Development of near and mid-ir ultrashort pulse laser systems at Q-Peak Evgueni Slobodtchikov Q-Peak, Inc.
Outline Motivation In search of Ti:Sapphire of infrared Yb:doped laser crystals Mid-IR laser crystals Pump lasers Fiber-coupled diode lasers Tm:fiber lasers CPA approach On the way to a working prototype Conclusion
Titan-ML first commercial mode-locked Ti:sapphire laser (1992) GVD COMPENSATING PRISMS BRF AO-MODULATOR Ti:SAPPHIRE CRYSTAL PUMP
Motivation Recent advances: high power cw fiber-coupled diode lasers, high-power Tm:fiber lasers, development of new laser materials. Ultimate goal: development of practical femtosecond laser sources in near (~1 μm-wavelength) and mid-infrared (IR) (2-5 μm wavelength) spectral ranges with high pulse energy output (above 1 mj). List of outside-of-lab applications for fs lasers is growing rapidly. Some of them are: LIBS Industrial micromachining Medical Imaging Rapid prototyping
In search of Ti:Sapphire for the infrared 1. Ti:S is the ideal crystal for generating of high power ultrafast laser pulses. BUT it can not be diode-pumped and requires expensive green pump lasers. 2. The spectral bandwidth of Ti:S is outstanding (>200 nm) and can support pulses as short as 5 fs pulses but such pulses don t survive in the real world (dispersion, nonlinear effects). 3. Most of real world applications need sub-ps pulses with only a few nm bandwidth. BUT systems must be compact, efficient, inexpensive and reliable.
Ytterbium doped crystals A very simple structure No undesired effects Weak quantum defect Low thermal load Diode pumping around 980 nm Broad emission spectrum (100 fs pulses possible) Yb 3+ 2 F 5/2 2 F 7/2 Crystal field N 2 980 nm 1.05 μm 2 F 5/2 N 1 2 F 7/2
Yb-doped laser media for near-ir lasing Material Δt (fs) λ central (nm) σ emission (x10-20 cm 2 ) τ fluo (ms) K (W/m/K) Yb:CALGO 47 1050 0.75 0.42 6.3-6.9 Yb:phosphate glass 58 1080 0.05 1.3 0.85 Yb:YVO 4 61 (KLM) 1050 0.14 0.3 5.2-5.11 Yb:BOYS 69 1062 0.3 1.1 1.8 Yb:SYS 70 1070 0.44 0.44 1.6-2.8 Yb:KYW 71 (KLM) 1025 3 0.6 3.3 Yb:GdCOB 89 1045 0.35 2.6 2.1 Yb:KGW 100 1037 2.8 0.35 3.3 Yb:CaF 2 150 1047 0.25 2.4 9.4
Absorption and emission spectrum of Yb: CaF 2
Mid-IR laser materials Co 2+ :MgF 2 (P. Moulton et al.) Tuning range 1.6-2.3 μm Continuous-wave output only at 77 K with 2.5 W output power Pulsed at room temperature with output up to 6 W Fe 2+ :ZnSe (W. Krupke et al.) Lasing around 4.5 μm, only pulsed around 100 K but recent work shows progress (Mirov et al.) Cr 2+ -doped II-VI media (W. Krupke et al.)
Cr:ZnSe is " Ti:Sapphire of mid- IR The fractional tuning range (Δλ/λ 0 ) of Cr:ZnSe is equal to 0.49 and is comparable with that of Ti:Sapphire (Δλ/λ 0 0.57); the material has a high gain cross section Gain spectrum Ti:Sapphire Cr:Forsterite Cr:YAG Cr:ZnS Cr:ZnSe Cr:CdSe Semi-log plot, keeping Δλ/λ 0 constant 700 1000 1400 2000 3000 4000 Wavelength (nm, log scale) Access to 2.0-3.2 µm region, smooth tunability with highest efficiency at Watt-level powers opens up new possibilities: superbroadband spectrum (>1100nm) high-power (>1 W) narrow-line sources high efficiency (>60%).
Properties in comparison Cr:ZnSe Cr:ZnS Ti:Sapphire Crystal structure Cubic Cubic-uniaxial Uniaxial Thermal conductivity 18 W/m C 17 W/m C 27 W/m C (cubic) 28 W/m C Thermooptics dn/dt 70 10-6 1/ C 46 10-6 1/ C 12 10-6 1/ C Third order nonlinearity n 2 180 10-20 m 2 /W at 1.6 µm*) <180 10-20 m 2 /W 3 10-20 m 2 /W Two-photon absorption band gap 2.83 ev 3.84 ev ~8 ev Second-order nonlinearity very high: 30 pm/v <30 pm/v absent Peak emission cross-section σ em 13 10-19 cm 2 at λ 0 2450 nm 14 10-19 cm 2 2350 nm 4.5 10-19 cm 2 780 nm Fluorescence bandwidth Δλ 1000 nm (50 THz) 800 nm (43 THz) 300 nm (130 THz) Relative bandwidth Δλ/λ 0 0.49 0.34 0.57 Peak pump cross-section σ abs 11 10-19 cm 2 at λ max 1780 nm 10 10-19 cm 2 1690 nm 0.65 10-19 cm 2 500 nm Lifetime at room temp. 6 µs 4.3 µs 3 µs I sat = hν/σ em τ 11 kw/cm 2 14 kw/cm 2 210 kw/cm 2 Direct diode pumping Yes Yes No
Pumping options for Cr 2+ -doped lasers Er:fiber Tm:YALO Tm:YAG Tm:fiber InGaAsP diodes Co:MgF 2 GaSb diodes Cr:ZnS Cr:ZnSe 1200 1400 1600 1800 2000 2200 2400 2600 Wavelength, nm I. Sorokina et all., International Symposium on Lasers and Nonlinear Optical Materials) Meeting, Keystone, CO, July 2003.
State-of-the-art in Cr:ZnSe lasers Laser characteristics Output parameter Reference CW, output power, W 5.26 Slobodtchikov et al., 2008 (to be published) CW, tuning range, nm 2000-3100 Sorokina et al., 2004 CW, efficiency, % 70 Mond et al., 2001 Pulsed, output power, W 18.5 @ 10 khz Carrig et al., 2004 Pulsed, output energy, mj 14 @ 200 μs Koranda et al., 2006 Pulsed, tuning range, nm 1880-3100 Demirbas et al., 2006 SBR mode-locked 80 fs @ 80 mw Sorokina et al., 2007 KLM mode-locked ~300 fs @ 50 mw Moskalev et al., 2008
Diode lasers for end-pumping of near-ir lasers
High-power FC diode lasers (~$100s/W)
Low-power FC diode lasers (~$50/W)
Fiber laser for end-pumping of mid-ir lasers
Rare-earth laser transitions in fibers Energy (wavenumber/10000) 1550 nm 1950-2050 nm
Recent advances in Tm-doped fiber-laser efficiencies show levels approaching Yb fibers Slope Efficiency (%) 100 90 80 70 60 50 40 30 20 10 2:1 limit 0 1995 2000 2005 2010 Date
Q-Peak Tm:fiber-laser testbed power meter 2050 nm output Single-ended pump Active fiber coil clamp Dichroic mirror HR at 2050 nm HT at 790 nm clamp focusing head focusing head 793-nm pump 400-um, 0.2 NA fiber delivery Heat sink Meniscus 2.5-cm R concave surface HR at 2050 nm HT at 790 nm Pump Laser A Pump Laser B
Output power of directly diode-pumped Tm:fiber laser (2007 result) 325 300 301 W 275 250 225 Output power (W) 200 175 150 125 100 75 61.8% slope 59.1% slope LMA HI2 fiber data conduction cooled Linear fit 50 LMA HI2 fiber data water cooled 25 Linear fit 0 0 50 100 150 200 250 300 350 400 450 500 550 600 Launched pump power (W)
100W prototype of directly diode-pumped Tm:fiber laser (NUFERN) NUFERN, presented at SSDLTR2007
Pumping options for Cr:ZnSe laser Femtosecond oscillator can be pumped directly by Tm:fiber laser. Similar to Ti:S due to short upper state life time (6 μs) the amplifier has to be pumped by high energy source. Ho:YLF laser can be used as an energy storage: can be pumped by Tm:fiber laser 2.05 μm output is ideally suited for high energy pumping of Cr:ZnSe with low thermal stress. InGaAsP diodes Er:fiber Co:MgF 2 Tm:YALO Tm:YAG 1200 1400 1600 1800 2000 2200 2400 2600 Wavelength, nm Tm:fiber Ho:YLF GaSb diodes Cr:ZnS Cr:ZnSe
Tm:fiber laser pumped single-crystal Ho:YLF oscillator Tm-fiber laser PBS λ/2 Pump to AMP #1 DM OC Master Oscillator Ho:YLF AO HR DM DM Dichroic Mirror, AOM Acousto-Optic Modulator, OC Output Coupler, HR High Reflector
Ho:YLF Oscillator 30 1 khz 0.5 khz 60 25 50 Energy per pulse, mj 20 15 10 0.5 khz 1 khz 40 30 20 Pulsewidth, ns 5 10 0 0 0 10 20 30 40 50 60 70 Input power, W Q-Peak, presented at SSDLTR 2006
CPA approach for ultrashort pulse generation
Setup of typical CPA Ti:Sapphire system SHG CW Nd:doped pump laser 5 W at 532 nm SHG Nd:doped Q-switched pump laser 5 mj, 1 khz, 532 nm femtosecond KLM mode-locked Ti:S laser 50 fs, 10 nj, 800 nm 100 MHz Pulse stretcher 100 fs 100 ps Ti:S regenerative amplifier 1 mj, 1 khz, 800 nm output 800 nm, 50-100 fs, 1 mj, 1 khz Pulse compressor
Our approach Chirped Pulse Amplification (CPA) technique provides a simple, reliable, working solution. Direct diode-pumping reduces number of sub-systems shrinking the size and increasing reliability. Significant gain narrowing in Yb: doped amplifier leads to longer pulse BUT simplifies the design of pulse compressor for smaller and more rugged system. Fiber-based pump lasers provide the possibility of designing a compact system, with low sensitivity to misalignment and therefore amenable to mobility as desirable for many applications. The high peak power of generated mid-ir laser pulses will make possible highly efficient OPG conversion in 3-5 μm wavelength range.
Near-IR diode-pumped Yb:doped CPA system Diode-pumped Yb:KGW oscillator 150 fs, 2.5 nj, 1050 nm, 100 MHz Pulse stretcher 150 fs 600 ps Diode-pumped Yb:CaF2 regenerative amplifier 1.5 mj, 250 Hz, 1050 nm output 1050 nm, 200-300 fs, 1 mj, 250 Hz Pulse compressor
Mid-IR fiber laser-pumped Cr:ZnSe CPA system 1.94 μm Tm:fiber CW pump laser 5 W 60 W 2.05 μm Ho:YLF Q-switched pump laser 17 mj, 1 khz 2.5 μm Femtosecond Cr:ZnSe laser 100 fs, 10 nj, 100 MHz Pulse stretcher 100 fs 100 ps 2.5 μm Cr:ZnSe regenerative amplifier 8 mj, 1 khz Output: 2.5 μm 100-200 fs ~ 1 mj 1 khz Pulse compressor
On the way to a working prototype
1.5 W, 170 fs Yb:KGW femtosecond laser The spectrum (9 nm FWHM) and the pulse duration (170 fs) of the output of femtosecond Yb:KGW laser.
Yb:CaF2 regenerative amplifier The experimental results so far: Pulse energy 0.68 mj at repetition rate 250 Hz. Pulse duration 600 fs with 3 nm FWHM bandwidth.
3D model of the packaged diode-pumped Yb:doped system Femtosecond oscillator Stretcher-compressor Regenerative amplifier Electronics section: 2 FC pump diodes, power supply control electronics, TE coolers air-cooling
Mid-IR CPA laser system
Multi wavelength and pulse format outputs of mid-ir system 1.94 μm Tm:fiber CW pump laser 5 W 60 W 2.05 μm Ho:YLF Q-switched pump laser 17 mj, 1 khz 2.5 μm Femtosecond Cr:ZnSe laser 100 fs, 10 nj, 100 MHz Pulse stretcher 100 fs 100 ps pico 2.5 μm Cr:ZnSe regenerative amplifier 8 mj, 1 khz Output for FEMTO and PICO config OPG femto Pulse compressor Output for NANO config ZGP based OPO
Conclusions Standard diffraction grating based pulse stretching/compression technique can be used in compact, transportable laser systems. We demonstrated a feasibility to construct a compact, efficient and inexpensive near-ir amplified femtosecond high power laser system based on Yb:doped crystals. For mid-ir generation Cr:ZnSe can be used to build a Ti:sapphirelike femtosecond source. Recent progress in the development of high-power directly diodepumped Tm:doped fiber lasers make power scaling of mid-ir laser system straightforward.
Acknowledgments Program is supported by SBIR Phase II programs from AFRL (DE) and ARL