1.1 WHAT IS A LASER? 3 energy upper level laser Q、 action population inversion FIGURE 1.2 lower level Population inversion between two quantum- mechanical energy levels. population inverted laser medium FIGURE 1.3 Laser amplification. input amplit相ed output light beam light beam these atoms into their higher quantum-mechanical energy levels.Practical laser materials can be pumped in many ways,as we will describe later in this text. For laser action to occur,the pumping process must produce not merely excited atoms,but a condition of population inversion (Figure 1.2),in which more atoms are excited into some higher quantum energy level than are in some lower energy level in the laser medium.It turns out that we can obtain this essential condition of population inversion in many ways and with a wide variety of laser materials-though sometimes only with substantial care and effort. Laser Amplification Once population inversion is obtained,electromagnetic radiation within a certain narrow band of frequencies can be coherently amplified if it passes through the laser medium(Figure 1.3).This amplification bandwidth will extend over the range of frequencies within about one atomic linewidth or so on either side of the quantum transition frequency from the more heavily populated upper energy level to the less heavily populated lower energy level. Coherent amplification means in this context that the output signal after being amplified will more or less exactly reproduce the input signal,except for a substantial increase in amplitude.The amplification process may also add some small phase shift,a certain amount of distortion,and a small amount of
4 CHAPTER 1:AN INTRODUCTION TO LASERS reflected light waves laser Inverted laser medium output beam 100% partially reflecting tranamitting mirror mirror FIGURE 1.4 Laser oscillation. amplifier noise.Basically,however,the amplified output signal will be a coherent reproduction of the input optical signal,just as in any other coherent electronic amplification process. Laser Oscillation Coherent amplification combined with feedback is,of course,a formula for producing oscillation,as is well known to anyone who has turned up the gain on a public-address system and heard the loud squeal of oscillation produced by the feedback from the loudspeaker output to the microphone input.The feedback in a laser oscillator is usually supplied by mirrors at each end of the amplifying laser medium,carefully aligned so that waves can bounce back and forth between these mirrors with very small loss per bounce(Figure 1.4).If the net laser amplification between mirrors,taking into account any scattering or other losses,exceeds the net reflection loss at the mirrors themselves,then coherent optical oscillations will build up in this system,just as in any other electronic feedback oscillator. When such coherent oscillation does occur,an output beam that is both highly directional and highly monochromatic can be coupled out of the laser oscillator,either through a partially transmitting mirror on either end,or by some other technique.This output in essentially all lasers will be both extremely bright and highly coherent.The output beam may also in some cases be extremely powerful.Just what we mean by“bright”and by“coherent"”we will explain later. REFERENCES The first stimulated emission devices,before lasers,were various kinds of masers,which operated on essentially the same basic physical principles,but at much lower frequencies and with much different experimental techniques.For an overview and unified approach to all these devices,see my earlier texts Microwave Solid-State Masers(McGraw-Hill, 1964)and An Introduction to Lasers and Masers (McGraw-Hill,1971). Some other good books on lasers can be found.A more elementary introduction, with good illustrations,is D.C.O'Shea,W.R.Callen,and W.T.Rhodes,Introduction to Lasers and Their Applications (Addison-Wesley,1977).A good general coverage is also given in O.Svelto,Principles of Lasers(Plenum Press,1982).Two well-known texts by A.Yariv are Introduction to Optical Electronics(Rinehart and Winston,1971)and the more advanced Quantum Electronics (Wiley,1975)
1.1 WHAT IS A LASER? 5 For full quantum-mechanical treatments of lasers,two good choices are M.Sargent III,M.O.Scully,and W.E.Lamb,Jr.,Laser Physics (Addison-Wesley,1977),and H. Haken,Laser Theory (Springer-Verlag,1983). A useful short bibliographic survey of laser references,aimed particularly at the college teacher,can be found in "Resource Letter L-1:Lasers,"by D.C.O'Shea and D.C.Peckham,Am.J.Phys.49,915-925 (October 1981). For more advanced information on various laser topics,the four-volume Laser Hand- book,edited by F.T.Arecchi and E.O.Schulz-Dubois (North-Holland,Amsterdam, 1972),provides an encyclopedic source with detailed articles on nearly every topic in laser physics,devices,and applications.If you'd like to look at some of the impor- tant original literature on lasers for yourself,well-chosen selections can be found in F.S.Barnes,ed.,Laser Theory (IEEE Press Reprint Series,IEEE Press,1972),or in D.O'Shea and D.C.Peckham,Lasers:Selected Reprints (American Association of Physics Teachers,Stony Brook,N.Y.,1982). If you would like to do experiments with a home-made laser or just see how one might be constructed,a useful collection of articles from the "Amateur Scientist"section of Scientific American has been reprinted under the title Light and Its Uses,with introduction by Jearl Walker (W.H.Freeman and Company,1980).Topics covered include simple helium-neon,argon-ion,carbon-dioxide,semiconductor,tunable dye, and nitrogen lasers,plus experiments on holography,interferometry,and spectroscopy. Problems for 1.1 1.Diagramming the electromagnetic spectrum.On a large sheet of paper lay out a logarithmic frequency scale extending from the audio range (say,f=10 Hz)to the far ultraviolet or soft X-ray region (say,A 100 A).Mark both frequency and wavelength below the same scale in powers of 10 in appropriate units,e.g., Hz,kHz,MHz,and m,mm,um.(You might also mark a "wavenumber"scale for 1/A in units of cm,and an energy scale for hw in units of ev.)Above the scale indicate the following landmarks(plus any other significant ones that occur to you): Audio frequency range (human ear)(20-15000 Hz) Standard AM and FM broadcast bands(535-1605 kHz,88-108 MHz) .Television channels 2-6 (54-88 MHz)and 7-13 (174-216 MHz) ·Microwave radar“g"and“X"bands(2-4and8-l2GHz) Visible region (human eye) Important laser wavelengths,including: HCN far-.IR laser(311,337,545,676,7444m) H2O far-IR laser (28,48,120 um) C02 laser(9.6-10.64m) C0 laser(5.1-6.5μm) HF chemical laser (2.7-3.0 um) Nd:YAG laser (1.06 um) He-Ne lasers (1.15 um,633 nm) GaAs semiconductor laser (870 nm) Ruby laser(694 nm)
6 CHAPTER 1:AN INTRODUCTION TO LASERS ,A 7065 He spectrum 6678 Red 5876 Yellow Green 5015 Transparent 4922 grating Blue 4713 Violet 4471 He discharge lamp FIGURE 1.5 Helium discharge spectrum observed through an inexpensive replica transmission grating. Rhodamine 6G dye laser(560-640 nm) Argon-ion laser(488-515 nm) Pulsed N2 discharge laser(337 nm) Pulsed H2 discharge laser (160 nm) 1.2 ATOMIC ENERGY LEVELS AND SPONTANEOUS EMISSION Our objective in this section is to give a very brief introduction to the concepts of atomic energy levels and of spontaneous emission between those levels.We attempt to demonstrate heuristically that atoms (or ions,or molecules)have quantum-mechanical energy levels;that atoms can be pumped or excited up into higher energy levels by various methods;and that these atoms then make spontaneous downward transitions to lower levels,emitting radiation at char- acteristic transition frequencies in the process.(Readers already familiar with these ideas may want to move on to Section 1.3.) The Helium Spectrum Figure 1.5 illustrates a simple experiment in which a small helium discharge lamp (or lacking that,a neon sign)is viewed through an inexpensive transmission diffraction grating of the type available at scientific hobby stores.(If you have never done such an experiment,try to do this demonstration for yourself
1.2 ATOMIC ENERGY LEVELS AND SPONTANEOUS EMISSION 7 blue green yellow red orange human y argon-ion laser He-Ne ruby laser laser 5876 4471 5015 6678 4713 4922 7065 4000 5000 6000 7000 wavelength, FIGURE 1.6 Helium spectral lines,four common laser lines,and human visual sensitivity. When viewed directly the discharge helium lamp appears to emit pinkish- white light.When viewed through the diffraction grating,however,each wave- length in the light is diffracted at a different angle.Upon looking through the grating,you therefore observe multiple images of the lamp,each displaced to a different discrete angle,and each made up of a different discrete wavelength or color emitted by the helium discharge.A strong yellow line at 5876A(or 588 nm) is particularly evident,but violet,green,blue,red,and deep red lines are also readily seen.These visible wavelengths are plotted in Figure 1.6,along with (as a matter of curiosity)the relative response of the human eye,and the wavelengths of four of the more common visible lasers. These different wavelengths are,of course,only a few of the discrete compo- nents in the fluorescence spectrum of the helium atoms.In the helium discharge tube a large number of neutral helium atoms are present,along with a small number of free electrons and a matching number of ionized helium atoms to conduct electrical current.The free electrons are accelerated along the tube by the applied electric field,and collide after some distance with the neutral he- lium atoms.The helium atoms are thereby excited into various higher quantum energy levels characteristic of the helium atoms.A small fraction are also ion- ized by the electron collisions,thereby maintaining the electron and ion densities against recombination losses,which occur mostly at the tube walls. After being excited into upper energy levels,the helium atoms soon give up their excess energy by dropping down to lower energy levels,emitting sponta- neous electromagnetic radiation in the process.This spontaneous emission or fluorescence is the mechanism that produces the discrete spectral lines