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Diode Laser-Pumped Solid-State Lasers

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Diode Laser-Pumped Solid-state Lasers directly?" The diode laser efficiently emits optical radiation into a Diode laser-pumped solid-state lasers are efficient, am- narrow spectral band. When the emission wavelength of the diode pact, all solid-state sources of coherent optical radiation. laser lies within the absorption band of the ion-doped solid-state Major advances in solid-state laser technology have his- laser medium, diode laser optical pumping can be very dcient with torically been preceded by advances in pumping technolo- little excess heat generation. Flash lamp pumping efficiency is gy. The helical flash lamps used to pump early ruby lasers limited by the broad spectral emission of the lamp and the less were superseded by the linear flash lamp and arc lamp efficient absorption of the lamp radiation by the solid-state laser now used to pump neodymium-doped yttrium-alumi- medium. Excess heat and power fluctuations of the lamp also num-garnet lasers. The latest advance in pumping tech- degrade the solid-state laser performance, as does the finite lamp nology is the diode laser. Diode laser-pumped neodymi- lifetime. The diode laser is essentially a continuous wave (cw) device um lasers have operated at greater than 10 percent with low energy storage capability, whereas the solid-state laser can electrical to optical efficiency in a single spatial mode and store energy in the long-lived metastable ion levels. The stored with linewidths of less than 10 kilohertz. The hi& energy can be extracted by rapid switching (Q-switching) to provide spectral power brightness of these lasers has allow:d peak power levels that are orders of magnitude greater than from the frequency extension by harmonic generation in nonlinear diode laser itself. Furthermore, the solid-state laser can collect the crystals, which has led to green and blue sources of output from several diode lasers to provide greater average power coherent radiation. Diode laser pumping has also been than is available from a single diode laser. The diode laser-pumped used with ions other than neodymium to produce wave- solid-state laser can operate at a variety of wavelengths not accessible lengths &om 946 to 2010 nanometers. In addition, Q- with diode lasers. The diode laser-pumped solid-state laser line- switched omation with kilowatt Deak Dowers and mode- width is fimdamentally orders of magnitude less than that of the locked opekon with l~-~icose;ondhse widths have been demonstrated. Progress in diode lasers and diode laser arravs ~romisesall solid-state lasers in which the GaAslGaAlAs Ell~pt~calGauss~an S~ngleax~al mode flash lam6 idreplaced by diode lasers for average power D~odelaser pump (809 nm) TEM,, output levels in excess of tens of watts and at a price that is competitive with flash lamppumped laser systems. Pow- er levels exceeding 1kilowatt appear possible within the next 5 years. Potential applications of diode laser- pumped solid-state lasers include coherent radar, global sensing fiom satellites, medical uses, micro ma^, and miniature visible sources for digital optical storage. OLID-STATE LASER DEVEIDPMENT HAS BEEN PACED BY THE improvement and discovery of pump sources. The helical Slamp, used to pump the first ruby laser, was replaced by the linear flash lamp and discharge arc lamp that are now used to pump vimally every neodymium-doped yttrium-aluminum-garnet (Nd:YAG) and neodymium glass (Nd:glass) laser system in the world. The next advance in solid-state laser technology promises to be improved pumping by means of diode lasers and diode laser arrays (1). The recent and rapid advances in the power and efficiency of diode lasers and diode laser arrays and their application to the pumping of solid-state lasers have led to a renaissance in solid-state ' laser- d&elopment (2). Advanced technology solid-state lasers pumped by diode lasers will make possible such diverse applications as coherent radar for global wind measurements, semiconductor circuit repair, and all solid-state color video projection. A often asked is, 'Why use the diode laser to pump another solid-state laser instead of using flash bps or the diode Fig. 1. (Top) Schematic of the diode laser-pumped monolithic Nd:yAG oscillator. (Bottom) Photograph of the three components that constitute the l-he author is a pmfeSSOrof ap lied ph ic- and provost and dm of rcscdat laser source: the diode laser, the gradient index-lens, and the Nd:YAG crystal Stanford University, Stanford, 943z. (5 mm long) with mirrors polished and coated on its ends. 742 SCIENCE, VOL. 239 diode laser source. The solid-state laser source also emits optical Fig. 2. Output power of the diode 2 radiation in a diffraction-limited spatial beam that is easily focused laser-pumped Nd:YAG monolithic oscillator versus input diode laser 5 mm Nd:YAG into a fiber or to a small spot. The result is that the diode laser- power. The 2.3-mW threshold and monolithic oscillator pumped solid-state laser provides spectral brightness "amplification" 25% slope efficiency are evidence z- that is essential for many applications that require a high degree of for efficient operation. - 25% slope :1 temporal and spatial coherence. eff~ciency 2.3 mW threshold Early Developments The high efficiency of the diode laser compared to a flash lamp, 1 A2 4 6 8 1 0 coupled with the fortuitous overlap of GaAs diode laser emission Diode laser power (mW) wavelength with the absorption band of the neodymium ion, has -- allowed the diode laser source to become a nearly ideal pump source Fig. 3. Energy level diagram for 4F512 -- -- - for solid-state lasers. However, it is only with recent improvements Nd:YAG show~ng the common 4F~n in diode laser power and efficiency that the replacement of the flash four-level 1064-nm transition and the quasi-three-level 946-nm tran- lamp by the diode laser and diode laser arrays has become a reality. sltlon. D~odclaser pumplng occurs 808 nm 946 nm 1064 nm In 1963, one year after the nearly simultaneous discovery of the at the 808-nm pump band. diode laser at four laboratories (3),Newman (4)recognized that the spectral overlap of the GaAs diode laser emission wavelength with the pump absorption band of the neodymium ion could lead to an 900 cm '= 7 ground efficient, compact, all solid-state laser. Keyes and Quist (5)success- f stateman fold fully tested the idea in 1964 by demonstrating GaAs diode laser pumping of uranium-doped CaF2 at 4 K. Early progress in diode laser-pumped solid-state lasers was slowed by the need for cryogen- ic cooling and by the low power levels of the diode lasers. The Diode Laser-Pumped Neodymium Lasers limited diode laser power led to the investigation of low power In 1982 Zhou et al. (15) demonstrated a diode laser-pumped threshold intrinsically doped neodymium lasers during the early miniature Nd:YAG laser with a linewidth of less than 10 kHz. The 1970s (6). It was not until 1972, nearly a decade after the low threshold and good slope efficiency, together with the potential pioneering experiment by Keyes and Quist, that Danielmeyer and for long operational life, pointed to the advantages of diode laser Ostermeyer (7)demonstrated diode laser pumping of Nd:YAG at pumping for the satellite-based global wind measurement transmit- room temperature. ter. The monolithic design, coupled with the small size of the diode laser and focusing optics, meant that the source was rugged and compact. Figure 1 shows a schematic and photograph of the components of the diode laser-pumped, 5 mm long, monolithic Recent Progress Nd:YAG oscillator. Figure 2 shows the Nd:YAG laser output power The suggestion by Conant and Reno (8)in 1974 that diode laser versus the diode la~er-~um~power for this early device.*~he25% arrays be used for pumping Nd:YAG lay dormant until 1980 when slope efficiency along with the 2.2-mW threshold power demon- greater powers and efficiencies from the heterojunction GaAlAs strated the potential for efficient operation with diode laser pump- diode lasers (9) allowed the group led by Rice at McDonnell ing. Douglas to demonstrate a diode laser array-pumped miniature The Stanford work was soon extended by Sipes (16) who Nd:YAG rod laser (10).The work at McDonnell Douglas was reported a diode laser linear array-pumped Nd:YAG laser with 8% motivated by the prospect of developing an efficient, long-lived, wall plug efficiency. Sipes took advantage of the recently introduced solid-state laser transmitter for satellite to satellite communication. commercial diode laser arrays invented and developed. at Xerox by The concept of global wind measurements from a satellite by Scifres, Burnham, and Streifer (17)and commercializcd in a joint means of coherent laser radar and the Doppler effect was proposed venture between Xerox and Spectra Physics. The early multistripe by Huffaker in 1978 (11).Atmospheric wind measurements with diode laser arrays introduced by Spectra Diode were capable of C02lasers in ground-based systems had proceeded well. However, producing in excess of 0.1 W of continuous wave output at 35% for satellite-based global wind measurements the laser transmitter electrical to optical efficiency. must meet strict weight and size criteria and provide 100 W of The possibility of diode. laser pumping of neodymium-doped average power at 10% electrical efficiency over a 4-year period of crystal hosts other than Nd:YAG was also explored. In early work operation. These requirements, which seemed impossible to laser Fan et al. (18)extended diode laser pumping to neodymium-doped technologists only a few years earlier, fit the admonition by Edwin lithium yttrium fluoride (Nd:YLF), which operates at 1047 nm.
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