Laser System Upgrade Overview RF Gun review @ 19, Nov, 2015.

Laser System Upgrade Overview

RF Gun review @ 19, Nov, 2015

Requirement of laser system for RF-Gun• Laser energy

– 500 mJ for Ir5Ce cathode

– 50 mJ for Ce2Te cathode

• 50 Hz, 2-bunch (96ns spacing)=> Regenerative amplifier + 96ns delay=> Large regenerative amplifier (unstable)

=> Multi-pass amplifier (Upgrade existing underground laser )

• Temporal pulse shaping to reduce energy spread=> Broadband laser crystal (Yb or Ti:Sapphire)

=> postponded until Phase-III• Continuous operation (limited cost / human resource)

– Support cost for commertial product is very high.(1,000-2,000$/day/person + a (margin) )

– No laser system company in Japan.– Recovering time.

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02

Charge(QE=1e- 5)@266nmCharge(QE=1e- 4)@266nmCharge(QE=1e- 3)@266nm

Required laser pulse energy

5nC

Ce2Te

ATF

Offline measurementP-polarization

Offline measurementS-polarization

Normal injection (old 3-2)Inclined injection

Charge( C )

Laser power ( J )

QE=10-3QE=10-4

QE=10-5

Current laser energy(500μJ)

How to generate 2-bunch with 96 ns spacing.• Amplification time of standard regenerative amplifier

(usually adopted in commertial product) is around 1 ms.

• Regenerative amplifier and 96 ns delay (ground laser)• 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 (upgrade existing underground

laser)

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 => 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.

Short term upgrade for laser development• Undeground Yb-Fiber(Upgraded) + Yb:YAG (Existing)

– Downgrade to 25Hz 2-loop amplifier => Fix configuration– Stability improvement : Improvements on Yb-Fiber laser.– Increase monitor points / quadrant detector etc..

• Ground Yb-Fiber + Nd:YAG / Yb:YAG– Installation of MENLO Orange oscillator(commertial)

However it doesn’t cover 1064nm.=> New stable oscillator for 1064nm was installed.

– Existing fiber amplifier is best fit to amplify 1064nm. (p.9) – Nd:YAG 10 Hz regenerative amplifier is under test.– Yb:YAG 50Hz regenerative amplifier is under construction.– Better room environment / Virtual cathode.– (Cryogenic Yb:YAG) => Postpond until Phase-III.

Collaboration / Visit• Collaboration

– Hamamatsu Laser Diode / Disk Laser– HiLASE Yb Fiber & Disk

Laser– IMS / Taira Lab. Laser Crystal / Bonding– Waseda Univ. Yb-Oscillator

• Visit– DESY / Flash Nd:YLF– J-Lab– etc,.

Spectrum compare

Upgrade of existing underground laser

Yb-doped fiber oscillator with

Transmission grating

Yb-doped fiber pre amplifier

Yb-doped fiber main amplifierPulse picker

Yb:YAG thin-disk multi-pass cavity amplifier

Yb:YAG thin-disk multipass amplifier ×3

SHG

Synchronization system

SHGRF GUN

51.9 MHz

2, 5, 25, 50 HzSingle or Double bunch

0.2 nJ @ 1037 nm

Yb-doped fiber main amplifier

Transmission grating pair

stretcher

2856 MHz triggerFrom Accelerator

Yb-doped fiber pre amplifier

>μJ @ 1037 nm

Pulse picker

10 MHz

Fix the configuration to 25 Hz operation

Improvement of Yb fiber laser• Fiber oscillator

– MENLO oscillator is installed and under operation.However it doesn’t cover 1064nm

– ANDI type oscillator was installed for the 1064nm.• Simplify and increase the pulse energy

– Semiconductor Optical Amplifier (SOA) for pulse picker (114 to 10 MHz) and amplification

• Multimode fiber amplifier– PCF fiber cutting method etc,.

Simplify the Yb:YAG solid laserNew high gain multi-pass amplifier (10-15 pass x 2 loop)

LD Laser Diode

INP

UT

OU

TP

UT

1p

ass 1

0-1

5p

ass

←

Laser stability

New laser system in the ground laser room

Menlo1030nm oscillator

Grating stretcher(Transmission)

SOApulse picker

Yb single mode fiber amp

Yb-doped double clad fiber amp

Repetition frequency114.24MHz

Wavelength selection1030nm & 1064nm

Repetition frequency 10MHz

Nd:YAG regenerative amplifier

EO modulepulse picker

Repetition frequency : 10Hz

Yb:YAG Thin diskregenerative amplifier

1030nm oscillator(ANDi type)

1064nm oscillator(ANDi type)

Output(1030nm)Output(1064nm)

ASE rejection grating pair

1030nm

1064nm

1064nm

Fiber part

96ns Delay => Combine => SHG => FHG

Strecher for Yb:YAG & Nd:YAG

2.0nm

Yb:YAG Nd:YAG

Center wavelength 1030 nm 1064 nm

Gain spectrum width ~2 ns ~0.5ns

Distance of the Stretcher 1.5 m (30 ps) 8 m (10ps)

Grating pair

10 ps

Gain spectrum of Yb:YAG

Distance of grating pair

Nd solid laser• 4-state laser is easy to operate• Small band width / Thermal lens effect• Crystal type

– Rod - Side pump: Thermal lens effect, High gain– Thin disk: Less thermal lens, Low gain

• Pumping Laser Diode– 808nm : Commercial product– 885nm : Direct pump, Higher LD power, Lower

cross section

Nd rod DPSS module○ 4-state laser is easy to operate.○ High power pump LD is available.○ High gain efficiency.○ Large crystal is available× Small gain bandwidth. (Gaussian)

1.1% Dia. 4×93mm AR/AR@1064nm

Existing DPSS module is designed for 10 Hz operation.50 Hz commercial DPSS module is available (expensive).

Nd:YAG regenerative amplifier

On going development.• Nd regenerative amplifier for the Phase-I

stable injection.• Yb regenerative amplifier is for Rui’s R&D

under collaboration with HiLASE.• Virtual cathode will be built on the optical

board.• Vacuum optical duct is under development.