HIGH-AVERAGE-POWER, DIODE-PUMPED SOLID STATE LASERS FOR ENERGY AND INDUSTRIAL APPLICATIONS. William F. KRUPKE

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1 Proceedings of the 6lh International Symposium on Advanced Nuclear Energy Research -INNOVATIVE LASER TECHNOLOGIES IN NUCLEAR ENERGY- HIGH-AVERAGE-POWER, DIODE-PUMPED SOLID STATE LASERS FOR ENERGY AND INDUSTRIAL APPLICATIONS William F. KRUPKE Laser Programs Directorate Lawrence Livermore National Laboratory University of California Livermore, CA Tel: (510) Progress at LLNL in the development high-average-power diode-pumped solid state lasers is summarized, including the development of enabling technologies.. Keywords: laser diode arrays, micro-optics, laser materials 1. INTRODUCTION Remarkable advances have been made recently in the performance of highaverage-power, diode-pumped solid state lasers (DPSSLs), including output power level, efficiency, beam-quality, and wavelength-diversity, etc. It is now anticipated that DPSSLs will play an important and increasing role in emerging industrial and energy applications. The realization of highperformance DPSSLs, in turn, has been made possible by advances in many constituent technologies: 1) high-duty-factor, low-cost high-average-power semiconductor laser diode pump arrays; 2) novel low-cost array output beam microoptic elements; 3) new and/or improved laser and nonlinear optical materials, and; 4) advanced resonator/laser cavity mode-control techniques and components. Recent progress made at the Lawrence Livermore National Laboratory in each of these technology areas is summarized in this paper, along with DPSSL performance levels attained. DPSSL work at LLNL, summarized here, has been performed primarily by the group led by Rich Solarz, that includes Geogr Albrecht, Ray Beach, Brian Comaskey, Steve Velsko, Nils Carlson, Charles Hamilton, Steve Sutton, Mark Emanuel, Jay Skidmore, Bill Benett, Barry Freitas, and Pat Reichert. Novel laser materials developments have been performed principally by Steve Payne, Laura DeLoach, and Larry Smith

2 2. SEMICONDUCTOR LASER DIODE PUMP ARRAY TECHNOLOGY The relatively recent development of high-power, high-efficiency laser diode arrays is the quintessential technological advance that launched the current renaissance in high power solid state lasers. At LLNL, we have focused on the development of a suite of prototype high-average-power, high-duty-factor DPSSLs for use in various military and civilian applications: 1) watt, near-diffraction-limited, high-repetition-rate, free-running and Q-switched neodymium doped crystals lasers for industrial-scale precisionmanufacturing applications, and as a source for soft x-ray lithography, 2) watt, high-repetition-rate, Q-switched, harmonically-doubled and tripled crystal lasers for tactical military applications, as dye laser pump sources for advanced laser isotope separation applications, and for large scale precision manufacturing applications, 3) 100-watt, direct-diode-pumped tunable crystal lasers for remote-sensing applications, and in the longer term, 4) 105 to >106 watt-class crystal lasers for (military) theater missile defense and (civilian) inertial confinement fusion power reactor applications. These lasers generally require 2-D diode pump arrays producing up to -,500 watts of average power per cm2, comprised of linear bar arrays emitting up to ~100 watts of average power per cm, with duty-factors ranging from 25% to 100% (CW). For the mentioned DPSSLs to be economically viable, we estimate that a pump array OEM manufacturing unit cost below a few dollars per average watt will be necessary. We conclude that this is a feasible goal, provided: 1) the basic manufactured unit produces about 100 watts of average power with adequately long operating life (typically >10,000 hours or >10l0 shots); and 2) a low-cost, highly-reliable, mass-manufacturable cooler is developed that is amenable to 2-D stacking at a high packing density (~500 watts average/cm2). To implement this low-cost, high-duty-factor pump array strategy, we adopted and significantly further developed the silicon microchannel cooler approach first reported by Tuckerman and Pease [1]. Fabrication of LLNL's thin (~800 microns) silicon-based microchannel coolers (see Figure 1) draws on lithographic processing techniques developed for large-scale integrated circuit manufacture. The design of cooler packages, and the achieved performance of LLNL array packages have been reported [2-5]. Array packages (1.8 cm of diode arrays) made with circa 1992 processing fixtures and techniques are operated at 40 watts of average power with projected lifetimes >10,000 hours. A summary of our manufacturing methods has been reported [5,6] along with associated manufacturing unit costs. Using our 1992 fully vertically-integrated processing technology and know-how, we were able to produce 40 watt average power arrays at a unit cost of ~3$/average watt (were an annual production rate sustained at -140 kw average per year). The newer (1993) higher performance 140 watt average power array packages are produced at a unit cost of ~l$/average watt (were an annual production rate sustained at -420 kw average per year). More recent array packages using improved laser

3 material produced at LLNL are now being operated at 100 watts CW from a one cm long bar array [5], (see Figure 2). 3. MICRO-OPTIC LENS TECHNOLOGY A second technology, crucial to the implementation of that new class of diode pumped solid state lasers possible only using unusually high pump fluxes (such as the so-called ground-state-depleted lasers [7] or various end-pumped lasers [8]), is that of low-cost micro-optical elements that are suitable for collimating the output beam from a linear bar array. Our approach to solving this problem is the "shaped fiber lens" [9]. Here (see Figure 3) a glass fiber preform is machined to the required (calculated) shape and pulled into a fiber (cylindrical) microlens with (typically) a 100 microns transverse dimension. The initial shape is maintained during the pulling process, the surface of the drawn lens is "fire-polished" to optical smoothness, and initial machining errors are reduced in magnitude by a factor equal to the ratio of initial to final size. These f/1 spherical-aberation-free micro-lenses are then attached to individual laser bar arrays (see Figure 4 [5]), collimating the "fast-axis" divergence angle (typically 60 degrees) of a laser bar to better than 10 milliradians. Linear bar arrays fitted with these cylindrical microlenses may be further integrated into 2-D array stacks (see Figure 5.) whose high average output power may be coupled to a fiber using a conventional lens, or be focused onto a small spot (laser rod end) using an optical "lens duct" (see Figure 6 [8]). Using the latter approach, pump fluxes in excess of 75 kw/cm 2 have been achieved while end-pumping a tunable Cr:LiSAF rod laser [5, 10]. 4. HIGH AVERAGE POWER DPSSL DEVICES These high average power array technologies have been utilized to design, fabricate, and demonstrate a number of high-performance laser devices: 1. A kilowatt average power Nd:YAG laser (free-running repetitively pulsed 2500 Hz) [11,12] 2. A 250 watt average power, electro-optically Q-switched diodepumped Nd:YAG power oscillator [13] 3. A 100 watt average power, frequency-doubled Nd:YAG laser [13,14] 4. A 24 watt average power frequency-doubled Nd:YOS laser [15] -155

4 5. NOVEL DPSSL LASER MATERIALS To further exploit the unique high spectral brightness of 2-D laser diode pump arrays, we have performed guided searches for novel solid state lasers materials whose spectro-kinetic, optical, thermal, and mechanical properties are more favorable to the design and construction of lasers suitable for emerging applications. For example, the family of tunable chromium-doped colqunrite laser materials (e.g. Cr:LiSAF [16]) may be diode pumped directly [17,18] using ubiquitous AlGaAs laser diodes or now emerging InGaAlP "red" diodes. Such compact efficient tunable lasers may be used to perform a variety of applicationre (medicine, remote-sensing, etc.). For a second example the family of fluorapatites (FAP) [19] doped with selected rare earth ions appear to be superior laser gain materials for such diverse applications: 1. Gain medium (Yb:FAP [19,20]) in a DPSSL driver for an inertial fusion energy power reactor [21]; See Figure Gain medium (Nd:S-VAP [22]) in a minilaser for use in fiber optic telecommunications sytems, and in optical data storage systems. Figure 1. Advanced Modular Diode Laser MicroChannel Heatsink [4,5]. -156

5 ; >? ^ * -,. * J > * - '" \l - ; :^v$ \«^>- -'\\\*~{r- ^>^& ^M * \? ^ W.*^ * '^^'IVv 50.0 ' '*, \«- s \s\> ^ ^j'};^*, \ *»^^S** *%^V- S^* ^'V'VX* ^ " ^ jv-v ««*SvJ* %% -i % i ^ ^ " i, * - " ' i ; ; i ;... : J...;,,f.. / r j. ;. / ;tf r-, yi I / : If'! j TJ! f J\ U* \ 1! f ; J '" a i i O.0 L. fc t / ; " LOO j^l^u^^jnanitilyfcswwjwdhjyw >»»**& ***<*&' H'C.i J Figure 2. Output Power.From a One Centimeter Long Diode Bar Units = cm f I (a) (b Figure 3. a) Ellipical Immersion Lens Preform Shape and Dimensions; b) Electron Microscope Photograph of a Pulled Shaped Fiber Lens (100 micron lens thickness)

6 Diode Laser ^en s Figure 4. Cross-sectional View of an LLNL Diode Array with a Collimating Cylindrical Shaped-Fiber Microlens Attached Figure 5. 2-D Stack of Microchannel-Cooled Linear Bar Arrays with Attached Shaped-Fiber Microlenses

7 .Diode assembly, Lens duct.laser rod. Potartxtr "^.Q-SYritCh Output coupler Figure 6. Optical Schematic of an End-Pumped DPSSL, Using a 2-D, Silicon MicroChannel Cooled Diode Pump Array, Fitted with Shaped Fiber Microlenses, and Coupled by a Lens Duct. Qas-eooled Gas-cooled gatnrn»dlum slabs /poekola coll (p~v Polorlzot Pulso Injoctlon Qo3-o0olod harmonic conversion Dlehrolc f^~~> mirror _ ' Output pulso 3<AOUtput beam Figure 7. Multi-pass Regenerative Amplifier Architecture for an Yb:FAP DPSSL Driver for an Inertial Fusion Energy Power Reactor [19-21]

8 REFERENCES 1) D. Tuckerman and R. Pease, "Heat Transfer Micros tructures for Integrated Circuits", IEEE Electron Device Lett. ED-2,126 (1981). 2) D. Mundinger, R. Beach, W. Benett, R. Solarz, W. Krupke, R. Staver, and D. Tuckerman, "Demonstration of High-Performance Silicon MicroChannel Heat Exchangers for Laser Diode Array Cooling", Appl. Phys. Lett. 53,1030 (1988). 3) W. J. Benett, B. Freitas, R. Beach, D. Ciarlo, V. Sperry, B. Comaskey, M. Emanuel, R. Solarz, and D. Mundinger, "MicroChannel Cooled Heatsinks for High Average Power Laser Diode Arrays", Proceedings SPIE, Laser Diode Technology and Applications. 1634,453 (1992). 4) R. J. Beach, W. Benett, B. Freitas, D. Mundinger, B. Comaskey, R. Solarz, and M. Emanuel, "Modular MicroChannel Cooled Heatsinks for High Average Power Laser diode Arrays", IEEE J. Quantum Electronics 28,966 (1992). 5) R. J. Beach, M. A. Emanuel, W. J. Benett, B. L. Freitas, D. Ciarlo, N. W. Carlson, S. B. Sutton, J. A. Skidmore, and R. W. Solarz, "Improved Performance of High Average Power Semiconductor Arrays for Applications in Diode Pumped Solid State Lasers', SPIE OE/LASE 1994, Laser Diode Technology and Applications Conference. 6) W. F. Krupke, R. W. Solarz, R. J. Beach, M. A, Emanuel, and W. J. Benett, "The Economics of High-Duty-Factor, High-Average-Power Diode-Pumped Solid State Lasers (DPSSLs)", Conference Proceedings, LEOS 6th Annual Meeting, San Jose, CA, November (1993). 7) W. F. Krupke and L. L. Chase, "Ground State Depleted Solid State Lasers: Principles, Characteristics, and Scaling Laws", Optics and Quantum Electronics, 22, SI (1990). 8) R. Beach, P. Reichert, W. Benett, B. Freitas, S. Mitchell, S. Velsko, J. Davin, and R Solarz, "Scalable Diode-End-Pumping Technology Applied to a 100 mj Q-s witched Nd:YLF Laser Oscillator", Optics Letters, 18,1326 (1993). 9) J. J. Snyder, P. Reichert, and T. Baer, "Fast Diffraction-Limited Cylindrical Microlenses", Appl. Opt., 30, 2743 (1991) 10) R. J. Beach, LLNL, private communication, December

9 11) B. J. Comaskey, G. Albrecht, R. Beach, S. Sutton, M. Emanuel, C. Petty, K. Jancaitis, W. Benett, B. Freitas, and R. Solarz, "A One Kilowatt Average Power Diode Pumped Nd:YAG Folded Zig-Zag Slab Laser", Diode Pumping of Average Power Solid State Lasers, Proceedings SPIE 1865,9 (1993). 12) B. Comaskey, R. Beach, G. Albrecht, W. Benett, B. Freitas, C. Petty, D. VanLue, D. Mundinger, and R. Solarz, High Average Power Diode Pumped Slab Laser", IEEE J. Quantum Electronics, 28, 992 (1992). 13) S. Velsko, C. Ebbers, B. Comaskey, G. Albrecht, and Scott Mitchell, "250 Watt Average Power Electro-Optically Q-Switched Power Oscillator", Technical Digest, Advance Solid State Lasers Conference, Salt Lake City, Feb ) S. Velsko, C. Ebbers, B. Comaskey, G. Albrecht, and S. Mitchell, "100 Watt Average Power at 0.53 Microns by External Frequency Conversion of an Electro-Optically Q-Switch,ed Diode Pumped Power Oscillator", Applied Phys. Letters, to be published. 15) B. Comaskey, G. Albrecht, S. Velsko, and B. Moran, "24 Watts Average Power at 0.53 Microns from an Externally Frequency Doubled Q- Switched Diode Pumped Nd:YOS Laser Oscillator", IEEE Journal of Quantum. Electronics, to be published. 16) S. A. Payne, L. L. Chase, H. W. Newkirk, L. K. Smith, and W. F. Krupke, "LiCaAlF6:Cr 3+ : A Promising New Solid State Laser Material", J. Quantum Electronics, 24, 2243 (1988). 17) S. A. Payne, W. F. Krupke, L. K. Smith, W. L. Kway, L. D. DeLoach, and J. B. Tassano, "752 nm Wing Pumped CnLiSAF Laser", J. Quantum Electronics, 28,1188 (1992). 18) R. Scheps, J. F. Myers, H. Serreze, A. Rosenberg, R. C. Morris, and M. Long, "Diode Pumped CnLiSAF Laser", Opt. Letters, 16, 820 (1991) 19) L. Deloach, S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway., and W. F. Krupke, "Evaluation of Absorption and Emission Properties ob Yb 3+ Doped Crystals for Laser Applications, IEEE J. Quantum Electronics, 29, 1179 (1993) 20) S. A. Payne, L. K. Smith, L. D. Deloach, W. L. Kway, J. B. Tassano, and W. F. Krupke, "Laser Optical and Thermomechanical Properties of Yb- Doped Fluorapatite", IEEE J. Quantum Electronics, to be published

10 21) C. D. Orth, S. A. Payne, and W. F. Krupke, :Diode Pumped Solid State Laser Driver for Inertial Fusion Energy Power Plants", ICF Quarterly Report, Vol 3, Number 4, July-September, 1993, Lawrence Livermore National Laboratory (UCRL-LR ) S. A. Payne, B. H. Chai, W. L. Kway, L. D. DeLoach, L. K. Smith, G. Lutts, R. Peale, X. X. Zhang, G. D. Wilke, and W. F. Krupke, "New High Cross Section Laser Crystal: Neodymium Doped Strontium Fluorovanadate", Post Deadline Paper PD-12, Conference on Lasers and Electro-Optics (CLEO), Baltimore, MD, May