Solid laser system

Solid laser system
2015.02.19
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
Best for RF-Gun
SHG(400nm) 40%
THG(266nm) 20%
FHG(200nm) 10%
Ti:Sapphire laser system for beam monitor.
Flash pumped
LD pumped
Absorption
CPA
η～0.5% Nd:YAG
Ti-Sapphire
Ti-Sapphire
Oscillator
Superconitnuum
broadning
Yb-Fiber
Frontend
Fluorescence
Laser schemes
940nm LD
OPCPA
1030nm
Yb:BOYS, Yb:CaF2
- Broadband
Oscillator
Pump
Amplifier
Nd:YAG
Yb:YAG
Ti:Sapphire
Wavelength
1064nm
1030nm
660-1100nm
Fluorescent time
230μs
960μs
3.2μs
Spectral width
0.67nm
9.5nm
440nm
Fourier minimum
Pulse width
2.48ps
165fs
2.59fs
807.5nm
941nm
488nm
1.5nm
21nm
200nm
76%
91%
55%
Wavelength
Spectral width
Quantum efficiency
Many commertial product.
- How to maintain continuously?
- How to generate 2-bunch ?
Yb:YAG Thin Disk
η～40%
Material
- Very high gain
- Critical incident angle
- Fiber laser is stable in principle.
- High efficiency
(long fluorecense lifetime)
- Low gain at room temperature
=> Lower temperature
Nd based solid laser
(3-2 DAW RF-Gun)
Nd based laser system
• Nd:YVO4 oscillator + Nd:YAG multi-pass amplifier
30 ps (10 mm)
Nd based laser system (renewed)
発振器
増幅部
増幅部
波長変換部
Yb solid laser
(A-1 RF-Gun)
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)
Large regenerative amplifier for 2-bunch operation
100ns (2-bunch)+20ns (Pockels cell speed) = 36m
=> round trip + polarization => resonator length > 9m : 2.25m×3 + 0.75m×4
Input
R=3m (f=1.5m)
λ/4
R=1.5m
f 4.5
R=3m
f9
R=1.5m
f 4.5
λ/4
λ/4
f=200mm
Output
f=75mm
2.25m
f=300mm
f=100mm
f 1.5
A1ハット内概要図
シャッター
発振器オシロ
Ch1(黄)に該当する。
出
入
口
制御
ラック
増幅器オシロ
Ch2(緑)に該当する。
ファイバー
アンプ
マルチパスアンプ
1段目
パルスピック
発振器
A
2段目
マルチパス
アンプ
ファイバー
アンプ
ストレッチャー
発振器
B
GR_A1へ
遮蔽扉
ファイバー
プリアンプ
3段目
マルチパス
アンプ
波長変換
1033nm
↓
532nm
4,5段目
マルチパス
アンプ
エ
レ
ベ
ー
タ
増幅器オシロ
Ch3(橙)に該当する。
出入口
Original multi-pass amplifier (5-pass)
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
New high gain multi-pass amplifier (10-15 pass x 2 loop) to simplify the laser
Laser Diode
OUTPUT
INPUT
10-15pass
1pass
←
Final amplifier
Laser Diode
Uniform pumping
is required.
Low gain G=1.3
=> Multi-pass
5pass
4pass
3pass
2pass
1pass
Main Yb:YAG Amplifier
Focused type multi-pass amplifier < 1mJ
- High gain
- Focused at crystal leads to avoid thermal lens effect.
UV conversion (BBO SHG+FHG)
=> 1 mJ maximum @ 258 nm
Typical charge distribution
Current situation:
- Instability
=>
- No spatial shaping
- No compressor
Non-Focused type amplifier > 10 mJ
- Low gain
- Uniform pumping is required.
Laser instability is caused by:
- ASE of fiber amplifier.
- Pointing fluctuation from fiber amplifier.
- Stability of pump laser
(Upgrade of charger is required)
- Separated optical table between fiber and solid
laser.
Wavelength conversion
：Telescope
:Mirror
:Wave Plate
:Lens
Laser diagnostics (Streak camera / Beam profile)
Power monitor
532nm
1033nm
BBO
Piezo mirror
From multi-pass amplifier
Inside Gun laser case
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.
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-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).

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