1. Laser System Upgrade Overview
RF Gun 19, Nov, 2015

2. 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.

3. Required laser pulse energy
Current laser energy(500μJ)
Charge( C )
Ce2Te
QE=10-4
QE=10-3
Offline measurement
S-polarization
Offline measurement
P-polarization
QE=10-5
5nC
Inclined injection
ATF
Normal injection (old 3-2)
Laser power ( J )

4. 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)

5. Properties of laser medium
Nd laser system for 3-2 RF-Gun
Nd-doped
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
(808nm)
Nd:YVO4
Nd:YAG
SHG(532nm) 40%
FHG(266nm) 20%
5HG(213nm) 3%
808nm
1064nm
Yb-doped
○ Wide bandwidth => pulse shaping
○ Long fluorescent time => High power
○ Fiber laser => Stable
○ Small state difference
× ASE
× Absorption
τ～900μs, 40%
Yb-glass
Yb:YAG
Yb:BOYS
SHG(520nm) 40%
FHG(260nm) 20%
5HG(208nm) 3%
LD Pump
(941/976nm)
941/976nm
1040nm
τ～3μs, 40%
Ti-doped
τ=200μs, 40%
Pump
40%
SHG(400nm) 40%
THG(266nm) 20%
FHG(200nm) 10%
Pump
(808nm)
Nd:YAG
SHG
Ti:Sapphire
808nm
1064nm
532nm
800nm
○ Very wide bandwidth
○ High breakdown threshold
× Low cross section
× Short fluorescent time => Q-switched laser is required for pumping
Ti:Sapphire laser system for beam monitor.
TW laser is based on Ti-Sapphire

6. 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.

7. Collaboration / Visit Collaboration Visit
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,.

8. Spectrum compare

9. Upgrade of existing underground lasernm
Yb-doped fiber oscillator with Transmission grating
Yb-doped fiber pre amplifier
Transmission grating pair stretcher
Yb-doped fiber pre amplifier
Pulse picker
51.9 MHz
10 MHz
2, 5, 25, 50 Hz
Single or Double bunch nm
Synchronization system
Yb-doped fiber main amplifier
Yb-doped fiber main amplifier
Pulse picker
2856 MHz trigger
From Accelerator
Yb:YAG thin-disk multi-pass cavity amplifier
Yb:YAG thin-disk multipass amplifier ×3
Fix the configuration to 25 Hz operation
RF GUN
SHG
SHG

10. 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,.

11. Simplify the Yb:YAG solid laser
New high gain multi-pass amplifier (10-15 pass x 2 loop) LD
Laser Diode
1pass
10-15pass
OUTPUT
INPUT ←

12. Laser stability

13. New laser system in the ground laser room
Fiber part
Repetition frequency
114.24MHz
Wavelength selection
1030nm & 1064nm
Repetition frequency
10MHz
1064nm oscillator
(ANDi type)
Grating stretcher
(Transmission)
SOA
pulse picker
Yb single mode
fiber amp
Menlo
1030nm oscillator
Repetition frequency : 10Hz
EO module
pulse picker
Yb-doped double clad
fiber amp
ASE rejection
grating pair
1030nm oscillator
(ANDi type)
1064nm
1064nm
1030nm
Nd:YAG
regenerative amplifier
Yb:YAG Thin disk
regenerative amplifier
Output(1064nm)
Output(1030nm)
96ns Delay => Combine => SHG => FHG

14. Strecher for 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)
Gain spectrum of Yb:YAG
2.0nm
Distance of grating pair
10 ps
Grating pair

15. 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

16. 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
Existing DPSS module is designed for 10 Hz operation.
50 Hz commercial DPSS module is available (expensive).

17. Nd:YAG regenerative amplifier

18. 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.