High-power, high-energy diode-pumped Tm:YLF-Ho:YLF laser Alex Dergachev, and Peter F. Moulton Q-Peak, Inc. 135 South Road, Bedford, Massachusetts 01730 Tel.: (781) 275-9535, FAX: (781) 275-9726 E-mail: dergachev@qpeak.com Acknowledgements: Lockheed Martin Laser Ultrasonic Technology Center
Outline Motivation Previous Results Tm:YLF Laser - Details Ho:YLF Laser Details ZGP OPO Conclusions
Motivation Development of a 2-um laser source: High-energy (up to 100 mj) High repetition rate (100-400 Hz) High beam quality (TEMoo) CW 1940 nm Tm:YLF laser 100 mj 2050 nm Ho:YLF laser 30 mj 3200 nm ZGP OPO 100 Hz - 400 Hz Possible applications : Pump source for other IR lasers Industrial Military
Approaches to diode-pumping of Ho-doped lasers 780-790-nm diode lasers Tm,Ho-laser 780-790-nm diode lasers Tm-laser Ho-laser 1900-nm diode lasers Ho-laser
Advantages of Tm-pumped Ho-laser Compared to diode-pumped Tm, Ho-co-doped laser: Eliminates upconversion from Tm-Ho interaction that reduces efficiency and creates additional heating in crystal Eliminates energy sharing between Tm and Ho that limits energy extraction in Q-switched mode Compared to direct-diode-pumped Ho-laser Can operate at much higher power due to the availability of highpower diodes for Tm:YLF pumping
References on resonantly pumped Ho lasers P.F. Moulton, Industry R&D related to 2-μm lidars, Second Review of 2-μm Solid State Laser Technology, NASA Headquarters, Washington, DC, May 18-19, 1992. R.C. Stoneman and L. Esterowitz, Opt. Lett. 17, 736 (1992). D.W Hart, M. Jani and N.P. Barnes, Opt. Lett. 21, 728 (1996). M. Petros, J. Yu, U. N. Singh and N.P. Barnes, High energy directly pumped Ho:YLF laser, in Advanced Solid State Lasers, OSA Technical Digest (Optical Society of America, Washington, DC, 2000), pp. 79-81. P.A. Budni, M.L. Lemons, J.R. Mosto, and E.P. Chicklis, IEEE J. Sel. Topics in Quantum Electron. 6, 629 (2000). P.A. Budni, M.L. Lemons, C.A. Miller, P.A. Ketteridge, L.A. Pomeranz, T.M. Pollak, P.G. Schuneman, K.L. Lanier, J.R. Mosto, and E.P. Chicklis, High power 1.9 micron pumped solid state holmium lasers, in Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of America, Washington, DC, 2000), p 564. L.D. DeLoach, S.A. Payne, L.L. Chase, L.K. Smith, W.L. Kway and W.F. Krupke, IEEE J. Quantum Electron. 29, 1179 (1993). W.F. Krupke and L.L. Chase, Optical and Quantum Electron. 22, S1 (1989).
Previous results Ho-lasers Tm:YLF pumped Ho:YAG P. A. Budni et al., High-power/high-brightness diode-pumped 1.9-µm Thulium and resonantly pumped 2.1-µm Holmium lasers, IEEE J. on Selected Topics in Quantum Electron., 6, 629-635 (2000). Tm:YLF pump 36 W CW output at 1.907 mm (σ-line) Multimode, M 2 ~ 2 Ho:YAG CW: 19 W QCW: 16 W at 15 khz
Ho:YLF vs Ho:YAG Why Ho:YLF? Long upper laser level lifetime ~ 15 ms Higher emission cross-section Naturally birefringent material Low dn/dt > weak thermal lensing Ho:YAG Isotropic Lifetime ( 5 I 7 ) 7 ms Strong thermal lensing Excellent thermo-mechanical properties
Gain Calculation 3.5% Tm:YLF 0.25 0.20 0.15 Gain coefficient, cm -1 0.10 0.05 0.00 1800 1825 1850 1875 1900 1925 1950 1975 2000-0.05-0.10 Inversion fraction 0.25, π 0.25, σ 0.15, π 0.15, σ -0.15-0.20-0.25 Wavelength, nm Polarized gain in Tm:YLF at two values of inversion fraction g(λ) = N [ p σ em (λ) - (1-p) σ abs (λ) ], where p inversion fraction, N - Tm-concentration
Pumping Ho:YLF with Tm:YLF laser 3.5 2.0% Ho:YLF 3 Absorption coefficient, cm-1 2.5 2 1.5 1 Ho abs - Pi Tm-tuning 0.5 0 1800 1850 1900 1950 2000 2050 2100 2150 Wavelength, nm
Experimental Set-Up Tm:YLF Laser Tm:YLF DL HR DL BRF Tm:YLF DL OC DL Tm:YLF Active Element: Rectangular slab: 22-mm long Clear aperture 2x6 mm.
Tm:YLF Dual GM Oscillator 1 pass 35 30 25 Output power, W 20 15 Slope efficiency 38% 10 5 0 0 20 40 60 80 100 120 140 160 Total diode power, W
Tm:YLF Dual GM Oscillator 3 passes 30 25 Output power, W 20 15 10 25% slope efficiency 5 0 0 20 40 60 80 100 120 140 160 180 Diode pump power, W
Calculations for Tm:YLF-pumped Ho:YLF laser at low pulse rates Crystal doping (%) 0.5 N 0 (cm -3 ) 7 x 10 19 Crystal length (cm) 3.6 Scaled pump fluence 1.6 Pump pulsewidth (msec) 15 Pump power (W) 20 average inversion fraction 0.44 η s pumping efficiency 0.52 F p (J/cm 2 ) 21.1 Pump energy (J) 0.3 Pump-beam radius (cm) 0.048 Stored energy in crystal (J) 0.16 g 0, Gain coefficient (cm -1 ) 0.36 G, Single-pass gain 3.7 Calculations are based on work by W.F. Krupke and L.L. Chase, Optical and Quantum Electron. 22, S1 (1989).
Schematic layout of the end-pumped Ho:YLF laser AOM Tm:YLF laser #2 DM OC Ho:YLF HR DM Tm:YLF laser #1 DM Dichroic Mirror, AOM Acousto-Optic Modulator, OC Output Coupler, HR High Reflector
CW Ho:YLF Laser Operation (TEM oo ) 25 20 Ho:YLF CW output, W 15 10 54% slope efficiency T oc 10% 15% 40% 70% 45% slope efficiency 5 0 0 10 20 30 40 50 60 Total Tm pump power, W
Ho:YLF Q-Switched Operation (TEMoo) 18 40 16 35 14 30 Output power, W 12 10 8 6 25 20 15 Pulse energy, mj P E 4 10 2 5 0 0 0 500 1000 1500 2000 2500 Repetition rate, Hz
Ho:YLF Pulsewidth vs repetition rate 25 20 Pulsewidth, ns 15 10 5 0 0 200 400 600 800 1000 1200 Repetition rate, Hz
ZGP OPO - Layout HR 3.2 um HT 2.05 and 5.7 um ZGP R=38% 3.2 um ZGP OPO: ZGP 1 cm-long Type I, 53 o -cut Flat/flat resonator Singly resonant cavity Pump double pass ~6 cm-long resonator OC 38% at 3.2 um HR 2.05 um HT 5.7 um
ZGP Operation 400 Hz 4.5 0.50 4 3.5 400 Hz 0.45 0.40 OPO output, W 3 2.5 2 1.5 1 Slope Efficiency 63% 0.35 0.30 0.25 0.20 0.15 0.10 Conversion efficiency Pout Eff. 0.5 0.05 0 0.00 0 1 2 3 4 5 6 7 8 9 10 Pump power, W
ZGP OPO Operation Pulse energy 14 12 Slope Efficiency: 50 Hz 60% 200 Hz 56% 400 Hz 63% OPO pulse energy, mj 10 8 6 4 50 Hz 200 Hz 400 Hz 2 0 0 5 10 15 20 25 30 Pump pulse energy, mj
Conclusions Development of an efficient Tm:YLF - Ho:YLF ZGP laser system: Ho:YLF laser: Highest (to the best of our knowledge) CW output of 21 W for 2-μm Ho:YLF laser Efficient Q-switched operation (up to 37 mj per pulse) Repetition rates in wide range from Hz to khz, particularly, in 100-400 Hz High beam quality TEMoo beam ZGP OPO Demonstrated > 10 mj (total) output at 50-400 Hz rep. rates