Yb:YAG thin disk multi-pass amplifier 2015.11.19 Mitsuhiro Yoshida

Properties of laser medium Nd laser system for 3-2 RF-Gun Nd-doped τ～200μs, 40% ○ 4-state laser is easy to operate. ○ High power pump LD is available. ○ Large crystal is available × Pulse width is determined by SESAM. (Gaussian) LD Pump (808nm) Nd:YVO4 Nd:YAG SHG(532nm) 40% FHG(266nm) 20% 5HG(213nm) 3% 808nm 1064nm Yb-doped ○ Wide bandwidth => pulse shaping ○ Long fluorescent time => High power ○ Fiber laser oscillator => Stable ○ Small state difference × ASE × Absorption τ～900μs, 40% Yb-glass Yb:YAG Yb:BOYS SHG(520nm) 40% FHG(260nm) 20% 5HG(208nm) 3% LD Pump (941/976nm) 941/976nm 1040nm τ～3μs, 40% Ti-doped τ=200μs, 40% Pump 40% SHG(400nm) 40% THG(266nm) 20% FHG(200nm) 10% Pump (808nm) Nd:YAG SHG Ti:Sapphire 808nm 1064nm 532nm 800nm ○ Very wide bandwidth ○ High breakdown threshold × Low cross section × Short fluorescent time => Q-switched laser is required for pumping Ti:Sapphire laser system for beam monitor. TW laser is based on Ti-Sapphire

Characteristics of Yb doped laser Long fluorescent lifetime～1ms Wideband High quantum efficiency X Quasi-three level => Absorption at room temperature X Small cross section Yb Base material Stimulated emission cross section [10-20cm2] Fluorescence lifetime [ms] Thermal conductivity [W/mK] Fluorescence spectral width [nm] Fourier minium [fs] Experimental records Pulse width [fs] Average power [W] YAG 2 0.95 11 9 120 340 0.11 136 0.003 730 16 810 60 KYW 3 0.7 3.3 24 50 71 0.12 KGW 25 47 112 0.2 176 1.1 glass 0.63 - 35 33 36 0.065 GdCOB 0.35 2.7 2.1 44 27 89 0.04 BOYS 2.5 1.8 19 69 0.08 86 0.3 YVO4 1.2 61 0.054 CaCdAlO4 0.55 6.9 0.038

Temperature dependence of Yb:YAG Improvement of thermal and emission property (Thermal lens effect)　 (Excitation density) GM+He 10 W/m/K , dn/dT = 8ppm/K @ 300K 25 W/m/K , dn/dT = 3ppm/K @ 150K ↑150K 1/6 Thermal lens Same gain @ 1/3 excitation density 　　→ 　　　↓ 　150K => 1/20 thermal lens Pertier 300K 30kW/cm2 300K P/P0 = exp(g0z) ～2 150K　　　→ g = 7 [cm-1]

Yb:YAG thin disk Laser at room temperature Yb:YAG disk 10 % doped 2mm thickness Yb disk laser 30% efficiency was achieved at room temperature Yb:YAG 940nm LD (2.4 kW / module)

Yb:YAG 10% dope, α=12/cm, 5kW/cm2, 25Hz 0.5t 1t

How to generate 2-bunch Amplification time of standard regenerative amplifier (usually adopted in commertial product) is around 1 ms. Two regenerative amplifier (not good) Large regenerative amplifier (built & failed) Unstable output energy due to low gain. Difficult to compensate thermal lens. High gain fast regenerative amplifier (built & failed) Difficult to reduce the ghost pulse from first bunch due to limted extinction ratio of pockels cell. Multi-pass amplifier (current configuration) More gain is required for the balanced 2-bunch. OPCPA (future candidate)

A-1 underground existing laser Entrance Elevator PCF fiber amp Fiber amp Strecher Pulse Picker 2nd stage 6-pass amp 3rd stage 5-pass amp 4th stage 5-pass amp Shield door GR_A1へ Shutter Control SHG 1033nm ↓ 532nm Oscillator A B 2-loop multi-pass amplifier

New high gain multi-pass amplifier (10-15 pass x 2 loop) to simplify the laser LD Laser Diode 1pass 10-15pass OUTPUT INPUT ←

5-pass amplifier To obtain higher gain, => Higher pumping density Thermal lens Focused type amplifier to avoid thermal lens. LD Balanced offset lens to avoid damage. Laser Diode 5pass 4pass 3pass 2pass 1pass

Final amplifier without focusing LD Final amplifier without focusing Laser Diode Uniform pumping is required. Low gain G=1.3 => Multi-pass 5pass 4pass 3pass 2pass 1pass

Laser stability

Wavelength conversion ：Telescope :Mirror :Wave Plate :Lens Laser diagnostics (Streak camera / Beam profile) Power monitor 532nm 1033nm BBO Piezo mirror From multi-pass amplifier

トンネル内 GR_A1 BOX 内部 532nm 266nm レーザーハットより :Wave Plate :Mirror 安全系シャッター BBO 結晶 リモートで角度を調整 266nm Cylindrical Lens ミラー リモートで X軸、Y軸を調整 テレスコープ リモートでレンズ位置を調整 安全系シャッター GR_A1 レーザープロファイルモニター 波長板で反射した光をモニターしている。

UV conversion efficiency improvement 【 Conversion efficiency of fundamental wave 】 Reference [1] Nd:YAG Laser [1] Pulse width : 3.5 ns Max Energy : 400 mJ/pulse single longitudinal mode single transverse mode (top-hat) Nd:YAG 1ω 2ω Nd:YAG 4ω Nd:YAG 5ω Crystal BBO CLBO 10 Hz 250 mJ 90.3 mJ 50.2 mJ 36.0 mJ conversion efficiency (%) 36.12 20.08 14.4 100 Hz 44.9 mJ 19.8 mJ 17.96 7.92 70.71 % 44.10 % Reference [2] QE = 9.10×10-4@213nm QE = 1.54×10-4@266nm ×6 【 QE of Ir5Ce photocathode 】 Photocathode: Ir5Ce compound Laser : 5th harmonics (CLBO) 【 The optimal combination 】 [1] K.Deki , et al., “CsLiB6O10 (CLBO)を用いた193nm光源の開発”, 光技術情報誌「ライトエッジ」No.18 [2] Yap YK, et al., "High-power fourth- and fifth-harmonic generation of a Nd:YAG laser by means of a CsLiB(6)O(10).", Opt Lett. 1996 Sep 1;21(17):1348-50.

Issues on Yb based laser system Yb-fiber oscillator 1030nm oscillator is not stable. Broadband oscillator is very stable => ASE reduction is required. Yb-fiber amplifier Lack of pulse energy Lifetime and stability of PCF fiber. Yb-disk amplifier: (Regenerative amplifiers were failed) => Multi-pass amplifier for 2-bunch operation. => More gain is required for balanced 2-bunch energy. 5 Hz => Soldered cryatal => 25 Hz operation => x 2 system => 50Hz before May 2015 Reduce thermal lens effect and simplify laser system => Focused type multipass amplifier x2 + Non-focused multipass amplifier => Cryogenic Yb laser at next summer Temporal shaping Compressor and Slit Stability improvement Casing of each block. Gas filled or vacuum laser transportation to improve pointing stability. Assemble on one large optical table (new laser room). Feedback (pointing / amplitude). Increase monitor points (pointing / power / beam pattern).