that operates like those used in high-speed 7stroboscopic7 photography4 ,when the lamp is flashed1 a bright beam of red light emerges from the ruby1 shining out through one end1 which has been only partially silvered4 ,illustration3 ,figure #ae ,a ruby laser system4 ,ruby ,flash lamp ,partially silvered end ,laser output ,power ,cooling ,the duration of this flash of red light is quite brief1 lasting only some #cjj millionths of a second1 but it is very intense4 ,in the early lasers1 such a flash reached a peak power of some #aj1jjj watts4 ,when ,maiman's device was successfully built and operating1 a public relations expert was called in to help introduce this revolutionary device to the world4 ,he took one look at the laser and decided that it was too small and insignificant looking and would not photograph well4 ,looking around the lab1 he spotted a larger laser and decided that that one was better4 ,dr4 ,maiman informed him in his best scientific manner that laser action had not been achieved with that one4 ,but the world of promotion won out1 and ,dr4 ,maiman allowed the larger device to be photographed on the assumption--or was it hope8--that he would be able to get it to operate in the future4 7,he did47 ,the device shown in ,figure #af is the true first laser4 ,the all-important crystal rod is seen at the center4 ,these crystals1 incidentally1 must be quite free of extraneous material2 hence they are artificially 8grown01 as shown in ,figure #ag4 ,the single large crystal is formed as it is pulled slowly from the 8melt01 after which it is ground to size and polished4 ,illustration3 ,figure #af ,dr4 ,maiman's first laser4 ,output was #aj1jjj watts4 ,illustration3 ,figure #ag ,an exotic crystal of the garnet family is 8grown0 from a melt at a temperature of #cdjj;,f4 ,,lasing--,a ,,new ,,word ,now we can begin to put together the various processes and equipment we have been discussing separately4 ,perhaps the best way to do this is to look again at the word laser and recall its meaning3 light amplification by stimulated emission of radiation4 ,our objective is to create a powerful1 narrow1 coherent beam of light4 ,let us see how to do this4 ,in ,figure #ah we imagine a laser crystal containing many atoms in the ground state 7white dots7 and a few in the excited state 7black dots74 ,pumping light 7wavy arrows in a7 raises most of the atoms to the excited state1 creating the required population inversion4 ,illustration3 ,figure #ah ,sequence of operations in a solid crystal laser4 7a7 ,pumping light raises many atoms to excited state4 7b7 ,lasing begins when a photon is spontaneously emitted along the axis of the crystal4 ,this stimulates other atoms in its path to emit4 7c7 ,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 end4 7a7 ,ruby crystal ,pumping light ,atom in ground state ,excited atom ,partial reflecting mirror ,full reflecting mirror 7b7 ,excited atom emits photon parallel to axis 7c7 ,lasing begins when an excited atom spontaneously emits a photon parallel to the axis of the crystal 7b74 7,photons emitted in other directions merely pass out of the crystal47 ,the photon stimulates another atom in its path to contribute a second photon1 in step1 and in the same direction4 ,this process continues as the photons are reflected back and forth between the ends of the crystal4 7,we might think of lone soldiers falling into step with a column of marching men47 ,the beam builds up until1 when amplification is great enough 7c71 it flashes out through the partially silvered mirror at the right--a narrow1 parallel1 concentrated1 coherent beam of light1 ready for4444 ,,some ,,interesting ,,applications ,application of lasers can be divided into two broad categories3 7#a7 commercial1 industrial1 military1 and medical uses1 and 7#b7 scientific research4 ,in the first case1 lasers are used to do something that has been done in another way up to now 7but not as well74 ,sometimes a laser solves a particular problem4 ,for example1 one of the first applications was in eye surgery1 for 8welding0 a detached retina4 ,the laser is particularly useful here because laser light can penetrate transparent objects such as the eye's lens 7,figure #ai71 eliminating the need to make a cut into the eye4 ,illustration3 ,figure #ai ,diagram of human eye showing laser beam focused on retina4 ,cornea ,lens ,optic ,nerve ,beam angle ,fovea centralis ,iris ,image ,retina ,surgeons have long wanted a better technique for treating extremely small areas of tissue4 ,a laser beam1 focused into a small spot1 performs perfectly as a lilliputian surgical knife4 ,an additional advantage is that the beam1 being of such high intensity1 can also sterilize or cauterize tissue as it cuts4 ,the narrowness of the laser beam has made it ideal for applications requiring accurate alignment4 ,perhaps the ultimate here is the #b-mile-long linear accelerator built by ,stanford ,university for the ,united ,states ,atomic ,energy ,commission4 8,arrow-straight0 would not have been nearly good enough to assure expected performance4 ,a laser beam was the only technique that could accomplish the incredible task of keeping the inch bore of the accelerator straight along its #b-mile length4 ,a remote monitoring system1 based on the same laser beam1 tells operators when a section of the accelerator has shifted out of line 7due for example to small earth movements7 by more than about _/ inch--and identifies the section4#ad ,figure #bj shows the #b-mile-long 8klystron gallery0 that generates the power for kicking the high-energy particles down the tube4 ,the gallery parallels the accelerator housing and lies #be feet beneath it 7,figure #ba74 ,the large tube houses the optical alignment system and supports the smaller accelerator tube above4 ,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 #ac4 ,precise alignment of the tube is achieved by aiming the laser at the center dot of the pattern4 ,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 tube4 ,it is the extreme coherence of the laser beam that makes this technique possible4 ,having heard that laser light has bored through steel and is being used in microwelding1 some have asked whether the laser will ever be used to weld bridge members and other structural girders4 ,this is missing the whole point of the laser3 ,it would be like washing your floor with a toothbrush 7even one with extra stiff bristles76 ,there would be no advantage to using lasers for large-scale welding2 present equipment for this operation is quite satisfactory and far less wasteful of input power4 ,the sensible approach is to use lasers where existing processes leave something to be desired4 ,until the advent of the laser1 for example1 there was no good way to weld wires #j4jja inch in diameter4 ,nor was there a good way to bore the tiny hole in a diamond that is used as a die for drawing such fine wire4 ,it used to take #b days to drill a single diamond4 ,with laser light the operation takes #b minutes--and there is no problem with rapid wear of a cutting tool4 ,so much for the first category of application4 ,in the second category1 namely use of the laser as a scientific tool1 we enter a more theoretical domain4 ,here we use coherent light as an extension of ourselves1 to probe into and to look at the world around us4 ,illustration3 ,figure #bj ,a laser beam was used 7and continues to be used7 for precise alignment of ,stanford ,university's #b-mile-long linear accelerator4 ,this view shows the aboveground portion during construction4 ,much experimental science is a matter of cooling1 heating1 grinding1 squeezing1 or otherwise abusing matter to see how it will react4 ,with each new tool--ultrafast centrifuges1 high- and low-pressure and extreme-temperature chambers1 intense magnetic fields1 atomic accelerators and so on--more has been learned about this still-puzzling world4 ,since coherent light is something new1 we can do things to matter that have not been done before1 and see how it reacts4 ,the laser is being used to investigate many problem areas in biology1 chemistry1 and physics4 ,for example1 sound waves of extremely high frequency can be generated in matter by subjecting it to laser light4 ,these intense vibrations may have profound effects on materials4 ,illustration3 ,figure #ba ,subterranean view of ,stanford accelerator housing4 ,alignment optics 7laser systems7 are housed in the large tube1 which also acts as support for the smaller accelerator tube above it4 ,illustration3 ,figure #bb ,laser beam spot as observed at the end of the accelerator4 ,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 reactions4 ,chemical reactions usually are set off by heat1 agitation1 electricity1 or other broadly applied means4 ,none of these energizers allow the fine control that the laser beam does4 ,its extremely fine beam can be focused to a tiny spot1 thus allowing chemical activity to be pinpointed4 ,but there is a second advantage3 ,the monochromaticity of coherent light also makes it possible to control the energy 7in addition to the intensity7 of the beam accurately by simply varying the wavelength4 ,thus it may be possible1 for instance1 to cause a reaction in one group of molecules and not in another4 ,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 plastics4 ,bell ,telephone ,laboratory scientists have changed the styrene monomer 7a 8raw0 plastic material7 to its final state1 polystyrene1 in this way4 ,the success of these and similar experiments elsewhere opens for exploration a vast area of molecular phenomena4 ,in another scientific application1 the laser is being used more and more as a teaching tool4 ,coherence is a concept that formerly had to be demonstrated by diagrams1 formulas1 and inference from experiments4 ,the laser makes it possible to see coherence 8in action01 along with many of the physical effects that result from it4 ,such phenomena as diffraction1 interference1 the so-called ,airy disc patterns1 and spatial harmonics1 always difficult to demonstrate to students in the abstract1 can now be seen quite concretely4 ,other interesting things can also be seen more plainly now4 ,at the ,los ,alamos ,scientific ,laboratory1 laser light is being used to 8look0 at plasmas2 the result of one such look is shown in ,figure #bc4 ,plasmas are ionized gaseous mixtures4 ,their study lies at the heart of a constant search by atomic scientists for a self-sustained1 controlled fusion reaction that can be used to provide useful thermonuclear power4 ,this kind of reaction provides the almost unlimited energy in the sun and other stars4 ,it is more efficient and releases less radioactivity than the other principal nuclear process1 fission1 which is used in atomic-electric power plants4#ae ,illustration3 ,figure #bc ,shadowgraph of deuterium discharge taken in laser light4 ,turbulence of the plasma is clearly seen4 ,westinghouse ,electric ,corporation scientists1 on the other hand1 have used the concentrated energy of the laser1 not to look at1 but to produce a plasma 7,figure #bd74 ,they blasted an aluminum target the size of a pinhead with a laser beam1 thereby vaporizing it and creating a plasma4 ,the calculated temperature in the electrically charged gas was #c1jjj1jjj centigrade4 ,this is pretty hot1 but still not hot enough for a thermonuclear reaction4 ,illustration3 ,figure #bd ,plasma heating by laser light4 ,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 7above #aj1jjj1jjj;,c7 that any material is vaporized instantly on coming into contact with it4 ,the only means developed so far to contain the plasma is an intense magnetic field1 or 8magnetic bottle02 containment has been accomplished for only a few thousandths of a second at most4 ,the objective of the ,westinghouse research1 which was supported by the ,atomic ,energy ,commission1 was to study in detail the interaction of the plasma with a magnetic field4 ,we do not have room to describe more applications in detail1 but it may be interesting to list a few other uses of lasers--some commercial and some still experimental3 --,earthquake prediction4 --,measurement of 8tides0 in the earth's crust under the sea4 --,laser gyroscopes4 --,highly accurate velocity measurement 7useful in certain assembly line and continuous manufacturing processes74 --,scanner for analyzing photographs of bubble chamber tracks and astronomical phenomena4 --,computer output and storage systems2 perhaps even complete optical data processing systems4 --,lightning-fast printing devices4 --,high-speed photography 7,figure #be74 --,missile tracking and accurate alignment of antennas4 --,automatic flaw spotter for big radio antennas4 --,aircraft landing aid for poor weather conditions4 --,fast1 painless dental drill4 --,cancer research4 ,illustration3 ,figure #be ,twenty-two caliber bullet and its shock wave are photographed from the image produced by a doubly exposed laser hologram4 ,the original hologram was exposed twice by a ruby laser within half a thousandth of a second as the bullet sped past at #b times the speed of sound4 ,a ,,multitude ,,of ,,lasers ,it is almost self-evident that no single device1 even one as incredible as the laser1 could accomplish all the feats mentioned in the preceding paragraphs4 ,after all1 some of these applications require high power but not extremely high monochromaticity1 while in others the reverse may be true4 ,yet1 by its very nature1 any laser produces a beam with one1 or at the most a few1 wavelengths1 and many different materials would be needed to provide the many different wavelengths required for all the tasks listed4 ,also1 the first laser was a pulsed device4 ,light energy was pumped in and a bullet of energy emerged from it4 ,then the whole process had to be repeated4 ,pulsed operation is fine for spot-welding and for applications such as radar-type rangefinding1 where pulses of energy are normally used anyway4 ,with lasers smaller objects can be detected than when using the usual microwaves4 ,but a pulsed process is not useful for communications4 ,in other words1 pulsing is good for certain applications but not for others4 ,and of course solid crystals are difficult to manufacture4 ,hence1 it was natural for laser pioneers to look hopefully at gases4 ,gas lasers would be easier to make--simply fill a glass tube with the proper gas and seal it4 ,but other advantages would accrue4 ,for one thing the relatively sparse population of emitting atoms in a gas provides an almost ideally homogeneous medium4 ,that is1 the emitting atoms 7corresponding to chromium in the ruby crystal7 are not 8contaminated0 by the lattice or host atoms4 ,since only active atoms need be used1 the frequency coherence of a gas laser would probably be even better than that of the crystal laser1 they reasoned4 ,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 gases4 ,this was an ingeniously contrived partnership whereby one gas did the energizing and the other did the amplifying4 ,gas lasers now utilize many different gases for different wavelength outputs and powers and provide the 8purest0 light of all4 ,an additional advantage is that the optical pumping light could be dispensed with4 ,an input of radio waves of the proper frequency did the job very nicely4 ,but most significant of all1 ,javan's gas laser provided the first continuous output4 ,this is commonly referred to as ,,cw 7continuous wave7 operation4 ,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 other4 ,when a non-expert thinks of a laser1 he is apt to think of power--blinding flashes of energy--as illustrated in ,figure #bf4 ,as we know1 this is only a small part of the capability of the laser4 ,nevertheless1 since lasers are often specified in terms of power output it may be well to discuss this aspect4 ,the two units generally used are joules and watts4 ,you are familiar with a watt and have an idea of its magnitude3 think1 for example1 of a #ae-watt or a #aej-watt bulb4 ,a watt is a unit of power2 it is the rate at which 7electrical7 work is being done4 ,illustration3 ,figure #bf ,high power is demonstrated as a laser beam blasts through metal chain4 ,the joule is a unit of energy and can be thought of as the total capacity to do work4 ,one joule is equivalent to #a watt-second1 or #a watt applied for #a second4 ,but it can also mean a #aj-watt burst of laser light lasting #j4a second1 or a billion watts lasting a billionth of a second4 ,in general1 the crystal 7ruby7 lasers are the most powerful1 although other recently introduced materials1 such as liquids 7see ,figure #bg7 and specially prepared glass1 are providing competition4 ,with proper auxiliary equipment1 bursts of several billion watts have been achieved2 but the burst lasts only about #ajj millionths of a second4 ,for certain uses1 that's just what is wanted3 a highly concentrated burst of energy that does its work without giving the material being 8shot0 a chance to heat up and spread the energy1 perhaps damaging adjacent areas4 ,illustration3 ,figure #bg ,active substance for a modern liquid laser is made in an uncomplicated #aj-minute procedure4 ,bluish powder of the rare earth1 neodymium1 is dissolved in a solution of selenium oxychloride and sealed in a glass tube4 ,since the joule gives a measure of the total energy in a laser burst it is not applicable to ,,cw output4 ,power in this area began low--in the milliwatt 7one thousandth of a watt7 region--but has been creeping up steadily4 ,a recent gas laser utilizing carbon dioxide has already reached #eej watts of continuous infrared radiation4 ,this is the giant #dd-;footer shown in ,figure #bh4 ,an advantage of gas 7and liquid7 lasers is that they can be made just about as large as one wishes4 ,by way of comparison1 the smallest gas laser in use is shown in ,figure #bi4 ,illustration3 ,figure #bh ,a giant #dd-;foot gas laser produces #eej watts of continuous power and is expected to reach #ajjj watts4 ,glowing of the tube comes from gas discharge1 not from laser light1 which is in the infrared region and cannot be seen4 ,one of the least satisfactory aspects of the laser has been its notoriously low efficiency4 ,for a while the best that could be accomplished was about #a4 ,that is1 a hundred watts of light had to be put in to get #a watt of coherent light out4 ,in gas lasers the efficiency was even lower1 ranging from #j4ja to #j4a4 ,in gas lasers this was no great problem since high power was not the objective4 ,but with the high-power solid lasers1 pumping power could be a major undertaking4 ,a high-power laser pump built by ,westinghouse ,research ,laboratories handles #gj1jjj joules4 ,in more familiar terms1 the peak power input while the pump is on is about #ajj1jjj1jjj watts4 ,for a brief instant this is roughly equal to all the electrical power needs of a city of #ajj1jjj people4 ,two relatively new developments have changed the efficiency levels4 ,one1 the carbon dioxide gas laser1 is quite efficient1 with the figure having passed #ae4 ,the second is the injection1 or semiconductor laser1 in which efficiencies of more than #dj have been obtained4 ,unless unforeseen difficulties arise this figure is expected to continue to rise to a theoretical maximum of close to #ajj4 ,illustration3 ,figure #bi ,a miniature gas laser produces continuous output in visible red region4 ,the semiconductor laser is to solid and gas lasers what the transistor was to the vacuum tube2 all the functions of the laser have been packed into a tiny semiconductor crystal4 ,in this case1 electrons and 8holes0 7vacancies in the crystal structure that act like positive charges7 accomplish the job done by excited atoms in the other types4 ,that is1 when they are stimulated they fall from upper energy states to lower ones1 and emit coherent radiation in the process4 ,aside from this the principle of operation is the same4 ,the device itself1 however1 is vastly different4 ,for one thing it is about the size of this letter 8o0 7,figure #cj74 ,for another1 it is self-contained2 since it can convert electric current directly into laser light--the first time this has been possible--an external pumping source is not required4 ,this makes it possible to modulate the beam by simply modulating the current4 7,a different approach has been to modulate a magnetic field around the device4 ,this1 it turns out1 can also be done with some newer solid crystal lasers47 ,an additional advantage offered by the semiconductor laser is simplicity4 ,there are no gases or liquids to deal with1 no glassware to break1 and no mirrors to align4 ,although it will not deliver high power1 it can already deliver enough ,,cw power for certain communications purposes4 ,its simplicity1 efficiency1 and light weight make it ideal for use in space4 ,illustration3 ,figure #cj ,a tiny injection laser works in infrared region4 ,the beam is visible because photo was taken with infrared film4 ,the laser itself is a tiny crystal of gallium arsenide inside the metal mount being held between the fingers4 ,,communications ,future deep space missions are expected to require extremely high data transmission rates 7on the order of a million bits#af per second7 to relay the huge quantities of scientific and engineering information gathered by the spacecraft4 ,higher data rates are necessary to increase both the total capacity and the speed of transmission4 ,by comparison1 the ,mariner-#d 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 missions4 ,the use of lasers would mean that results could be transmitted to earth in seconds instead of the #h hours it took for the photos to be sent from ,mariner-#d4 ,one of the problems to be solved in using lasers for deep space communication1 oddly enough1 is that of pointing accuracy4 ,since the beam of laser energy is narrow1 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 precision4 ,aiming a gun at a target #ej yards away is one thing2 aiming a laser from an unmanned spacecraft #ajj million miles away is quite another4 ,it is believed1 however1 that present techniques can cope with the problem4 ,another peculiarity of laser communication is that it will probably be accomplished faster and more readily in space than here on earth4 ,powerful though laser light may be1 it is light and is therefore impeded to some extent by our atmosphere even under good conditions4 ,data transmissions of #bj and #cj miles have already been accomplished in good weather with lasers4 ,but if you have ever tried to force a searchlight beam or shine automobile headlights through heavy fog1 rain1 or snow1 you will appreciate the magnitude of the problem under these conditions4 ,the use of infrared frequencies helps to some extent1 since infrared is somewhat more penetrating1 but the poor-weather problem is a serious one4 ,a possible solution is the use of 8light pipes01 similar to the wave guides already in use for certain microwave applications over short distances4 ,but as often happens1 new developments create new needs2 how1 for example1 can we get the laser beam to stay centered in the pipe and follow curves8 ,a series of closely spaced lenses1 about #ajjj per mile1 probably would accomplish this1 but too much light would be lost by scattering from the many lens surfaces4 ,scientists are experimenting with a new kind of 8lens01 one that uses variations in the density of gases to focus and guide the beam automatically4 ,since there are no surfaces in the path of the light beam1 and since the gas is transparent and free of turbulence1 the laser beam is not appreciably weakened or scattered as it travels through the pipe4 ,illustration3 ,figure #ca ,laser light beam being guided through a 8light pipe0 by a gas 8lens04 ,heating coil 7lower left7 or mixture of gases 7lower right7 are two possible ways of maintaining proper density gradient in the gas4 ,figure #ca shows how the gas focusing principle might be used to guide a beam through a curving pipe4 ,the shading represents the density of the gas4 ,several means have been developed to keep the gas denser in the center than around the outside4 ,when the pipe curves1 the light beam starts moving off the axis of the pipe4 ,the gas then acts like a prism1 deflecting the light beam in the direction of the curvature of the 8prism04 ,in communication between distant space and earth1 a light pipe might be a little cumbersome2 hence it may prove necessary to set up an intermediate orbiting relay station that will1 particularly in cases of poor weather1 intercept the incoming laser beam and convert it to radio frequencies that can penetrate our atmosphere with greater reliability4 ,powering space-borne lasers will1 of course1 be a problem4 ,indeed one of the major unsolved problems in production of spacecraft and long-term satellites is the provision of an adequate supply of power4 ,fuel cells and solar cells have helped but do not give the whole answer4#ag ,one other approach has already been developed3 a sun-pumped laser4 ,sunlight focused onto the side of the laser 7see ,figure #cb7 provides the pumping power1 enabling the device to put out #a watt of continuous infrared radiation1 enough for special space applications4 ,descendents of this device could produce visible light if this is deemed desirable4 ,another approach1 using chemical lasers1 is even more intriguing and may have greater consequences4 ,chemical lasers will derive their energy from their internal chemistry rather than from the outside4 ,a mixture of two chemicals may be all that is needed to initiate laser action aboard a spacecraft or satellite4 7,chemical lasers also offer the promise of even greater concentrations of power than have been achieved heretofore1 which may make them useful in plasma research47 ,with all these possibilities1 it may still be that spacecraft will need more power than is available on board4 ,the narrow beam of the laser offers one more fascinating possibility1 especially in the case of satellites relatively near earth4 ,the light of a laser might actually be used to beam energy to a receiver1 either for immediate use or storage4 ,it would then become possible to 8refuel0 satellites at will1 giving them much greater capabilities4 ,if available laser power is great enough1 laser beams might even be used to push satellites back into their proper orbits when they begin to wander off course1 as they almost invariably do after a while4 ,illustration3 ,figure #cb ,artist's rendering of sun-pumped laser as it would operate in space4 ,the sun's rays are collected by a parabolic reflector and are focused on the laser's surface by two cylindrical mirrors4 ,sun ,parabolic ,collector ,hyperbolic-cylindric secondary mirror ,semi-circular-cylindric tertiary mirror ,laser beam ,a ,,laser ,,in ,,your ,,future8 ,atomic energy1 only a scientific dream a few short years ago1 is now providing needed power in many parts of the world4 ,in the same way1 the laser1 also an atomic phenomenon1 has made its way out of the laboratory and into the fields of medicine1 commerce1 and industry4 ,if it hasn't touched your life as yet1 you need only be patient4 ,it will4 ,indeed the most exciting probability of all is that lasers undoubtedly will change our lives in ways we cannot even conceive of now4 ,illustration3 ,figure #cc ,tiny hole drilled in paper clip demonstrates remarkable capability of laser beam4 ,paper clip is #a inches long4 ,hole 7top7 was drilled by the laser microwelder shown in ,figure #a4 ,,suggested ,,references ,books ,,abc's of ,masers and ,lasers1 ,allan ,h4 ,lytel1 ,howard ,w4 ,sams and ,company1 ,inc41 ,publishers1 ,indianapolis1 ,indiana #dfbjf1 #aiff1 #if pp41 #b4be4 ,the ,laser3 ,light ,that ,never ,was ,before1 ,ben ,patrusky1 ,dodd1 ,mead and ,company1 ,new ,york #ajjaf1 #aiff1 #abh pp41 #c4ej4 ,masers and ,lasers1 ,manfred ,brotherton1 ,mc,graw-,hill ,book ,company1 ,new ,york #ajjcf1 #aifd1 #bbd pp41 #h4ej4 ,masers and ,lasers1 ,h4 ,arthur ,klein1 ,j4 ,b4 ,lippincott ,company1 ,philadelphia1 ,pennsylvania #aiaje1 #aifc1 #ahd pp41 #c4ie4 ,the ,story of the ,laser1 ,john ,m4 ,carroll1 ,e4 ,p4 ,dutton and ,company1 ,inc41 ,new ,york #ajjjc1 #aifd1 #aha pp41 #c4ie4 ,quantum ,electronics3 ,the ,fundamentals of ,transistors and ,lasers1 ,john ,r4 ,pierce1 ,doubleday and ,company1 ,inc41 ,new ,york #ajjag1 #aiff1 #ach pp41 #a4be4 ,lasers and ,their ,applications1 ,kurt ,r4 ,stehling1 ,the ,world ,publishing ,company1 ,cleveland1 ,ohio #ddajb1 #aiff1 #aib pp41 #f4jj4 ,understanding ,lasers and ,masers1 ,stanley ,leinwoll1 ,hayden ,book ,companies1 ,new ,york #ajjaa1 #aifd1 #if pp41 #a4ie4 ,atomic ,light3 ,lasers1 ,richard ,b4 ,nehrich1 ,jr41 ,glenn ,i4 ,voran1 and ,norman ,f4 ,dessel1 ,sterling ,publishing ,company1 ,inc41 ,new ,york #ajjaf1 #aifg1 #acf pp41 #c4ie4 ,articles--,general and ,historical ,advances in ,optical ,masers1 ,a4 ,l4 ,schawlow1 ,scientific ,american1 #bji3 #cd 7,july #aifc74 ,the ,evolution of the ,physicist's ,picture of ,matter1 ,p4 ,a4 ,m4 ,dirac1 ,scientific ,american1 #bjh3 #de 7,may #aifc74 ,filling in the ,blanks in the ,laser's ,spectrum1 ,f4 ,m4 ,johnson1 ,electronics1 #ci3 #hb 7,april #ah1 #aiff74 ,the ,amateur ,scientist--,how a persevering amateur can build a gas laser in the home1 ,c4 ,l4 ,stong1 ,scientific ,american1 #baa3 #bbg 7,september #aifd74 ,the ,amateur ,scientist--,homemade ,laser1 ,c4 ,l4 ,stong1 ,scientific ,american1 #bac3 #ajh 7,december #aife74 ,the ,amateur ,scientist--,how to make holograms and experiment with them or with ready-made holograms1 ,c4 ,l4 ,stong1 ,scientific ,american1 #baf3 #abb 7,february #aifg74 ,the ,maser1 ,james ,p4 ,gordon1 ,scientific ,american1 #aii3 #db 7,december #aieh74 ,the ,quantum ,theory3 ,early ,years to #aibc1 ,karl ,darrow1 ,scientific ,american1 #ahf3 #dg 7,march #aieb74 ,laser's ,bright ,magic1 ,t4 ,meloy1 ,national ,geographic ,magazine1 #acj3 #heh 7,december #aiff74 ,infrared and ,optical ,masers 7original paper71 ,a4 ,l4 ,schawlow and ,c4 ,h4 ,townes1 ,physical ,review1 #aab3 #aidj 7,december #ae1 #aieh74 ,laser ,market ,enters ,era of ,practicality1 ,w4 ,mathews1 ,electronic ,news1 #aa3 #a 7,april #ah1 #aiff74 ,lasers1 ,a4 ,k4 ,levine1 ,american ,scientist1 #ea3 #ad 7,march #aifc74 ,lasers1 ,a4 ,l4 ,schawlow1 ,science1 #adi3 #ac 7,july #b1 #aife74 ,lasers and ,coherent ,light1 ,a4 ,l4 ,schawlow1 ,physics ,today1 #ag3 #bh 7,january #aifd74 ,the ,laser's ,dazzling ,future1 ,l4 ,lessing1 ,fortune1 #fg3 #ach 7,june #aifc74 ,optical ,masers1 ,a4 ,l4 ,schawlow1 ,scientific ,american1 #bjd3 #eb 7,june #aifa74 ,optical ,pumping1 ,a4 ,l4 ,bloom1 ,scientific ,american1 #bjb3 #gb 7,october #aifj74 ,research on ,maser-,laser ,principle ,wins ,nobel ,prize in ,physics1 ,j4 ,p4 ,gordon1 ,science1 #adf3 #hig 7,november #ac1 #aifd74 ,resource ,letter ,,mop-#a on ,masers 7,microwave through ,optical7 and on ,optical ,pumping1 ,h4 ,w4 ,moos1 ,american ,journal of ,physics1 #cb3 #ehi 7,august #aifd71 extensive bibliography4 ,available from ,american ,institute of ,physics1 #cce ,east #deth ,street1 ,new ,york #ajjag4 ,enclose stamped return envelope4 ,advances in ,holography1 ,k4 ,s4 ,pennington1 ,scientific ,american1 #bah3 #dj 7,february #aifh74 ,applications of ,laser ,light1 ,d4 ,r4 ,herriott1 ,scientific ,american1 #bai3 #adj 7,september #aifh74 ,holography for the ,sophomore ,laboratory1 ,r4 ,h4 ,webb1 ,american ,journal of ,physics1 #cf3 #fb 7,january #aifh74 ,laser ,light1 ,a4 ,l4 ,schawlow1 ,scientific ,american1 #bai3 #abj 7,september #aifh74 ,the ,modulation of ,laser ,light1 ,d4 ,f4 ,nelson1 ,scientific ,american1 #bah3 #ag 7,june #aifh74 ,articles--,special ,subjects ,biological ,effects of ,high ,peak ,power ,radiation1 ,s4 ,fine et al41 ,life ,sciences1 #c3 #bji 7#aifd74 ,the ,interaction of ,light with ,light1 ,j4 ,a4 ,giordmaine1 ,scientific ,american1 #baj3 #ch 7,april #aifd74 ,chemical ,lasers1 ,george ,c4 ,pimental1 ,scientific ,american1 #bad3 #cb 7,april #aiff74 ,color ,laser ,stores ,data1 ,j4 ,eberhart1 ,science ,news1 #ij3 #ea 7,july #bc1 #aiff74 ,communication by ,laser1 ,stewart ,e4 ,miller1 ,scientific ,american1 #bad3 #ai 7,january #aiff74 ,guidelines for ,selecting ,laser ,materials1 ,r4 ,h4 ,hoskins1 ,electronic ,design1 #ac3 ,m#bi 7,july #ai1 #aife74 ,holography3 ,the ,picture ,looks ,good1 ,j4 ,blum1 ,electronics1 #ci3 #aci 7,april #ah1 #aiff74 ,how ,dangerous ,are ,lasers81 ,l4 ,h4 ,dulberger1 ,electronics1 #ce3 #bg 7,january #bf1 #aifb74 ,injection ,lasers1 ,r4 ,w4 ,keyes1 ,industrial ,research1 #f3 #df 7,october #aifd74 ,laser ,potential in ,deep-,space ,link ,grows1 ,b4 ,miller1 ,aviation ,week and ,space ,technology1 #hd3 #ga 7,january #ca1 #aiff74 ,laser ,retinal ,photocoagulator1 ,n4 ,s4 ,kapany et al41 ,applied ,optics1 #d3 #eag 7,may #aife74 ,laser ,welding in ,electronic ,circuit ,fabrication1 ,j4 ,p4 ,epperson1 ,electrical ,design ,news 7,,edn71 #aj3 #h 7,october #aife74 ,the ,light ,that ,slices ,inch into ,millionths1 7use of interferometry in industry71 ,steel1 #aeh3 #ch 7,february #bh1 #aiff74 ,the ,optical ,heterodyne--,key to ,advanced ,space ,signaling1 ,s4 ,jacobs1 ,electronics1 #cf3 #bi 7,july #ab1 #aifc74 ,photography by ,laser1 ,e4 ,n4 ,leith and ,j4 ,upatnieks1 ,scientific ,american1 #bab3 #bd 7,june #aife74 ,liquid ,lasers1 ,alexander ,lempicki and ,harold ,samelson1 ,scientific ,american1 #baf3 #ha 7,june #aifg74 ,plasma ,experiments with a #egj-k,j ,theta-,pinch1 ,f4 ,c4 ,yahoda1 et al41 ,journal of ,applied ,physics1 #ce3 #bcea 7,august #aifd74 ,a ,sun-,pumped ,,cw ,one-,watt ,laser1 ,c4 ,g4 ,young1 ,applied ,optics1 #e3 #iic 7,june #aiff74 #c-;,d ,image ,made at ,home1 ,science ,news1 #ij3 #ahe 7#aj ,september #aiff74 ,scanning with ,lasers1 ,robert ,a4 ,myers1 ,international ,science and ,technology1 #fe3 #dj 7,may #aifg74 ,booklets ,applications of ,lasers to ,information ,handling1 ,the ,perkin-,elmer ,corporation1 ,norwalk1 ,connecticut #jfheb1 #aiff1 #cb pp41 free4 ,reprint of five talks given by company personnel4 ,laser ,interferometer1 ,airborne ,instruments ,laboratory1 ,division of ,cutler-,hammer1 ,inc41 ,deer ,park1 ,long ,island1 ,new ,york #aagbi1 #aife1 #bj pp41 free4 ,collection of article reprints4 ,laser3 ,the ,new ,light1 ,bell ,telephone ,laboratories1 ,murray ,hill1 ,new ,jersey #jgiga1 #ai pp41 free4 ,full color1 nontechnical brochure presents some background1 principles1 and applications of the laser4 ,illustration3 ,argon laser1 which emits high-power blue-green beam continuously1 has application in signal processing1 communications1 and spectroscopy4 ,this unit is being beamed through prisms that separate its several discrete wavelengths of light1 displayed on card at left foreground4 ,,footnotes #a,sometimes referred to as hertz 7abbreviated ,hz71 for the #aith ,century ,german physicist ,heinrich ,hertz2 #ajjj ,hz #ajjj cps4 #b;,devised in ,france and officially adopted there in #agii1 the metric system uses the meter as the basic unit of length and has been proposed for all measurements in this country4 #c,named for the ,swedish physicist ,anders ,j4 ,angstrom4 #d,the wavelength1 indicated by the ,greek letter 7lambda7 is related to frequency 7f7 in the proportion 7in meters7 #cjj1jjj1jjj_/;f4 7,the number #cjj1jjj1jjj is the velocity of light in meters per second47 #e,microwaves are radio waves with frequencies above #ajjj megacycles per second4 #f,ten to #cj1jjj1jjj kilocycles per second2 this is low in the electromagnetic spectrum1 but not low in terms of the radio spectrum1 which has a low-frequency classification of its own4 #g,primitive as early radios were by today's standards1 they brought a new era to communication at the time4 ,unmodulated ,,cw 7continuous wave7 transmissions and crystal receivers were used to summon rescuers in the ,titanic disaster of #aiab1 for example4 #h;,energy h 7,planck's constant7 frequency4 ,planck's constant is the energy of #a quantum of radiation1 and equals #f4fbeef #aj erg-sec4 #i;,each photon carries #a quantum of radiation energy1 which is a unit equal to the product of the radiation frequency and ,planck's constant 7see footnote page #ae74 #aj;,einstein was awarded the ,nobel ,prize in #aiba for his #aije explanation of the photoelectric effect 7in terms of quanta of energy7 and not for his relativity theory4 #aa;,einstein's theoretical explanation applies in the case of stimulation of a single atom4 ,in practical stimulation1 directionality is enhanced by stimulating many atoms in phase4 #ab;,an atomic clock is a device that uses the extremely fast vibrations of molecules or atomic nuclei to measure time4 ,these vibrations remain constant with time1 consequently short intervals can be measured with much higher precision than by mechanical or electrical clocks4 #ac,the #aiff ,nobel ,prize in ,physics was awarded to ,prof4 ,alfred ,kastler of the ,university of ,paris for his research on optical pumping and studies on the energy levels of atoms4 #ad,see ,accelerators1 a companion booklet in this series1 for a full account of the ,stanford 8,atom ,smasher04 #ae;,for descriptions of fission and fusion processes1 see ,controlled ,nuclear ,fusion1 ,nuclear ,reactors1 and ,nuclear ,power ,plants1 other booklets in this series4 #af;,a bit is a digit1 or unit of information1 in the binary 7base-of-two7 system used in electronic data transmission systems4 #ag,see ,,snap1 ,nuclear ,space ,reactors and ,power from ,radioisotopes1 other booklets in this series1 for descriptions of nuclear sources of power for space4 ,this booklet is one of the 8,understanding the ,atom0 ,series4 ,comments are invited on this booklet and others in the series2 please send them to the ,division of ,technical ,information1 ,u4 ,s4 ,atomic ,energy ,commission1 ,washington1 ,d4 ,c4 #bjede4 ,published as part of the ,,aec's educational assistance program1 the series includes these titles3 ,accelerators ,animals in ,atomic ,research ,atomic ,fuel ,atomic ,power ,safety ,atoms at the ,science ,fair ,atoms in ,agriculture ,atoms1 ,nature1 and ,man ,books on ,atomic ,energy for ,adults and ,children ,careers in ,atomic ,energy ,computers ,controlled ,nuclear ,fusion ,cryogenics1 ,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 ,terms1 ,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 ,,snap1 ,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 booklet1 or of no more than three different booklets1 may be obtained free by writing to3 ,,usaec1 ,p4 ,o4 ,,box #fb1 ,,oak ,,ridge1 ,,tennessee #cghcj ,complete sets of the series are available to school and public librarians1 and to teachers who can make them available for reference or for use by groups4 ,requests should be made on school or library letterheads and indicate the proposed use4 ,students and teachers who need other material on specific aspects of nuclear science1 or references to other reading material1 may also write to the ,oak ,ridge address4 ,requests should state the topic of interest exactly1 and the use intended4 ,in all requests1 include 8,zip ,code0 in return address4 ,printed in the ,united ,states of ,america ,,usaec ,division of ,technical ,information ,extension1 ,oak ,ridge1 ,tennessee ,transcriber's ,notes --,silently corrected a few typos4 --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,in the text versions only1 text in italics is delimited by underscores4