Download Solid laser system

Yb:YAG thin disk
multi-pass amplifier
2015.11.19
Mitsuhiro Yoshida
Properties of laser medium
Nd-doped
Nd laser system for 3-2 RF-Gun
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
SHG(532nm) 40%
FHG(266nm) 20%
5HG(213nm) 3%
Nd:YVO4
Nd:YAG
(808nm)
808nm
1064nm
Yb-doped
○ Wide bandwidth => pulse shaping
τ～900μs,
○ Long fluorescent time => High power
Yb-glass
○ Fiber laser oscillator => Stable
LD Pump
○ Small state difference
Yb:YAG
(941/976nm)
× ASE
Yb:BOYS
941/976nm
× Absorption
Ti-doped
Pump τ=200μs, 40%
Pump
(808nm)
40%
SHG(520nm) 40%
FHG(260nm) 20%
5HG(208nm) 3%
1040nm
τ～3μs, 40%
40%
Absorption
Fluorescence
Nd:YAG
SHG
Ti:Sapphire
808nm
1064nm
800nm
532nm
○ Very wide bandwidth
○ High breakdown threshold
TW laser is based on Ti-Sapphire
× Low cross section
× Short fluorescent time => Q-switched laser is required for pumping
Material
Nd:YAG
Yb:YAG
Ti:Sapphire
Wavelength
1064nm
1030nm
660-1100nm
Fluorescent time
230ms
960ms
3.2ms
Spectral width
0.67nm
9.5nm
440nm
2.48ps
165fs
2.59fs
807.5nm
941nm
488nm
1.5nm
21nm
200nm
76%
91%
55%
Fourier minimum
Pulse width
Wavelength
Spectral width
Quantum efficiency
SHG(400nm) 40%
THG(266nm) 20%
FHG(200nm) 10%
Ti:Sapphire laser system for beam monitor.
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
Stimulated
Fluoresce
Fluorescence
emission
Thermal
Yb
nce
spectral
conductivity
cross
width
Base material
lifetime [W/mK]
section
[nm]
[ms]
[10-20cm2]
YAG
2
0.95
11
9
Fourier
minium
[fs]
120
Experimental records
Pulse
Average
width
power
[fs]
[W]
340
0.11
136
0.003
730
16
810
60
71
0.12
112
0.2
176
1.1
KYW
3
0.7
3.3
24
50
KGW
3
0.7
3.3
25
47
glass
0.63
2
-
35
33
36
0.065
GdCOB
0.35
2.7
2.1
44
27
89
0.04
BOYS
0.2
2.5
1.8
60
19
69
0.08
86
0.3
YVO4
-
1.2
-
-
-
61
0.054
CaCdAlO4
0.55
-
6.9
-
-
47
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
300K
150K
Pertier
300K
30kW/cm2
P/P0 = exp(g0z) ～2
→ g = 7 [cm-1]
Yb disk laser
350
30% efficiency was achieved
at room temperature Yb:YAG
300
250
Eout (mJ)
Yb:YAG disk
10 % doped
2mm thickness
Yb:YAG thin disk Laser
at room temperature
200
150
100
50
0
0
200
400
600
800
Epump (mJ)
940nm LD (2.4 kW / module)
1000
1200
1400
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
Shutter
Entrance
Control
PCF
fiber amp
2-loop multi-pass
amplifier
Pulse Picker
Oscillator
A
2nd stage
6-pass amp
Fiber amp
Elevator
Strecher
Oscillator
B
3rd stage
5-pass amp
Fiber amp
SHG
1033nm
↓
532nm
4th stage
5-pass amp
Entrance
GR_A1へ
Shield door
New high gain multi-pass amplifier (10-15 pass x 2 loop) to simplify the laser
Laser Diode
OUTPUT
INPUT
10-15pass
1pass
←
5-pass amplifier
To obtain higher gain,
=> Higher pumping density
 Thermal lens
 Focused type amplifier to
avoid thermal lens.
Balanced offset
lens to avoid
damage.
5pass
Laser Diode
4pass
3pass
2pass
1pass
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
:Wave Plate
:Mirror
レーザーハットより
安全系シャッター
532nm
Cylindrical Lens
テレスコープ
リモートでレンズ位置を調整
ミラー
リモートで X軸、Y軸を調整
266nm
BBO 結晶
リモートで角度を調整
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
Reference [1]
Nd:YAG Laser [1]
Pulse width : 3.5 ns
Max Energy : 400 mJ/pulse
single longitudinal mode
single transverse mode (top-hat)
Reference [2]
【 Conversion efficiency of fundamental wave 】
Nd:YAG Nd:YAG Nd:YAG Nd:YAG
1ω
2ω
4ω
5ω
BBO CLBO CLBO
250 mJ 90.3 mJ 50.2 mJ 36.0 mJ
Crystal
10 Hz
70.71 %
conversion
36.12 20.08
14.4
efficiency (%)
100 Hz
250 mJ 90.3 mJ 44.9 mJ 19.8 mJ
44.10 %
conversion
36.12 17.96
7.92
efficiency (%)
【 QE of Ir5Ce photocathode 】
QE = 1.54×10-4@266nm
QE = 9.10×10-4@213nm
×6
【 The optimal combination 】
Photocathode: Ir5Ce compound
Laser : 5th harmonics (CLBO)
[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.
• Yb-fiber oscillator
Issues on Yb based laser system
– 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).