High power fiber lasers and amplifiers

Transcription

1 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 +49 (0) (0)

2 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

3 Solid-State Laser Concepts temperature [K] rod power dependent thermal lensing and thermal stress-induced birefringence disk slab fiber reduced thermo-optical distortions

4 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

5 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

6 Power evolution of single-mode fiber lasers Continuous-wave output power [W] 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 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)

7 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 Wellenlänge [nm] Source:

8 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 Wellenlänge [nm] Source:

9 Photonic Crystal Fibers Double clad fibers Active core Pump cladding guiding properties of the core guiding of the pump light Polymer coating

10 Index control of doped fiber cores V 2π 2 2 = a n core nclad λ 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 ~ NA ~ 0.02 Δn ~ NA ~ 0.06 significantly larger single-mode core possible

11 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, Bridge width [nm] high numerical aperture inner cladding no radiation has contact to coating material

12 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, (2005)

13 Rod-type photonic crystal fiber laser slope efficiency ~ 78% Output power [W] Launched pump power [W] 320 W continuous-wave, >10 mj ns-pulses extracted

14 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

15 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

16 High power continuous-wave fiber laser Output power [W] slope efficiency of 75 % measurement 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 kw laser output 75% slope efficiency (below 1.5 kw, above wavelength drift of pump diode)

17 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): (2007)

18 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

19 Q-switching of fiber lasers pump 976 nm > 1010 nm < 990 nm PCF end caps on Yb-doped rod-type fiber R = 0.08 output Q-switching element AOM air-cooled > 1010 nm < 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 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.

20 Q-switching of fiber lasers output 100 khz Average output power [W] up to 100 W down to 12 ns Launched pump power [W] Pulse duration [ns] output characteristics vs. rep. rate Average output power [W] mj energy Intensity [a.u.] 1,0 0,8 0,6 Δτ = 8 ns 0,4 0,2 0, Time [ns] 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, , 2007

21 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, 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, (2007) Current production costs:~300

22 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

23 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

24 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 : ps 2 Repetition rate : 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 , 2007.

25 High-energy femtosecond fiber laser - Results Autocorrelation trace Extra-cavity compression Intensity (a. u.) Intensity (a. u.) Delay time (ps) 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

26 High-energy femtosecond fiber laser - Results Optical Spectrum Extra-cavity compression Intensity (a. u.) Wavelength (nm) Intensity (a. u.) 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

27 Ultra-short pulse fiber amplification systems Higher average power Higher pulse energy Higher repetition rate Pulse energy [µj] mw 1 W 10 W 100 W repetition rate [khz]

28 Ultra-short pulse fiber amplification systems Higher average power Higher pulse energy Higher repetition rate Pulse energy [µj] mw 1 W 10 W 100 W fiber-based systems* 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)

29 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, Time delay [ps] Reduction of pulse quality

30 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

31 State of the art FCPA System Schematic Setup Yb:KGW oscillator Stretcher - Compressor - Unit Output AOM Pre-amplifier Main amplifier ISO

32 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

33 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

34 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

35 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, (2005)

36 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)

37 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

38 State of the art FCPA System Output characteristics Compressed output power [W] slope efficiency = 46% Compressed output power [W] total slope efficiency = 25% Launched pump power [W] Launched pump power [W] 100 W 200 khz -> 0.5 mj pulse energy 50 W 50 khz -> 1 mj pulse energy

39 State of the art FCPA System -20 highest power highest pulse energy Intensity, log scale [a.u.] Wavelength [nm] Intensity, log scale [a.u.] Wavelength [nm] Wavelength [nm] Wavelength [nm] ASE supression better than 30 db Extended measurement shows no sign of Stimulated Raman Scattering (1st stokes expected at 1080nm)

40 State of the art FCPA System Autocorrelation SH Intensity [a.u.] 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 mJ, 800 fs Time delay [ps] => pulse peak power ~ 1 GW!

41 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] slope efficiency 66% 710 W max. output (pump power limited) 570 W/m with no thermal degradation (passively cooled) Launched pump power [W] 1 kw, 1 MHz, 1 mj, sub-1 ps!

42 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

43 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