Sec. 1.1 A Brief History excellent reviews of the history of this era have been publis1t"62'3'l and only the rnajor points are summarized here. For a variety of reasons, it is easier to obtain amplification of electromagnetic waves in the microwave spectrum. Thus, the first devices to demonstrate gain (see the Appendix for definitions of common laser terms and Section 2.2 for more information on gain) by osers stirnulated emission (see Section 1.4) were microwave devices termed masers (Microwave Amplification by the Stimulated Emission of Radiation).' The idea of using stimulated emission as a means of amplifying electromagnetic radi- ation in the microwave spectrum seems to have been independently conceived of by Charles H. Townes at Columbia University, Joseph Weber at the University of Maryland,5 and Alexander M. Prokhorov and Nikolai G. Basov at the Lebedev Physics Institute (Moscow).6 The first maser (a 24-GHz ammonia device) was operated by James P. 6ordon, Townes, and Herbert J. Zniger at Columbia in 1954.7 The successful operation of the ammonia maser immediately generated discussion as to whether these principles could be applied to visible wavelengths. At the time, there were a number of issues that suggested that it would be quite difficult to construct a visible maser. The three major issues were: 1) increased pumping requirements as the wave- length decreases, 2) creating a single mode cavity (as is traditional for masers) at visible wavelengths, and 3) locating materials possessing visible transitions with a sufficiently high Objectlves quantum efficiency (see Section 2.1). In 1951, Fabrikant filed a patent entitled "A method for the amplification of elec- distinguished by the fact that o To calculate the transition wavelength between two states from the transition energy tromagnetic radiation (ultraviolet, visible, infrared and radio) means of auxiliary radiation (and to calculate the transition energy from the transition wavelength). the amplified radiation is passed through a medium which, by other means, generates excess concentrations, in comparison with the equilibrium con- o To calculate the population ratio between two states in thermal equilibrium. of . centration of atoms, other particles, or systems at upper energy states corresponding to the o To understand the differences between energy, average power, peak power' energy excited states." The pateniwas filed on June 18, 1951, but not granted until 1959.8 For a density, average power density, and peak power density (as they apply to lasers); and variety of reasons, this patent had little impact on either the Soviet Union or the international to calculate these quantities given the appropriate parameters. development of lasers. linewidth and to convert a linewidth given in frequency o To understand the concept of In 1954, Robert H. Dicke developed the idea of using a short excitation pulse to given length (meters). (Hz) to a linewidth in produce a population inversion (see Section 1.4). This inversion would then generate laser (given the o To draw a rough picture of the longitudinal mode spectmm of a an intense burst of amplified spontaneous emission.e This idea plus the idea of using frequency p.r"."terr for the laser) and to distinguish between the linewidths of each a Fabm-Perot etalon (see Section 3.2) as a resonant cavity (see Chapter 4) appear of the modes and the tinewidth of the gain curve. o To calculate the frequency spacing between longitudinal modes. 2Jeff Hecht, Laser Pioneers, revised ed. (San Diego, CA: Academic Press, 1992)' 1.1 A BRIEF HISTORY 3Jban L. Bromberg, The laser in America 1950-197A (Cambridge, MA: MIT Press, l99l). aWilliam Broad, Srar Warriors (New York: Simon and Schuster' 1985)' very difficult to say, "On this day the idea of a laser was As with most scientific fields, it is 5J. Weber, Trans. IRE Prof. Group on Electron Dev. PGED-3:l (1953)' concept lie as far back as 1940, when Valentin A- Fabrikant conceived." Roots of the laser 6N. G. Basov and A. M. Prokhorov, JETP 2'l:431 (1954). the possibility of "molecular amplification" in his doctoral thesis.' However, speculated on 7J. P. Gordon, H. J. Zeiger, and C' H. Townes, Phys. Rev. 95:282 (1954)' of the maser and laser in the late 1940s. A number of it is traditional to place the origin sV. A. Fabrikant referenced by Jeff Hecht, Laser Pioneers, revised cd. (San Diego, CA: Acadcmic Press. 1g2), pp. 12-13 as being cited in Mario Bertolotti, Masers and lasers: A Historical Approach (Bristol, England: rV. by Jeff Hecht, Laser Pioneers, reviscd ed. (San Diego, CA: Academic Press' A. Fabrikanr, refcrcnced Adam Hilger Ltd., 1983). (Bristol, England: 1992), pp. 5-6, as being cited in Mario Bertolotti, Masers and Lasers: A Historical Approach eR .H. Dicke, Phys. Rev.93:99 (1954). Adam Hilger Ltd., 1983). 2 4 Introduction to Lasers ChaP. 1 Sec. 1.2 The Laser Market 5 in his 1958 patent entitled "Molecular Amplification and Creneration Systems and and published on October l, 1960.17 Soon after, the Bell Laboratories group published an- Methods."lo othei paper reporting laser action in "red" ruby.l8 In 1957, Gordon Gould conceivetl of the idea of using a Fabry-Perot cavity as part This flurry of Physical Review Letter papers from the Bell Laboratories group caused of a laser structurc. He documented his ideas in his laboratory notebook under ttrc title much confusion as to who was responsible for the first demonstration of laser action. Many of "laser" (Light Amplification by Stimulated Emission of Radiation). He had his no0e- scientists assumed that the Bell Laboratories group had obtained laser action prior to Maiman book notarized on November 13, L957, by a candy-store clerk. Gould then attempted to at Hughes. Although Maiman published in Physical Review somewhat later,le uncertainty acquire a patent on these ideas. After a great deal.of legal fuss, he obtained four patents: remained for several Years. oni in 1977 on optically pumped laser amplifiers,ll one in 1979 on a broad range of laser The majority of early laser schemes were complex. Maiman's demonstration of laser applications,l2 on" in 1987 on electrical discharge pumped lasers,l3 and one in 1988 on action in a simple and elegant experiment dramatically altered the direction of laser develop- Brewster angle windows for lasers.la Many U.S. laser companies still pay royalties under ment. Soon after Maiman's demonstration of laser action in ruby, Sorokin and Stevenson of license agreements on these patents. IBM switched to a flashlamp pumped rod design for their uranium-doped calcium fluoride In 1958, Schawlow and Townes wrote a seminal paper entitled "Infrared and Optical laser. This laser lased on its first try in November 1960.20 A few weeks later, Sorokin Masers,"ls discussing the various aspects of constructing an optical maser. In this paper and Stevenson obtained laser action in samarium-doped calcium fluoride.2l Although doped a number of questions regarding the practicality of lasers were discussed. The required . calcium fluoride is not a commonly used laser material today, these experiments were the pumping power was calculated (10 mW for a l-cm cube) and shown to be practical, the first demonstrations of laser action in a four-state material. ; question of using a multimode cavity was discussed, and a number of schemes for mode The floodgates were now open to laser development. In rapid succession, a hodt of selection (including a long cavity Fabry-Perot) were presented. The possibility for three- new materials were found to lase. On December 12, 1960, Ali Javan, William R. Bennett, andfour-stale (see Section 2.1) solid-state lasers, linewidth (see Section 2.1), and tunability Jr., and Donald Herriott obtained laser action in a helium-neon gas mixture.22 In 1963, C, (see Chapter 11) were also briefly mentioned. Kumar N. Patel obtained laser action in carbon dioxide.23 In 1964, Joseph Geusic, H. M. The appearance of this paper caused a great deal of interest in the scientific com- Marcos, and Le Grand Van Uitert obtained laser action in Nd:YAG;24 and William Bridges munity. A number of laser programs were initiated, mostly distinguished by the choice obtained laser action in argon-ion.2s (Carbon dioxide lasers are discussed in Section 12.1, of the laser material. Peter P. Sorokin and Mirek Stevenson at IBM focused on cal- Nd:YAG lasers are discussed in Chapter 10, and argon-ion lasers in Section 9.2.) cium fluoride doped with a rare earth, Theodore Maiman at Hughes pursued ruby, and ln re-reading Schawloq and Townes' paper today, there is a clear feeling that visible AIi Javan at BelI Laboratories worked with a hefium-neon mixnrre. (Ruby as a laser ma- maser (laser) construction was expected to be very difficuh. To eYeryone's surprise, it tumed terial is discussed in Section ll.3 and helium-neon as a laser material is discussed in Sec- . out to be quite straightforward. Today, lasers have been demonstrated in solid, liquid, gas, tion 9.1.) and plasma materials at virtually every wavelength (see Figures l.l and 1.2). On May 16, 1960, Maiman and his coworkers achieved laser action in a "pink" (low chromium concentration) ruby rod. Maiman submitted his paper to Physical Review then sent a paper to Naturet6 and arranged a press Letters, where it was rejected. Maiman 1.2 THE LASER MARKET conference to discuss the development. The result was received with skepticism, because paper, ruby had been rejected as a laser material due to a low in Schawlow's Presumed Lasers are used in a number of commercial and research applications.