Development of near and mid-ir ultrashort pulse laser systems at Q-Peak. Evgueni Slobodtchikov Q-Peak, Inc.

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1 Development of near and mid-ir ultrashort pulse laser systems at Q-Peak Evgueni Slobodtchikov Q-Peak, Inc.

2 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

3 Titan-ML first commercial mode-locked Ti:sapphire laser (1992) GVD COMPENSATING PRISMS BRF AO-MODULATOR Ti:SAPPHIRE CRYSTAL PUMP

4 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

5 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.

6 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 nm 1.05 μm 2 F 5/2 N 1 2 F 7/2

7 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 Yb:phosphate glass Yb:YVO 4 61 (KLM) Yb:BOYS Yb:SYS Yb:KYW 71 (KLM) Yb:GdCOB Yb:KGW Yb:CaF

8 Absorption and emission spectrum of Yb: CaF 2

9 Mid-IR laser materials Co 2+ :MgF 2 (P. Moulton et al.) Tuning range 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.)

10 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 ); 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 Wavelength (nm, log scale) Access to 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%).

11 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 / C / C / C Third order nonlinearity n m 2 /W at 1.6 µm*) < m 2 /W 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 cm 2 at λ nm cm nm cm nm Fluorescence bandwidth Δλ 1000 nm (50 THz) 800 nm (43 THz) 300 nm (130 THz) Relative bandwidth Δλ/λ Peak pump cross-section σ abs cm 2 at λ max 1780 nm cm nm cm 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 kw/cm 2 Direct diode pumping Yes Yes No

12 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 Wavelength, nm I. Sorokina et all., International Symposium on Lasers and Nonlinear Optical Materials) Meeting, Keystone, CO, July 2003.

13 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 Sorokina et al., 2004 CW, efficiency, % 70 Mond et al., 2001 Pulsed, output power, W 10 khz Carrig et al., 2004 Pulsed, output energy, mj 200 μs Koranda et al., 2006 Pulsed, tuning range, nm Demirbas et al., 2006 SBR mode-locked mw Sorokina et al., 2007 KLM mode-locked ~ mw Moskalev et al., 2008

14 Diode lasers for end-pumping of near-ir lasers

15 High-power FC diode lasers (~$100s/W)

16 Low-power FC diode lasers (~$50/W)

17 Fiber laser for end-pumping of mid-ir lasers

18 Rare-earth laser transitions in fibers Energy (wavenumber/10000) 1550 nm nm

19 Recent advances in Tm-doped fiber-laser efficiencies show levels approaching Yb fibers Slope Efficiency (%) :1 limit Date

20 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

21 Output power of directly diode-pumped Tm:fiber laser (2007 result) W Output power (W) % slope 59.1% slope LMA HI2 fiber data conduction cooled Linear fit 50 LMA HI2 fiber data water cooled 25 Linear fit Launched pump power (W)

22 100W prototype of directly diode-pumped Tm:fiber laser (NUFERN) NUFERN, presented at SSDLTR2007

23 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 Wavelength, nm Tm:fiber Ho:YLF GaSb diodes Cr:ZnS Cr:ZnSe

24 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

25 Ho:YLF Oscillator 30 1 khz 0.5 khz Energy per pulse, mj khz 1 khz Pulsewidth, ns Input power, W Q-Peak, presented at SSDLTR 2006

26 CPA approach for ultrashort pulse generation

27 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, fs, 1 mj, 1 khz Pulse compressor

28 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.

29 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, fs, 1 mj, 250 Hz Pulse compressor

30 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 fs ~ 1 mj 1 khz Pulse compressor

31 On the way to a working prototype

32 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.

33 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.

34 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

35 Mid-IR CPA laser system

36 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

37 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.

38 Acknowledgments Program is supported by SBIR Phase II programs from AFRL (DE) and ARL