The 2nd International Advisory Committee
for the RIKEN-RAL Muon Facility
Nov 5, 2008
Nishina Hall, RIKEN Nishina Center, Wako, JAPANHigh Intensity Laser
for Ultraslow Muon Production
Satoshi Wada
RIKEN,ASI, Ohmori Microfabrication Lab.

Background
One of the key technology breakthroughs needed to the precise measurement of thethe anomalous magnetic moment of a muon is the substantial improvement of theLyman alpha laser pulse energy (@122 nm) by at least two orders of magnitude.
Since the presently available laser ionized muons at RIKEN-RAL has 1 µJ/pulse,
We require a new laser system with>100 µJ pulse energy at 122 nm
VUV laser technology:
There is no established method for generating high-power VUV light,
because no suitable laser material or nonlinear crystal is available.
However, the progress of laser technology has been remarkable. In this work, we
challenge high-intensity coherent Lyman-α generation based onlaser technology developed in RIKEN.

VUV :vacuum ultraviolet (wavelength less than 200 nm)

Objectives
Proposal for high-intensity Lyman-α coherent light (122 nm)
generation for ultraslow muon production.
We newly introduce a hybrid laser system with a laser
diode, solid-state laser and fiber laser.
The required specifications are as follows;
Wavelength:

122 nm(122nm)Pulse energy :100 µJ/pulse(<1µJ/pulse)

Spectral width:

80 GHz(800GHz)

Repetition rate:

50 Hz(25Hz)

Core technologies I
New laser crystal and crystal growth methods
Conventional technique(Czochralski method)
Problems:
Evaporation of materials
Mixing of crucible materials
It causes poor uniformity of
doping ion concentration in thecrystal

Floating zone (FZ) method
No crucible is required.
High O2 pressure atmosphereis realized.
Homogeneous crystals are obtained.

Seed crystal
Melting zone(Floating zone)

Halogen lamp
(Infrared source)
Feed rodGrown crystal< Image of furnace in the FZ method >A new laser crystal: Nd:GdVO4
Nd:YAG

Nd: 0.5%

A-axis
Nd: 1.0%

Nd:YVO4

Nd:GdVO4a-axisc-axisa-axisc-axisThermalconductivity
Emissioncross-section
Lifetime ofupper state

Nd: 2.0%

CrystalgrowthHighlydoping
Wavelength

1064
1064

1064
1064106３1063

・Nd:GdVO4 shows good optical and thermal characteristics for 1µm laser.
・The fundamental radiation of 1062.75 nm can be efficiently amplified .
・The Fifth harmonics of 1062.75 nm is 212.55 nm.
・The wavelength matches to 2-photon resonance of Kr.gas.

Core technologies II
Solid-state Laser and Fiber Laser Engineering

Pratical Laser fabrication systemwith ultrashort-pulse Yb laser
Japan Laser Focus World
2008.6

Laser guide star at
Subaru telescope on
top of Mt. Mauna Kea
in Hawaii

Fiber laser technology

Breakthrough of VUV LaserLaser material science
FZ method, new laser crystals
Nonlinear crystal

Integration

Laser engineering
Solid-state lasers, fiber lasers
Nonlinear optics, hybrid lasers

High intensity Lyman-α laser

Breakthrough of muon sciencePractical Design
2-photon resonance 4wave mixing in Kr gas
KrωLy-α = 2 ω1 - ω22πc/ω1 = 212.55 nm,2πc/ω2 = 815 ~ 850 nm
2πc/ωLy-α= 122.21 ~ 121.46 nm (Ly-α)

Ly-α generation in Kr

Efficiency in small-signal region: ηKr 4p55p

ω1212.55 nm

ω1212.55 nm
Kr 4p6ω2815~ 850 nm

100 µJ@122 nmωLy-α121.5~ 122.2 nm
Lyman αη∝ χ3P12P2 ・e(-Δk)
P1: Power @ ω1
P2: Power @ ω2

Estimated pump energy based on
the previous lasersystem.
Pump energy: P1 = 100ｍJ、 P2 = 100ｍ
JKey points :increase of pump energy andsatisfaction of phase matching condition.

Schematic Diagram
Pump laser 1：2-photon resonance at 212.55 nm
DFB-LD

LD pump Nd:GdVO4
Multiamp

0.1 mJ
Fiber
amp system

5 HG

1J@1062.55 nm

100 mJ@212.55 nm

KrωLy-α = 2 ω1 - ω2

Kr 4p55pω1212.55 nm

Pump laser 2： tunable from 815-850 nm
DFB-LD

100 mJ@815-850 nm

LD pump Cr:LiSAF
Multistage-amp. system

ω1212.55 nm
Kr 4p6100 µJ
ω2@122 nm
815~ 850 nm

ωLy-α
121.5~ 122.2 nm
Lyman αPump Laser 1
DFB Diode Laser
All-Solid-State MultiAmplifier 1J

Voltage Ramp
Fiber Laser MOPA System
DFB
DiodeLaser

SeedPulse

0.1 mJ

Nd:GdVO4
Yb:YAG
10mJ
AmplifierAmplifierCurrent Driver 1062.75 nm
Telescope
λ/2Nonlinear Frequency Conversion

212.55 nm (5ω)
100 mJ

BeamSplitter

CLBOω

CLBO2ω + 2ω 2ω

ω + 4ω 4ωωω2ωLBO

ω+ωLens

Pump Laser 2
DFB Laser System

Cr:LiSAF Regenerative Amplifier

Voltage Current
DFBRamp Driver Diode Laser

Trigger Pulse

Pockels
Cell
λ/4

LD

λ/2

Thin FilmPolarizer (TFP)

100 mJ@815-850 nm

Faraday Isolator

TFPLD
Cr: LiSrAlF6 (Cr:LiSAF)

Additional
Cr:LiSAF MultiAmplifier
Examination
Single-pass
Double-pass
Regenerative amplifierImprove of efficiency
LD pump solid-state amp.
Electric powerLD

Nd:GdVO45HG

212.55 nm

Typical eff.

50%

40%

10%

2.5%

212.55 nm generation at RIKEN-RAL
Electric power

Lamp

Nd:YAG

2HG

532 nmTypical eff.

10%

20%

50%

1%

532 nmOPO

4HG

212.55 nm

20%

5%

0.01%

Problems
Damage of optics
Damage of optics by high intensity UV pump laserand coherent VUV radiation.
Improvement of optics or,Introduction of gas jet without window
Satisfaction of phase matching condition in nonlinear process.
High Intensity pump laser brakes phase matching condition.
because of variation of index by changing population.

Optimization of focusing geometry by simulation and experiment

Research schedule200820092010

10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9Laser 1Harmonicgeneration
Laser 2VUV generationin Kr cell

Ship to RAL

Summary
We propose a new Lyman-α laser with wavelength 122 nm
based on a hybrid laser system.
Our goal is to realize substantial improvement of pulse
energy at 122 nm by at least two orders of magnitude.
Detailed specifications are as follows;
Pulse energy
Repetition rateLine width
Wavelength
Pulse width: >100 µJ: 50 Hz: <80 GHz: 122 nm: 1-2 ns第1励起レーザー
第1励起レーザー：2光子励起用212.55 nmレーザー
DFB半導体レーザー

LD励起Ndバナデートレーザー
第5高調波発生
マルチアンプシステム

100 mJ@212.55 nm
ファイバー
MOPAシステム
分布帰還型(DFB)レーザー

ファイバー増幅器系

全固体マルチアンプ系

非線形周波数変換

温度制御による波長選択

MOPA(Master Oscillator
Power Amplifier)システム

Nd:GdVO4系+Yb:YAG系

第2高調波発生(SHG: 2 ω)
第4高調波発生(4th HG: 4 ω)
第5高調波発生(5th HG: 5 ω)

λ = 1062.75 nm (ω)
印加電流の調整によって繰り返し速度
パルス幅の制御が可能
パルス幅: 1 ns
スペクトル幅: 2 GHz

活性イオン:
直線偏光
自己位相変調の回避
出力エネルギー= 0.1 mJ まで増幅

前段
Nd:GdVO4アンプ系
利得の中心: 1062.9 nm
利得バンド幅: 2 nm
出力エネルギー= 10 mJ まで増幅
後段Yb:YAGアンプ系
出力エネルギー= 1 J まで増幅

SHG
非線形光学結晶LBO
変換効率: 70%
4th HG 非線形光学結晶CLBO
変換効率: 20%

5th HG
非線形光学結晶: CLBO
変換効率: 10%=100 mJ@212.55 nm

第2励起レーザー
第2励起レーザー：815-850 nmレーザー
DFB半導体レーザー

100 mJ@815-850 nm
LD励起Cr:LiSAF
MOPAシステム

分布帰還型(DFB)レーザー

Cr:LiSAFレーザー

温度制御による波長同調

LD励起全固体システム

λ = 815-850 nm
印加電流の調整によって繰り返し速度パルス幅を制御可能
パルス幅: 1 ns
スペクトル幅: 2 GHz

直線偏光
波長同調域: 700-950 nm
出力エネルギー= 100 mJレベル

VUV Generation in Krypton
Time1985

1990

2000

~ 2008

Research Group

Univ. of Maryland

Imperial College

SRI International

RIKEN

Bonin et al.

Marangos et al.

Faris et al.

MethodPump Laser 1

Two-photon resonant four-wave mixing
ArF Excimer laser?

XeCl Excimer laser

Nd:YAG?

+ Dye laser+ Dye laser

Input Wavelength 216.67 nm

212.55 nm

193 nmInput Energy

0.18 mJ~ 20 mJ

Input Wavelength 723.97 nm

843 nm355 nmInput Energy

0.5 mJ

0.72 mJ?

VUV Wavelength

127.4 nm121-123 nm

121.6 nm121.5-122.2 nm

110 ± 60 nJ

7 µJ

1 µJ

5 x 10-4?

≈ 10-4JOSA B 7, 1254 (1990).

JOSA B 17, 1856 (2000).

Pump Laser 2

Nd:YAG laser

XeCl Excimer laser+ Freq. conversion+ Dye laser

Dye laser (Tunable)

1 mJ

VUV Energy
Conversion

10-5

Efficiency(determined by SFG)

Reference

JOSA B 2, 527 (1985).