Laser Overview 2015 02 19 Mitsuhiro Yoshida Requirement

Laser Overview 2015. 02. 19 Mitsuhiro Yoshida

Requirement of laser system for RF-Gun • Laser energy – 500 m. J for Ir 5 Ce cathode – 50 m. J for Ce 2 Te cathode • 50 Hz, 2 -bunch (96 ns spacing) => Difficult to adopt commertial products using regenerative amplifier. => Multi-pass amplifier or Special regenerative amplifier • Temporal pulse shaping to reduce energy spread => Broadband laser crystal (Yb or Ti: Sapphire) • Continuous operation (limited cost / human resource) – Support cost for commertial product is very high. (10 -20万円=1, 000 -2, 000$/day/person + a (margin) ) – No laser system company in Japan. – Recovering time.

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

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)

Energy spread reduction using temporal manipulation Energy spread of 0. 1% is required for Super. KEKB synchrotron injection. 15 n. C t 5 n. C 10 n. C 20 n. C Gaussian t Square 5 n. C electron 5 n. C 10 n. C 15 n. C 20 n. C 15 n. C Primary beam for positron production

Nd-doped Properties of laser medium Nd laser system for 3 -2 RF-Gun τ～ 200μs, 40% ○ 4 -state laser is easy to operate. SHG(532 nm) 40% LD Pump Nd: YVO 4 ○ High power pump LD is available. FHG(266 nm) 20% (808 nm) Nd: YAG ○ Large crystal is available 5 HG(213 nm) 3% × Pulse width is determined by SESAM. 808 nm 1064 nm (Gaussian) Yb-doped ○ ○ × × Wide bandwidth => pulse shaping τ～ 900μs, Long fluorescent time => High power Yb-glass Fiber laser => Stable LD Pump Small state difference Yb: YAG (941/976 nm) ASE Yb: BOYS 941/976 nm Absorption Ti-doped Pump τ=200μs, 40% Pump (808 nm) ○ ○ × × 40% SHG(520 nm) 40% FHG(260 nm) 20% 5 HG(208 nm) 3% 1040 nm Best for RF-Gun τ～ 3μs, 40% Nd: YAG SHG 808 nm 1064 nm Ti: Sapphire 532 nm 800 nm Very wide bandwidth High breakdown threshold TW laser is based on Ti-Sapphire Low cross section Short fluorescent time => Q-switched laser is required for pumping SHG(400 nm) 40% THG(266 nm) 20% FHG(200 nm) 10% Ti: Sapphire laser system for beam monitor.

Flash pumped LD pumped Absorption CPA Fluorescence Laser schemes η～ 0. 5% Nd: YAG Ti-Sapphire Oscillator Superconitnuum broadning Yb-Fiber Frontend 940 nm LD OPCPA 1030 nm Yb: BOYS, Yb: Ca. F 2 - Broadband Oscillator Pump Amplifier Nd: YAG Yb: YAG Ti: Sapphire Wavelength 1064 nm 1030 nm 660 -1100 nm Fluorescent time 230μs 960μs 3. 2μs Spectral width 0. 67 nm 9. 5 nm 440 nm 2. 48 ps 165 fs 2. 59 fs 807. 5 nm 941 nm 488 nm 1. 5 nm 21 nm 200 nm 76% 91% 55% Fourier minimum Pulse width Wavelength Spectral width Quantum efficiency Many commertial product. - How to maintain continuously? - How to generate 2 -bunch ? Yb: YAG Thin Disk η～ 40% Material - Very high gain - Critical incident angle - Fiber laser is stable in principle. - High efficiency (long fluorecense lifetime) - Low gain at room temperature => Lower temperature

Yb Fiber Laser Yb laser system (Present status) Yb Fiber Oscillator Yb Fiber Amplifier frep=52 MHz Amplitude modulator x 2 ASE problem frep=52 MHz→ 10 MHz Yb Fiber Amplifier x 2 (20 W + 70 W pump) Fiber strecher & wavelength selection Pulse picker Yb PCF Amplifier (70 W pump) frep=10 MHz→ 50 Hz λ 0→ 1030 - 1040 nm Yb: YAG laser Pulse Compression Yb: YAG Multipass Amplifier Yb: YAG 5 -pass Amplifier × 4 stage Yb: YAG Regenerative Amplifier Yb: YAG 5 -pass Amplifier 7 m. J @ 1030 nm Wavelength conversion Failed 0. 7 m. J @ 258 nm

Yb Fiber Oscillator Yb fiber oscillator: - 30 fs is possible using non-linear polarization rotation. - 52 MHz is not suitable => 114 MHz will be installed Transmission grating pair ← Remaining problem: - LD was sometimes broken. - Bunch structure at higher output power. => SESAM is not effective. => Replace some components. - 1030 nm oscillator is not stable - 1030 nm component of broadband oscillator is small => large ASE WDM Yb Fiber ← Improved items in FY 2013: - Transmission grating => Stable modelock (higher efficiency) - Super-invar breadboard => Improve thermal stability. - Piezo mirror on large lead block => Reduce vibration. - Broadband oscillator (No. 2) is stable. LD

Fiber laser system (Ideal and reality) Standalone Oscillator 52 or 114 MHz SM-fiber • 80 preamplifier Many constraints in our accelerator MHz is best oscillator frequency – But 52 or 114 MHz must be chosen. Fiber EO SM-fiber preamplifier Strecher SM-fiber preamplifier • Reduction of ASE 10 MHz – Multi stage amplification and pulse picking is required. • Pulse energy A few 100 k. Hz – No lower synchronization frequency (Only 10 MHz is available) 50 Hz => 10 m. J is expected, however much lower than this. (A few 100 n. J) • Stability – Lifetime of PCF MM-fiber preamplifier PCF pamplifier Double EO to pick up 2 -buch 50 Hz PCF pamplifier Yb: YAG solid state laser Much higher gain of Yb: YAG amplifier is required.

Main Yb: YAG Amplifier Focused type multi-pass amplifier < 1 m. J - High gain - Focused at crystal leads to avoid thermal lens effect. UV conversion (BBO SHG+FHG) => 1 m. J maximum @ 258 nm Typical charge distribution Current situation: - Instability => - No spatial shaping - No compressor Non-Focused type amplifier > 10 m. J - Low gain - Uniform pumping is required. Laser instability is caused by: - ASE of fiber amplifier. - Pointing fluctuation from fiber amplifier. - Stability of pump laser (Upgrade of charger is required) - Separated optical table between fiber and solid laser.

Cryogenic Yb: YAG • Improvement of thermal and emission property (Thermal lens effect)　 (Excitation density) GM+He 10 W/m/K , dn/d. T = 8 ppm/K @ 300 K 25 W/m/K , dn/d. T = 3 ppm/K @ 150 K ↑ 150 K 1/6 Thermal lens Same gain @ 1/3 excitation density 　　→ 　　　↓ 　150 K => 1/20 thermal lens 300 K P/P 0 = exp(g 0 z) ～ 2 150 K　　　→ g = 7 [cm-1] Pertier 300 K 30 k. W/cm 2

• Yb-fiber oscillator Issues on Yb based laser system – 1030 nm 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 => 50 Hz before May 2015 – Reduce thermal lens effect and simplify laser system => Focused type multipass amplifier x 2 + 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).