High-energy laser source of RFgun
Xiangyu Zhou

Contents
Laser system, undergroundYb-doped fiber & Thin-disk Yb:YAG @ 1030 nm

New laser system, on the groundYb-doped fiber & Rod Nd:YAG @ 1064 nm
Yb-doped fiber & Thin-disk Yb:YAG @ 1030 nm

Requirement of laser system for RF-Gun
 Laser energy

320nC

 500 µJ for Ir5Ce cathode 50 µJ for Ce2Te cathode5nC 10nC

2.5t

Gaussian2
σE/E (%)

 50 Hz, 2-bunch (96ns spacing) Temporal pulse shaping to reduceenergy spread=> Broadband laser crystal(Yb or Ti:Sapphire) Continuous operation=> High power pump LD(Yb or Nd)

15nC

1.5

1t
5nC
10nC
15nC
20nC

Square0.500510 5nC electron
15FWHM (ps)

20 15nC

2530

Primary beam for positron production

 No laser system company in Japan.
 Support cost for commercial productis very high.

Yb-doped ◎Nd-doped △Ti-doped ×○ Wide bandwidth => pulse shaping○ Long fluorescent time => High power○ Fiber laser oscillator => Stable○ Small state difference
× ASE× Absorption○ 4-state laser is easy to operate.
○ High power pump LD is available.
○ Large crystal is available
× Pulse width is determined by SESAM.
(Gaussian)○ Very wide bandwidth○ High breakdown threshold× Low cross section
× Q-switched laser is required for pumpingYb-doped Solid-state laser
fiberdisk

Fiber amplifierThin-disk amplifier○ Long gain media○ Compact○ No need cooling○ Good beam quality
× Pulse energy

× Thin gain media (low gain)
× Cooling system
× Thermal lens effect○ High pulse energyUnderground laser system
0.2 nJ @ 1030 nm
Yb-doped fiber
oscillator withTransmission grating

51.9 MHz

Synchronization
system2856 MHz trigger
From Accelerator

Yb-dopedfiber preamplifier

Transmission
grating pairstretcher

Yb-dopedfiber preamplifierPulsepicker
10 MHz>μJ @ 1030 nm

2, 5, 25 Hz
Single or Double bunch

Yb-doped fiber
main amplifier

Pulse picker

Yb:YAG thin-disk
multipass amplifier ×3

Yb:YAG thin-disk multipass cavity amplifier

RF GUN

Yb-doped fiber
main amplifier

SHGSHG

Fiber Oscillator & Amplification
WDM
couplerYb fiber Oscillator

PIEZO

SMF

LDPumpλ/4••Collimator

51.9 MHz
StableGrating pair
1000-grooves/mm

SMF

P.D.

250mW
976 nmλ/4

Yb-doped fiber

OutputIsolator

Polarizing

λ/4

λ/2

Collimator

Low power amplifier

High power amplifier
Isolator

Collimator

Yb-doped Fiber

Yb-doped Double
cladding Fiber Collimator
PM4μm

Dichroic mirror

WDM
coupler

HR@976 nm
HT@1050 nm
Ɵ=20°

750 mW
LDPump••Low lossCompact•

75 W
LDPumpHigh amplify efficiency

Yb:YAG thin-disk amplifier
Laser DiodeOUTPUT

INPUT

10-15pass

1pass

Yb:YAGdisk
10 % doped
0.5mm
thickness

Cavity multi-pass amplifier (15 pass x 2 loop) tosimplify the laser
To obtain higher gain,=> Higher pumping density⇒ Focused type amplifier toavoid thermal lens.

←Laser
Diode

Balanced offset
lens to avoiddamage.
940nm LD (2.4 kW / module)
5pass

4pass

3pass

2pass

1pass

Temperature of the Yb:YAG thindisk amplifier10% dope, α=12/cm, 5kW/cm2, 25Hz
0.5t

Repetition rate limitation of 25 Hz is reached!

1t

Spectral Range Adjustment
For chirped pulse, the pulse shaping in the time domain can be controlled
by adjusted the spectral shaping of the pulse.
Width of the slit

Spatially disperseddSpatial Mask
~200fs~30ps

Chirptl

Pulse shaping

Distance of thegrating pairt

Optimized pulse shape by streak camera(After adjustment 2015.12.17)

ω：~10 mJ, 2ω： ~2 mJ, 4ω： 230 μJ
Due to the Non-linear effect was occurred, Output power limitation is reached!

Layout of A1 laser room
EntranceControlrack

PCF fiberamplifier

Yb-doped
Fiber
Oscillator
120 μJ
Thin-disk Multi-passcavity amplifier

EO Pulse
pickerPCF fiber
amplifier
SOA Pulse
pickerThin-disk
multi-pass
amplifier

0.2 nJ
Grating pairYb-doped
Fiber
Oscillator
2

Thin-disk
multi-pass
amplifierstretcherFiber
pre-amplifier

Streak camera

SHG
1033nm↓532nm

ElevatorFiberpreamplifierThin-disk
multi-pass
amplifier

Entrance
10 mJ
3 mJ

Sheld
To GR_A1

Laser line underground
TunnelRF-Gun

2nd SHG stage

Laser Hut

Energy loss of the UV in the air: >50%

Laser line to RF-Gun

SHG for the 4ω

Motorized Translation Stages

Stability of lasers (3 days)

Elevator maintenanceLaser off

Stability of lasers (4/26—6/9)

Last laser Maintenance

Trig stoped
No signal, LD work on

Summary of the underground laserYb-doped fiber & Thin-disk Yb:YAG hybrid
system @ 1030 nm
Development of Underground Laser isfinished.
The stability of laser is improved.
The laser is employed for the Phase-I study.
The limitation of the output power (10 mJ)
and repetition rate (25 Hz) is reached.

Laser line on the groundKLY Gallery

Laser HutTunnel

RF-GunLaser line to RF-Gun

Yb fiber laser system on the ground
Fiber part
Repetition frequency
114.24MHz

1030-64 nm
ANDi typeoscillator
Menlo
1030nm oscillator(commercial)

Wavelength selection
1030nm & 1064nm

Grating stretcher(Transmission)

Repetition frequency
10MHzSOA
pulse picker

Yb-doped double clad
fiber amp

Yb single mode
fiber amp

ASE rejection
grating pair1064nm1030nm10Hz

EO module
pulse picker

Yb-doped PCF
fiber amp

1064nmNd:YAG
regenerative amplifierYb:YAG Thin disk
regenerative amplifier

Output(1064nm)

Output(1030nm)

New System

Grating pairFiber+Yb:YAG Thin-disk
Amplifier @1030 nm

Yb fiberOscillator
Spatially dispersed

Fiber+Nd:YAG RodAmplifier @1064 nm

Issue of the underground laser
Complex structure (Stability, Maintenance)
50 Hz double bunch repetition rate
10 MHz oscillator
High energy fiber amplifier
Yb:YAG & Nd:YAG Regenerate amplifierAmplifier efficiency & Non-linear effectChirped Pulse Amplification (CPA) system

Oscillator of 10 MHz repetition rateRepetition rate

Cavity length

Fiber length

114.24 MHz

2.6 m

~1 m

10.38 MHz28.9 m~20 m

Chirp Pulse Amplification (CPA)
Under ground Yb laser system
Yb fiber
Oscillator

StretcherThin-disk
amplifierYb PCFamplifierHarmonicgeneration

30 ps

On the ground Yb laser system
Yb fiber
Oscillator

Stretcher
2 ns

Yb PCFamplifierThin-disk
amplifierCompressor
30 psHarmonicgeneration

Grating pair compressor
Canon transmission grating
Size: 180mm×50mm
Groove density: 1740 lines/mm
Diffraction efficiency: 95%

180 mm2ns pulse can be dechirped

ΔƟl

Vacuum Chamber

63.65°ΔλΔƟl

Fiber amp

20 nm

4.5°1020 mm

Yb:YAG GainSpectrum

2 nm

0.45°

10173 mmYb:YAG thin disk regenerative amplifier• For picosecond and nanosecond laser pulse, the regenerative amplifier isvery powerful• Strong amplification factor can help relieve stress of multi-pass amplifier
stages and reduce thermal effect• New dichroic mirror with high optical quality and high damage thresholdLow temperature Yb:YAG thin disk laser• Low temperature laser operation feasibility analysis– Available high gain is possible– Low amplified spontaneous emission (ASE)– Simpler and more stable (compare with cryogenic laser)• Current low temperature laser operation– Peltier cooling plates: 23 oC  -36 oC– Small vacuum chamber– Home made laser soldering laser head
– 50 Hz repetition rate testYb fiber and Nd:YAG multi-stage laser @ 3-2
To Linac Tunnel

Laser shutter

Wavelength
convertion

AmpPulsepicker

Strecher &Fiberpreamp
Fiberoscillator
Optical surface plateDoorwayLaser Hut・Laser energy
4.0mJ@1064nm
550uJ@532nm
120uJ@226nm
・Under beam comissioning

Nd:YAG Diode-pumped solid-state(DPSS) Regenerative amplifier○ 4-state laser is easy to operate.
○ High power pump LD is available.
808nm : Commercial product○ High gain efficiency.
○ Large crystal is available
× Small gain bandwidth. (Gaussian)

1.1% Dia. 4×93mm
AR/AR@1064nm

̶Plan A̶

DevelopedDeveloping

Yb fiber + CPA + Yb:YAG

114 MHz fiber
oscillator

Grating stretcher
To 30 ps

10 MHz ANDi
type oscillator

Stretcherto 2 nsSOApulse picker

Yb single mode
fiber amp

EO module
pulse picker

Yb-doped PCF
fiber amp
10Hz

SHG × 2

Grating pairCompressor
to 30 ps

High-energy Yb-doped
ROD PCF fiber amp
1030nmOutputYb:YAG Thin disk
regenerative amplifier< mJ
1064nmNd:YAG
regenerative amplifier

Plan BYb fiber + Nd:YAG
114 MHz fiber
oscillator

Grating stretcher
To 30 ps

10 MHz ANDi
type oscillator

Stretcherto 2 nsSOA
pulse picker

Yb single mode
fiber amp

EO module
pulse picker

Yb-doped PCF
fiber amp
10Hz

SHG × 2

Grating pairCompressor
to 30 psHigh-energy Yb-doped
ROD PCF fiber amp
1030nmOutputYb:YAG Thin disk
regenerative amplifier< mJ1064nm

Nd:YAG
regenerative amplifierSummaryLaser system, underground Yb-doped fiber & Thin-disk Yb:YAG @ 1030 nm• Development of Underground Laser is finished.
• The stability of laser is improved.
• The limitation of the output power and repetition rate is reached.
• The laser is employed for the Phase-I study.

New laser system, on the ground Yb-doped fiber & Rod Nd:YAG @ 1064 nm• Fiber parts are finished.
• Nd regenerative amplifier for the Phase-II stable injection.

 Yb-doped fiber & Thin-disk Yb:YAG @ 1030 nm• 10 MHz oscillator to simplify the laser system• High-energy fiber ROD amplifier μJmJ• CPA system to avoid non-linear effect• Yb regenerative amplifier for the Phase-III