1. Yb:YAG Regenerative Amplifier for A1 Ground Laser Hut Rui Zhang ACCL Division V, RF-Gun Group Nov 20, 2015 SuperKEKB Injector Laser RF Gun Review

2. Outline Yb:YAG regenerative amplifier 2 of 11 Back ground and introduction – Feasibility analysis and corporation – Current Yb laser system of A1 ground laser hut Yb:YAG regenerative amplifier – Cavity design – Experimental setup introduction Next step work – Ring cavity – Cryogenic experiment Summary

3. Feasibility analysis and corporation Yb:YAG regenerative amplifier 3 of 11 Feasibility analysis – In the picosecond or femtosecond domain, the regenerative amplifier is very powerful – Strong amplification factor can help relieve stress of multi-pass amplifier stages and reduce thermal effect Corporation with HiLASE (http://www.hilase.cz/en/) – Beamline B of HiLASE gets excellent laser amplification by use of Yb:YAG thin disk regenerative amplifier 1 – The parameters of HiLASE beamline B are very similar with our experiment 1.M. Chyla, et. al “Optimization of beam quality and optical-to-optical efficiency of Yb:YAG thin- disk regenerative amplifier by pulsed pumping,” Opt. Letts., 39 1441, 2014. 2.M. Divoky, et. al “Overview of the HiLASE project: high average power pulsed DPSSL systems for research and industry,” High Power Laser Science and Engineering, 2 e14, 2014. Parameters of thin disk beamline B 2 Method Regenerative amplifier with one thin disk laser head Seed repetition rate50 MHz Seed pulse width500 ps Seed output power2 W @ 1030 nm Pump power1 kW @ 969 nm Repetition rage of pump1 kHz Achieved energy45 mJ Next milestone energy100 mJ Under development Add second thin disk laser head into regenerative amplifier Schematic of the current status of HiLASE beam line B

4. Menlo 1030nm oscillator Grating stretcher (Transmission) SOA pulse picker Yb single mode fiber amp Yb-doped double clad fiber amp Repetition frequency 114.24MHz Wavelength selection 1030nm & 1064nm Repetition frequency 10MHz Nd:YAG regenerative amplifier EO module pulse picker Repetition frequency 10Hz Yb:YAG Thin disk regenerative amplifier 1030nm oscillator (ANDi type) 1064nm oscillator (ANDi type) Output@1030nmOutput@1064nm ASE rejection grating pair 1030nm 1064nm Fiber part Current laser system of A1 ground laser hut Yb:YAG regenerative amplifier 4 of 11 YAG crystal part

5. Yb:YAG regenerative amplifier: Cavity design Yb:YAG regenerative amplifier 5 of 11 Timing and cavity length Seed laser repetition rate: 10 MHz  100 ns Pockels cell rise time: 10-15ns  Length>3 m As to linear cavity: cavity length > 1.5 m Regenerative amplifier cavity length is 1.6 m Match of pump size and laser spot size Pump size: 6 x 6 mm For getting higher gain, laser spot size should be bigger on the surface of Yb:YAG disk 10 ns

6. Yb:YAG regenerative amplifier: Cavity design Yb:YAG regenerative amplifier 6 of 11 Cavity design case I: Low threshold cavity oscillation and high output power Output laser beam pattern is excellent Bad tolerance for thermal lens effect Strange beam focus occurred and the dichroic mirror was damaged

7. Yb:YAG regenerative amplifier: Cavity design Yb:YAG regenerative amplifier 7 of 11 Cavity design case II: Low threshold cavity oscillation and high output power Big output laser beam spot Very excellent tolerance for thermal lens effect Suitable to seed laser injection Small beam size on the surface of Yb:YAG

8. Yb:YAG regenerative amplifier: Experimental setup Yb:YAG regenerative amplifier 8 of 11 Yb:YAG disk soldering laser head Old design −Strong residual stress −Adhesion between soldering composite and heatsink decreased the heat removal New design −Weak residual stress −More efficient thermal management can be realized −Suitable for Peltier cooler in vacuum chamber Copper plate with AuSn coating Yb:YAG thin disk/Cu soldering composite He-Ne laser beam pattern He-Ne beam pattern reflected by soldering composite under 1 J pump Yb:YAG laser head achieved by AuSn soldering He-Ne laser beam patternHe-Ne beam pattern reflected by laser head under 1 J pump He-Ne beam pattern reflected by laser head under 3 J pump

9. Yb:YAG regenerative amplifier: Experimental setup Yb:YAG regenerative amplifier 9 of 11 Repetition rate: 10 MHz Power: 160 mW Pulse width: 30 ps Repetition rate: 5 Hz Peak power: 10 kW Pulse width: 600 μs Small divergence along the fast axis direction 10 ns Thickness of Yb:YAG disk: 1.0 mm Size: 12.7 x 12.7 mm Doping concentration: 10 a.t.% High quality copper heatsink by water cooling Cooling temperature: 16 Centigrade Cavity length: 1.65 m

10. Next step work Yb:YAG regenerative amplifier 10 of 11 Ring cavity (design is completed) – Long cavity length and bigger laser beam size inside cavity – Cycle amplification and one-way transmission, lower ASE intensity (I ASE ~1/L), excellent contrast ratio – Compare with linear cavity, more gain can be achieved under the same saturation flux – Lower losses associate with thermally induced aberrations – More stored energy can be obtained because of absence of spacing hole burning Cryogenic experiment – Peltier cooler can be adopted to current disk laser head – All the experiment equipment are available

11. Summary Yb:YAG regenerative amplifier 11 of 11 Different cavities (linear and ring) are designed and built for purchasing high gain and more stable operation in A1 ground laser hut High quality Yb:YAG disk laser head has been achieved Yb:YAG regenerative amplifier cavity is completed and amplification is going to be carried out Ring cavity regenerative amplifier and cryogenic experiment are carrying out at the same time Long term harmonious corporation with HiLASE THANK YOU!