1. Yb:YAG thin disk multi-pass amplifier
Mitsuhiro Yoshida

2. 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

3. 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

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

5. 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)

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

7. 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)

8. 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

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

10. 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

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

12. Laser stability

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

14. 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。

15. 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 =
QE = ×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 Sep 1;21(17):

16. 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).