1. CryoYb:YAG-1 DJR 12/6/2015 MIT Lincoln Laboratory Cryogenically cooled solid-state lasers: Recent developments and future prospects * T. Y. Fan, D. J. Ripin, J. D. Hybl, J. T. Gopinath, A. K. Goyal, D. A. Rand, S. J. Augst, and J. R. Ochoa MIT Lincoln Laboratory * This work is sponsored by the Missile Defense Agency’s Airborne Laser Directorate, DARPA, and HEL-JTO under Air Force contract number FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors, and are not necessarily endorsed by the United States Government.

2. MIT Lincoln Laboratory CryoYb:YAG-2 DJR 12/6/2015 Outline Cryogenic laser background The case for power scalability and high efficiency in Yb lasers Laser demonstration results Summary

3. MIT Lincoln Laboratory CryoYb:YAG-3 DJR 12/6/2015 Motivation Goal: Many laser applications require: –High average power –Near-diffraction-limited beam quality –Low weight and volume –Low cost Challenge # 1: Average power and beam quality of solid-state lasers is generally limited by thermo-optic effects –Thermo-optic distortion –Thermally induced birefringence Challenge # 2: Cost, size, and weight of solid-state laser systems are generally limited by low efficiency –Lower efficiency systems require more pump lasers, larger power supplies, and larger cooling systems Cryogenic solid-state lasers can effectively address these challenges

4. MIT Lincoln Laboratory CryoYb:YAG-4 DJR 12/6/2015 Approaches to Generate High- Brightness from Solid-State Lasers Optimize gain-element geometry for low thermo-optic distortion –Thin-disk, slab lasers Compensate for thermo-optic distortion outside of gain element –Deformable mirror driven by feedback loop –Phase-conjugate mirror to reverse phase distortions Guide beam to maintain beam quality while spreading heat –Fiber, waveguide lasers Combine multiple lower-power lasers –Coherent or wavelength beam combining Ceramic materials to scale size, provide spatially varying properties Cryogenic cooling is complementary to many other solid-state-laser power-scaling approaches

5. MIT Lincoln Laboratory CryoYb:YAG-5 DJR 12/6/2015 Outline Cryogenic laser background The case for power scalability and high efficiency in Yb lasers Laser demonstration results Summary

6. MIT Lincoln Laboratory CryoYb:YAG-6 DJR 12/6/2015 Materials Properties Values of thermo-optic properties of dielectric crystals substantially improve at lower temperatures for higher-power laser operation –Higher thermal conductivity and diffusivity (scales like 1/T) –Generally smaller coefficient of thermal expansion (CTE) (goes to 0 at T = 0) –Generally smaller dn/dT dn/dT is affected by CTE and bandgap changes with temperature Cryogenic materials properties are needed in order to perform modeling and simulation and assess power scalability but only limited properties data exists below 300 K

7. MIT Lincoln Laboratory CryoYb:YAG-7 DJR 12/6/2015 Distortion (OPD)Depolarization  h  fractional thermal load   thermal conductivity  thermal expansion dn/dT  change in refractive index with temperature FOM d =  / [  h  dn/dT  ]FOM b =  /  h  Thermo-Optics Improve with Cooling Larger material FOM’s give less OPD and less stress-induced birefringence Key material properties ( , , dn/dT) scale favorably at lower temperature in bulk single crystals Properties of Undoped YAG Thermal Conductivity (W/m K) CTE (ppm/K), dn/dT (ppm/K) Temperature (K) 100 K300 K  (in W/mK) 4711 dn/dT(ppm/K) 0.97.9  (ppm/K) 2.06.2 Relative FOM d (300-K Nd:YAG = 1) 87 (Yb:YAG) 1 (Nd:YAG) Relative FOM b (300-K Nd:YAG = 1) 31 (Yb:YAG) 1 (Nd:YAG) Un-doped YAG Figures of Merit

8. MIT Lincoln Laboratory CryoYb:YAG-8 DJR 12/6/2015 Thermo-Optic Properties of Host Crystals Thermo-optic properties of single-crystal laser hosts generally improve at cryogenic temperatures Improvement in thermal conductivity is present but reduced for high-doping levels Thermal ConductivityYb:YAG Thermal Conductivity Undoped Hosts Aggarwal et al, JAP (2005) Fan et al, JSTQE (2007)

9. MIT Lincoln Laboratory CryoYb:YAG-9 DJR 12/6/2015 Efficiency Improves at Cryogenic Temperatures Cryo-cooling allows efficient use of gain media –Yb:YAG has high intrinsic efficiency (quantum defect ~ 9%) –Yb:YAG is four-level system at low temperatures Broad absorption band maintained at low temperature –Efficient diode pumping possible –Reliable temperature-tune-free operation Yb:YAG Absorption Spectrum Absorption Coefficient (cm –1 ) 900 Wavelength (nm) 0 2 4 6 8 10 920940960980100010201040 Laser Wavelength 77 K 300 K Pump Array Energy Levels in Yb:YAG Laser: 1030 nm Pump: 940 nm Energy 3k B T @ 300K, 9k B T @ 100K

10. MIT Lincoln Laboratory CryoYb:YAG-10 DJR 12/6/2015 Thermal Sources for Yb:YAG Lasers Typical measured heat load is 0.3 W dissipated per W output –9% of absorbed pump power dissipated in Yb:YAG by quantum defect –Additional contribution to cold-tip thermal load from trapped fluorescence Modest amounts of liquid nitrogen are required –A 10-kW laser (3000 W of heat) will consume 1 LPM of L N 2 Fluorescence Laser Output Quantum Defect Unabsorbed Pump Untrapped Trapped Pump Photons Cooled Yb:YAG Absorbed Pump

11. MIT Lincoln Laboratory CryoYb:YAG-11 DJR 12/6/2015 Outline Cryogenic laser background The case for power scalability and high efficiency in Yb lasers Laser demonstration results Summary

12. MIT Lincoln Laboratory CryoYb:YAG-12 DJR 12/6/2015 Typical Laser Breadboard Layout Pump Lasers Polarizers Yb:YAG cryogenically cooled in LN 2 cryostat Efficient end-pumping with high-brightness diode pump lasers Yb:YAG crystal mounted to copper for heat-sinking Laser Output Beam Profile Output Coupler LN 2 Dewar Yb:YAG Crystal

13. MIT Lincoln Laboratory CryoYb:YAG-13 DJR 12/6/2015 494-W CW Power Oscillator 494-W CW power 71% optical-optical efficiency M 2 ~ 1.4 at 455 W OC reflectivity = 25%, L = 1 m, Near-flat-flat resonator Limited by available pump power Near-Field Profile at 275 W (CW) Laser Output Output Coupler LN 2 Dewar Yb:YAG Crystals Fiber- Coupled Pump Laser High Reflector Dichroic Mirror Polarizers Fan et al, JSTQE (2007)

14. MIT Lincoln Laboratory CryoYb:YAG-14 DJR 12/6/2015 255-W (CW) Single-Pass Amplifier 255-W (CW) generated by amplifying 110-W (CW) in a single- pass amplifier M 2 ~ 1.1 measured from amplifier 54% optical-optical efficiency of single-pass amplifier Beam size ~ 0.9-mm radius 255-W (CW) Average Power Near-Field Beam Profile M 2 ~ 1.1 Amplifier Performance Thin-Film Polarizers /4 waveplate 150-W Diode Modules 110-W (CW) Power Oscillator Dewar and Crystal (Identical to Oscillator) Polarization Isolator Ripin et al, IEEE JQE (2005)

15. MIT Lincoln Laboratory CryoYb:YAG-15 DJR 12/6/2015 High-Average-Power Short-Pulse Laser Joint MIT Campus-Lincoln effort demonstrated 287-W ps-class laser Hong et al, Optics Letters (2008)

16. MIT Lincoln Laboratory CryoYb:YAG-16 DJR 12/6/2015 Ultrafast Cryo-Yb Lasers Relatively simple and inexpensive to generate high average power Hosts available for picosecond and femtosecond operation Key attributes are –Large bandwidth at cryogenic temperature –Favorable thermo-optics Examples of possible gain media: –Yb:YAG – ps-class –Yb:YLF (LiYF 4 ) – <100-fs class –Yb:YSO (Y 2 SiO 5 ) - <50-fs class

17. MIT Lincoln Laboratory CryoYb:YAG-17 DJR 12/6/2015 Candidates for Ultrashort Pulse Lasers Laser Parameter Nd:Glass 300 K Ti:Al 2 O 3 300 K Nd:YAG 300 K Yb:YAG 100 K Yb:YLF 100 K Yb:YSO 100 K Thermal Conductivity (W/mK) ~ 130113920 Thermal Expansion (ppm/K) ~ 1056.223 dn/dT (ppm/K) ~ 3117.90.9-1.8 (n e ) Quantum-Limited Thermal Load per Unit Output Power 0.18 ( p = 870 nm) 0.52 ( p = 532 nm) 0.32 ( p = 808nm) 0.11 ( p = 940 nm) 0.09 ( p = 940 nm) Nominal Gain Bandwidth (nm) 203000.51.517>50 I sat (kW/cm 2 ) at laser 162402.61.25.714 Expected Efficiency

18. MIT Lincoln Laboratory CryoYb:YAG-18 DJR 12/6/2015 Cryogenic Yb:YLF Provides Path to High-Power Short-Pulse Lasers Direct diode-pumping for simplicity and ease of use Thermo-optic effects scale favorably at cryogenic temperatures 4-level laser with small quantum defect for high efficiency Yb:YLF Gain Spectrum YLF Properties ~17 nm FWHM Thermal Conductivity (W/m K) dn e /dT (ppm/K) 0 10 20 30 40 0 -2 -4 -6 -8 Temperature (K) 100150200250300 Data from Aggarwal et al. (2005) Fan et al. (2007)

19. MIT Lincoln Laboratory CryoYb:YAG-19 DJR 12/6/2015 >200-W Yb:YLF Laser High-power cw Yb:YLF laser shows the potential for power scaling fs sources Pump at 960-nm, output at 995 nm with 44% R output coupler M 2 of 1.1 at 60 W, M 2 of 2.6 at 180 W –Multi-transverse mode operation at higher power Absorption Spectrum 960-nm pump 400-µm fiber Yb:YLFLN 2 Dewar Output Coupler R = 44% Dichroic Focusing Optics 20 cm Laser Schematic Output Power at 995 nm 68% slope Pump Feature Zapata et al. (2010)

20. MIT Lincoln Laboratory CryoYb:YAG-20 DJR 12/6/2015 Summary Cryogenically cooled Yb:YAG lasers enable high- average-power with excellent beam quality –High efficiency and low thermo-optic distortion Laser designs relatively simple and inexpensive Further power scaling –Increase pump power –Combine cryogenic cooling with orthogonal power-scaling approaches