High power fiber lasers and amplifiers Jens Limpert Friedrich-Schiller University Jena, Institute of Applied Physics, Jena, Germany and Fraunhofer Institut of Applied Optics and Precission Engeniering, Jena, Germany Jens Limpert Jens.Limpert@uni-jena.de +49 (0) 36 41. 947 811 +49 (0) 36 41. 947 802
Outline of the talk High power fiber lasers and amplifiers Properties of Fiber Lasers Advanced Fiber Designs Selected Experiments of High Power Fiber Lasers Conclusion and Outlook
Solid-State Laser Concepts temperature [K] rod 347 -- 352 342 -- 347 337 -- 342 333 -- 337 328 -- 333 324 -- 328 319 -- 324 315 -- 319 310 -- 315 288 power dependent thermal lensing and thermal stress-induced birefringence disk slab fiber reduced thermo-optical distortions
Double-clad fiber laser active core cladding mirror mirror pump light active core inner cladding (pump core) laser radiation refractive index profile laser radiation outer cladding n
Properties of Rare-Earth-Doped Fibers active core cladding mirror mirror pump light no or reduced free space propagation immune against thermo-optical problems excellent beam quality high gain efficient, diode-pumped operation laser radiation
Power evolution of single-mode fiber lasers Continuous-wave output power [W] 3500 3000 2500 2000 1500 1000 500 5 W 9,2 W 30 W 110 W 485 W 1360 W 800 W 170 W 150 W 3000 W 2500 W 2000 W 135 W 1530 W 1300 W 600 W 200 W 0 1992 1994 1996 1998 2000 2002 2004 2006 Year V. Fomin et al, 3 kw Yb fibre lasers with a single-mode output, International Symposium on High Power Fiber Lasers and their applications (2006)
Performance-limiting effects End-facet damage Thermal effects Available pump power Non-linear effects Laser Stokes-Signal Intensität [w.e.] Ausgangsleistung 100 W 110 W 120 W 1050 1075 1100 1125 1150 1175 1200 Wellenlänge [nm] Source: www.specialtyphotonics.com
Performance-limiting effects End-facet damage Thermal effects Available pump power Non-linear effects Laser Stokes-Signal Intensität [w.e.] Ausgangsleistung 100 W 110 W SRS 120 W P threshold = const. A g R L eff eff 1050 1075 1100 1125 1150 1175 1200 Wellenlänge [nm] Source: www.specialtyphotonics.com
Photonic Crystal Fibers Double clad fibers Active core Pump cladding guiding properties of the core guiding of the pump light Polymer coating
Index control of doped fiber cores V 2π 2 2 = a n core nclad λ 2.405 n n n core n core n eff-01 n eff clad n eff-01 n Clad 0 Microstructured Fiber r 0 a Step-index Fiber r Δn ~ 1 10-4 NA ~ 0.02 Δn ~ 1 10-3 NA ~ 0.06 significantly larger single-mode core possible
The air-cladding region NA 1,0 0,8 0,6 0,4 0,2 350 nm 380 nm 400 nm 25 µm 380 nm Yb-doped core Pump core area Coating 0,0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Bridge width [nm] high numerical aperture inner cladding no radiation has contact to coating material
rod-type photonic crystal fiber pump core (200 µm) active core (80 µm) air-clad 8 µm core 1.5 mm outer cladding rod-type fiber: 30 db/m Pumplichtabsorption, 71 µm Modenfelddurchmesser, M 2 ~ 1.2 Limpert et. al., "High-power rod-type photonic crystal fiber laser," Opt. Express 13, 1055-1058 (2005)
Rod-type photonic crystal fiber laser 350 300 slope efficiency ~ 78% Output power [W] 250 200 150 100 50 0 0 50 100 150 200 250 300 350 400 450 Launched pump power [W] 320 W continuous-wave, >10 mj ns-pulses extracted
Rare-earth doped photonic crystal fibers Design freedom to tailor optical, mechanical and thermo-optical properties Advantages of PCF higher index control larger SM cores shorter fibers possible (0.5 m) comparable heat dissipation intrinsically polarizing without drawbacks
Fiber laser systems all fiber laser CW, Q-switched low fs source power light source ultra-short fiber oscillators Pump light coupling, e.g. tapered fiber bundles single pass amplifiers fiber integrated mirror, e.g. fiber bragg grating (R=1) Pump Properties of Fiber Lasers active medium saturable absorber Advanced Fiber Designs Fiber Fiber Fiber advanced fiber amplifiers dispersion compensation (R<1) Selected Experiments of High Power Fiber Lasers Conclusion and Outlook
High power continuous-wave fiber laser Output power [W] 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500 250 0 slope efficiency of 75 % measurement 0 500 1000 1500 2000 2500 3000 3500 4000 launched pump power [W] fiber length 15 m, forced air-cooling two side pump coupling of 2 kw fiber temperature max. 120 C beam quality M² < 1.4 at 2 kw max. 2.53 kw laser output 75% slope efficiency (below 1.5 kw, above wavelength drift of pump diode)
Scaling approach: Incoherent Combining Δλ ΔΘ = Λ cos ( Θ Litt ) transmission grating Λ= 800 nm (in near and far field) 153 W Combining-efficiency 95 % Degree of Polarization 98% Polarizing PCF, 1.5 m, 40 µm core Scalable while maintaining beam quality S. Klingebiel,F. Röser,B. Ortac, J. Limpert, A. Tünnermann, Spectral beam combining of Yb-doped fiber lasers with high efficiency, JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS 24 (8): 1716-1720 (2007)
Combining of pulsed fiber lasers two beams (ΔΤ ~ 10ns): M²x =1.17 M²y =1.10 M²x =1.22 M²y =1.13 Intensity [a.u] Time [50 ns/div] M²x =1.17 M²y =1.18 SPIRICON TM M 2 measurement scaling of MW peak power pulsed fiber sources beyond self-focusing limit of 4 MW
Q-switching of fiber lasers pump diode @ 976 nm HR @ > 1010 nm HT @ < 990 nm PCF end caps on Yb-doped rod-type fiber R = 0.08 output Q-switching element AOM air-cooled HR @ > 1010 nm HT @ < 990 nm Fiber parameter: outer diameter: up to 2 mm 60 µm Yb-doped core, A eff ~ 2000 µm 2, 180 µm (NA ~ 0.6) inner cladding 30 db/m pump absorption @ 976 nm, (0.5 m absorption length) Microscope image of the rod-type photonic crystal fiber and close-up to the inner cladding and core region.
Q-switching of fiber lasers output characteristics @ 100 khz Average output power [W] 100 75 50 up to 100 W down to 12 ns 25 0 0 50 100 150 Launched pump power [W] 120 100 80 60 40 20 0 Pulse duration [ns] output characteristics vs. rep. rate Average output power [W] 10 8 6 4 pulse @ 2.5 mj energy Intensity [a.u.] 1,0 0,8 0,6 Δτ = 8 ns 0,4 0,2 0,0-20 -10 0 10 20 30 40 50 Time [ns] 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 20 40 60 80 100 Repetition rate [khz] Pulse energy [mj] O. Schmidt, J. Rothhardt, F. Röser, S. Linke, T. Schreiber, K. Rademaker, J. Limpert, S. Ermeneux, P. Yvernault, F. Salin, A. Tünnermann, Millijoule pulse energy Q-switched short-length fiber laser, Optics letters Vol.32, No.11, 1551-1553, 2007
Quasi-monolithic, passively Q-switched microchip laser f=30mm f=20mm a) b) ( c -Unmatched simplicity -No moving parts in the resonator -Simple gluing technique AR@808nm, HR@1064nm 808nm, 0.5W 100µm, 0,22NA 1µJ, 50ps, 40kHz 0.5µJ, 110ps, 170kHz a) 0.2mm, 3% doped Nd:YVO4, AR@808nm, T=10% @ 1064nm b) Spin on glass glue, c) SESAM, ΔR=20%, TR= 320ps, -Spin on glass glue *high dielectric strength *high transparency 3mm D. Nodop, J. Limpert, R. Hohmuth, W. Richter, M. Guina, and A. Tünnermann, "High-pulse-energy passively Q-switched quasi-monolithic microchip lasers operating in the sub-100-ps pulse regime," Opt. Lett. 32, 2115-2117 (2007) Current production costs:~300
Fiber based amplification of ps-µchip lasers low cost micro-chip laser Isolator 35 µm, 1.5 m Pump diode amplified signal 60 ps, 0.75 µj, 50 khz 60 ps, 80 µj, 50 khz peak power: 1.33 MW micromachining
Ultra-short pulse generation (R=1) Pump DC (R<1) AM SA ultra-short pulsed laser active medium (AM) -e.g. Yb-doped fiber Theory: dissipative, nonlinear system: saturable absorber (SA) -favours pulse against noise background -initiates mode-locking dispersion compensation (DC) - keeps the pulse short during roundtrip gain loss Dispersion Nonlinearity
High-energy femtosecond fiber laser dispersion compensation free HR Pump diode 975 nm L1 M1 L2 Yb-doped rod-type fiber L3 M2 λ 4 Optical Isolator Output HR HR HR SAM L4 Modulation depth 30% Fast relaxation time 200 fs Slow relaxation time 500 fs Total cavity dispersion : + 0.012 ps 2 Repetition rate : 10.18 MHz B. Ortaç, O. Schmidt, T. Schreiber, J. Limpert, A. Tünnermann, A. Hideur, High-energy femtosecond Yb-doped dispersion compensation free fiber laser, Optics Express, vol. 15, pp. 10725 10732, 2007.
High-energy femtosecond fiber laser - Results Autocorrelation trace Extra-cavity compression 1.0 1.0 Intensity (a. u.) 0.8 0.6 0.4 0.2 Intensity (a. u.) 0.8 0.6 0.4 0.2 0.0-20 -10 0 10 20 Delay time (ps) 0.0-6 -4-2 0 2 4 6 Delay time (ps) Output pulse duration = 4 ps Compressed pulse duration = 400 fs Single pulse characterization: Average output power: 2.7 W Compression efficiency: 75 % Energy per pulse: 265 nj Energy per pulse: 200 nj Peak power: 500 kw
High-energy femtosecond fiber laser - Results Optical Spectrum Extra-cavity compression 10 0 1.0 Intensity (a. u.) 10-1 10-2 10-3 1010 1020 1030 1040 1050 Wavelength (nm) Intensity (a. u.) 0.8 0.6 0.4 0.2 0.0-6 -4-2 0 2 4 6 Delay time (ps) Spectral bandwidth = 8.4 nm Compressed pulse duration = 400 fs Single pulse characterization: Average output power: 2.7 W Compression efficiency: 75 % Energy per pulse: 265 nj Energy per pulse: 200 nj Peak power: 500 kw
Ultra-short pulse fiber amplification systems Higher average power Higher pulse energy Higher repetition rate 10000 Pulse energy [µj] 1000 100 10 100 mw 1 W 10 W 100 W 1 10 100 1000 repetition rate [khz]
Ultra-short pulse fiber amplification systems Higher average power Higher pulse energy Higher repetition rate 10000 Pulse energy [µj] 1000 100 10 100 mw 1 W 10 W 100 W fiber-based systems* 1 10 100 1000 repetition rate [khz] * Röser et. al., Millijoule pulse energy high repetition rate femtosecond fiber chirped-pulse amplification system, Opt. Lett. 32, 3495 (2007) * Röser et. al., 90 W average power 100 μj energy femtosecond fiber chirped-pulse amplification system, Opt. Lett. 32, 2230 (2007)
Influence of self-phase modulation (SPM) Nonlinear phase: Accumulated nonlinear phase (B-integral): 2 ( z, T ) γ A( z, T ) z φ = SPM NL 2 π B = λ Simulated autocorrelation traces L 0 n 2 I ( z) dz SHG intensity [a.u.] 1,0 0,8 0,6 0,4 0,2 B = 0 B = 0.6 B = 8 B = 16 0,0-4 -2 0 2 4 Time delay [ps] Reduction of pulse quality
Chirped Pulse Amplification (CPA) D. Strickland and G. Mourou, Compression of amplified optical pulses, Opt. Comm. 56, 3, 219 (1985). mode-locked laser grating stretcher amplifier grating compressor reduced pulse peak power during amplification reduced nonlinearity enhanced damage threshold
State of the art FCPA System Schematic Setup Yb:KGW oscillator Stretcher - Compressor - Unit Output AOM Pre-amplifier Main amplifier ISO
State of the art FCPA System Schematic Setup Yb:KGW oscillator 9.7 MHz, 1.6 W, 400 fs, 1030 nm Stretcher - Compressor - AOM Pre-amplifier Unit ISO Output Main amplifier
State of the art FCPA System Schematic Setup Yb:KGW oscillator 9.7 MHz, 1.6 W, 400 fs, 1030 nm Stretcher - Compressor - AOM Pre-amplifier Output Unit 1740 l/mm dielectric gratings Main amplifier ISO
State of the art FCPA System Schematic Setup Yb:KGW oscillator 9.7 MHz, 1.6 W, 400 fs, 1030 nm Stretcher - Compressor - AOM Pre -amplifier 40 µm core PCF, 1.2 m Output Unit 1740 l/mm dielectric gratings Main amplifier 80 µm core PCF, 1.2 m ISO
State of the art FCPA System Rod-type photonic crystal fiber pump core (200 µm) active core (80 µm) air-clad 8 µm core 1.5 mm outer cladding High pump light absorption (30dB/m) -> short fiber length + large mode area ( >100x of standard fiber) -> ultralow Nonlinearity Limpert et. al., "High-power rod-type photonic crystal fiber laser," Opt. Express 13, 1055-1058 (2005)
State of the art FCPA System 200/80 Rod-type PCF, 1.2m length Near field image Beam quality-measurement MFD = 71 µm -> MFA ~ 4000 µm 2 M 2 x = 1.17, M2 y = 1.26 (Spiricon, 4σ method)
State of the art FCPA System Schematic Setup Yb:KGW oscillator 9.7 MHz, 1.6 W, 400 fs, 1030 nm Stretcher - Compressor - AOM Pre -amplifier 40 µm core PCF, 1.2 m Output Unit 1740 l/mm dielectric gratings Main amplifier 80 µm core PCF, 1.2 m ISO multilayer dielectric reflection gratings average power scalable 70% compressor throughput efficiency
State of the art FCPA System Output characteristics 100 50 Compressed output power [W] 80 60 40 20 slope efficiency = 46% 0 0 50 100 150 200 250 Compressed output power [W] 40 30 20 10 total slope efficiency = 25% 0 0 50 100 150 200 Launched pump power [W] Launched pump power [W] 100 W compressed @ 200 khz -> 0.5 mj pulse energy 50 W compressed @ 50 khz -> 1 mj pulse energy
State of the art FCPA System -20 Spectrum @ highest power Spectrum @ highest pulse energy -20-30 Intensity, log scale [a.u.] -30-40 -50-60 -70-40 -60-80 1000 1020 1040 1060 1080 Wavelength [nm] Intensity, log scale [a.u.] -30-40 -50-60 -70-40 -50-60 -70-80 1000 1020 1040 1060 1080 Wavelength [nm] -80 1020 1030 1040 1050 Wavelength [nm] -80 1020 1030 1040 1050 Wavelength [nm] ASE supression better than 30 db Extended measurement shows no sign of Stimulated Raman Scattering (1st stokes expected at 1080nm)
State of the art FCPA System Autocorrelation SH Intensity [a.u.] 1.0 0.8 0.6 0.4 Δτ AC = 1.23 ps Δτ AC = 1.2 ps Δτ AC = 0.93 ps 50 khz / 1 mj (B-Integral 7) 200 khz / 500 µj (B-Integral 4.7) At low pulse energy 0.2 0.0-10 -5 0 5 10 1mJ, 800 fs Time delay [ps] => pulse peak power ~ 1 GW!
Average power scalability of main amplifier Schematic Setup cw cavity HR R=10% 80 µm core Rod-type PCF diode laser DM 1.2 m Length DM diode laser Output power [W] 800 700 600 500 400 300 200 slope efficiency 66% 710 W max. output (pump power limited) 570 W/m with no thermal degradation (passively cooled) 100 0 0 200 400 600 800 1000 1200 Launched pump power [W] 1 kw, 1 MHz, 1 mj, sub-1 ps!
Laser trepanning with high average power on copper Copper (Cu 99.9%) Thickness: 0.5 mm Rep.rate.: 975 khz Pulse Energy : 70 μj Focal length: 80mm Fluence: ~ 2.32 J/cm 2 Number of rounds: 50 trepanning radius rotating speed breakthrough time 75 µm 106 rounds/s 75 ms
Conclusion and outlook Success story is based on RE-doped fibers are a power scalable solid-state laser concept! significant progress in fiber manufacturing technology recent developments of reliable high power all solid-state pump sources RE-doped fibers are good for Outstanding performance in a variety of operation regimes continuous-wave: >10 kw diffraction-limited ultra-short pulse: >1 kw, 1 MHz, 1 mj, sub-1 ps