Yb:YAG thin disk multi-pass amplifier

2015.11.19Mitsuhiro Yoshida

Properties of laser medium

LD Pump(808nm)

Nd:YVO4Nd:YAG

SHG(532nm) 40%FHG(266nm) 20%5HG(213nm) 3%

τ～ 200μs, 40%Nd-doped

Yb-doped

Ti-doped

LD Pump(941/976nm)

Yb-glassYb:YAGYb:BOYS

1064nm808nm

941/976nm 1040nm

τ～ 900μs, 40%

Pump(808nm)

Nd:YAG SHG40%

1064nm808nmTi:Sapphire

532nm 800nm

τ～ 3μs, 40%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)

○ Wide bandwidth => pulse shaping○ Long fluorescent time => High power○ Fiber laser oscillator => Stable○ Small state difference × ASE× Absorption

○ Very wide bandwidth○ High breakdown threshold× Low cross section× Short fluorescent time => Q-switched laser is required for pumping

Pump

TW laser is based on Ti-Sapphire

SHG(520nm) 40%FHG(260nm) 20%5HG(208nm) 3%

SHG(400nm) 40%THG(266nm) 20%FHG(200nm) 10%

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

Fourier minimumPulse width

2.48ps 165fs 2.59fs

Wavelength 807.5nm 941nm 488nm

Spectral width 1.5nm 21nm 200nm

Quantum efficiency 76% 91% 55%

Fluo

resc

ence

Abs

orpt

ion

Nd laser system for 3-2 RF-Gun

Ti:Sapphire laser system for beam monitor.

Characteristics of Yb doped laser• Long fluorescent lifetime ～ 1ms• Wideband• High quantum efficiencyX Quasi-three level

=> Absorption at room temperatureX Small cross section

YbBase material

Stimulatedemission

cross section

[10-20cm2]

Fluorescence

lifetime[ms]

Thermalconductivity

[W/mK]

Fluorescencespectral width[nm]

Fourierminium

[fs]

Experimental recordsPulse 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 3 0.7 3.3 25 47112 0.2

176 1.1

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 1969 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)

300K30kW/cm2

Pertier

GM+He

300K P/P0 = exp(g0z) ～ 2150K 　　　→ g = 7 [cm-1]

10 W/m/K , dn/dT = 8ppm/K @ 300K25 W/m/K , dn/dT = 3ppm/K @ 150K ↑150K 1/6 Thermal lens

Same gain @ 1/3 excitation density 　　→　　　↓

　 150K => 1/20 thermal lens

Yb:YAG thin disk Laser at room temperature

0

50

100

150

200

250

300

350

0 200 400 600 800 1000 1200 1400

Epump (mJ )Eou

t (m

J)

30% efficiency was achieved at room temperature Yb:YAG

Yb disk laser

940nm LD (2.4 kW / module)

Yb:YAG disk10 % doped 2mm thickness

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

Entrance

ElevatorPCF

fiber amp

Fiber amp

Strecher

Puls

e Pi

cker

2nd stage6-pass amp

3rd stage5-pass amp

4th stage5-pass amp

Shield doorGR_A1へ

Shutter

Control

Fiber amp

SHG1033nm

↓532nm

OscillatorA

OscillatorB

2-loop multi-passamplifier

New high gain multi-pass amplifier (10-15 pass x 2 loop) to simplify the laser

LD Laser Diode

INPUT

OUTPUT

1p

ass 1

0-1

5p

ass

←

1pass

LD

Laser Diode

2pass3pass

4pass5pass

5-pass amplifier

Balanced offsetlens to avoiddamage.

To obtain higher gain,=> Higher pumping density Þ Thermal lens Þ Focused type amplifier to

avoid thermal lens.

Final amplifier without focusing LD

Laser Diode

1pass

2pass

3pass

4pass

5pass

Uniform pumpingis required.

Low gain G=1.3=> Multi-pass

Laser stability

:Mirror

:Lens

From multi-pass amplifier

1033nm532nm

BBO

:Wave Plate

：TelescopeWavelength conversion

Power monitor

Piezo mirror

Laser diagnostics (Streak camera / Beam profile)

:Mirror:Wave Plateトンネル内 GR_A1 BOX 内部

レーザーハットより

532nm

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

　 Nd:YAG1ω

Nd:YAG 2ω

Nd:YAG 4ω

Nd:YAG 5ω

Crystal 　 BBO CLBO CLBO10 Hz 250 mJ 90.3 mJ 50.2 mJ 36.0 mJ

conversion efficiency (%) 　 36.12 20.08 14.4

100 Hz 250 mJ 90.3 mJ 44.9 mJ 19.8 mJconversion

efficiency (%) 　 36.12 17.96 7.92

Reference [1]Nd:YAG Laser [1]

Pulse width : 3.5 nsMax Energy : 400 mJ/pulse single longitudinal mode single transverse mode (top-hat)

70.71 %

44.10 %

[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.

Reference [2]

QE = 9.10×10-4@213nm

QE = 1.54×10-4@266nm×6

【 QE of Ir5Ce photocathode 】

Photocathode: Ir5Ce compoundLaser : 5th harmonics (CLBO)

【 Conversion efficiency of fundamental wave 】

【 The optimal combination 】

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).