1. Characteristic evaluation of new laser crystals Rui Zhang ACCL Division V, RF-Gun Group Feb 20, 2015 SuperKEKB Injector Laser RF Gun Review

2. Outline Introduction to current Yb laser system of SuperKEKB injector laser RF gun New Yb doped crystal candidates – Cascade laser configuration – Thermal and laser characteristic for new crystals – New Yb doped crystal disk laser – Gain test under cryogenic environment Summary Characteristic evaluation of new laser crystals

3. Introduction to current Yb laser system Characteristics of Yb doped laser Long fluorescent time Wide bandwidth High power commercial pump LD source High quantum efficiency Suitable for fiber laser system Best candidate for RF-Gun

4. Yb-doped fiber oscillator Yb-doped fiber pre amplifier Yb-doped fiber amplifier Transmission Grating Stretcher EO Pulse picker double Yb:YAG thin- disk 6-pass amplifier Yb:YAG thin- disk 5-pass amplifier SHG Synchronization system 2856 MHz trigger from accelerator SHG RF Gun 51.9 MHz 1 nJ @ 1035 nm Yb-doped fiber main amplifier BBO Yb:YAG thin- disk 5-pass amplifier Yb-doped fiber pre amplifier Yb:YAG thin- disk 2-pass amplifier Yb:YAG thin- disk 1-pass amplifier Introduction for current Yb laser system Current laser system

5. New Yb doped crystal candidates Cascade laser configuration Cascade Yb laser is a good candidate Tiny thermal lens effect Less re-absorption Higher excitation density Easy to maintain the LD pump system For 1035 nm band laser – Absorption – Low gain 1035 nm pump source: Q-switch laser

6. New Yb doped crystal candidates Cascade laser Pump laser 940 nmSeed laser 1050 nm10.4% Pump laser 976 nmSeed laser 1050 nm7.1% Pump laser 1035 nmSeed laser 1050 nm1.43% Superb thermal management! Experimental setup for testing the gain of cascade laser Gain test of cascade laser Quantum defect of Yb cascade laser Current situation: Emission cross section of new crystals  High dopant and longer effective length Polarization of pump laser and seed laser  Transmission grating and pin hole Overlap between pump laser and seed laser  Higher energy of pump laser

7. New Yb doped crystal candidates Thermal and laser characteristic for new crystals Yb:YAGYb:KGWYb:BOYSYb:CaF 2 Absorption bandwidth at the zero-line wavelength (nm) 941968981975979.8 Absorption bandwidth (nm, FWHM) 1833.566 Peak absorption cross section (10 -21 cm 2 ) 8.09.4 120 (a-axis) σ 7.3 π 9.4 5.4 Fluorescent lifetime (ms)0.950.751.12.4 Room temperature laser wavelength (nm) 1030102610661049 Emission cross section at laser wavelength (10 -21 cm 2 ) 2022 28 (a-axis) 21.7 Emission bandwidth (nm)10206070 Experimental tuning range (nm) 1020-10831020-10701017-10861000-1072 Mean refractive index1.822.031.751.43 Thermal conductivity (W/m/K) 113.31.86.0 Thermal expansion coefficient (10 -6 /K) 6.73.8 5 (a) 20 (c) 18.9 Young Modulus (GPa)280115.8-76 Yb:CaF 2 Yb:KGW

8. New Yb doped crystal candidates Available new crystals Yb:CaF 2 CrystalYb:KGWYb:BOYSYb:CaF 2 ShapeCuboid Disk Dimension (mm)4×4×205×5×18 Dia. 12.7 T: 2 Doping concentration (a.t. %) 5.0 10.510.0 Coating√×× Quantity 2 25 Next experiments: Gain of cascade laser Gain under cryogenic environment

9. New Yb doped crystal candidates New Yb doped crystal disk laser Yb:YAGYb:CaF 2 Fluorescence Room temperature laser wavelength (nm) 10301049 Fluorescent lifetime (ms)0.952.4 Spectral width (nm)1070 Fourier minimum pulse width (fs) 16523 Absorption Absorption bandwidth at the zero- line wavelength (nm) 941979 Absorption bandwidth (nm, FWHM) 186 Peak absorption cross section (10 -21 cm 2 ) 8.05.4 Mean refractive index1.821.43 Thermal Thermal conductivity (W/m/K)116.0 Thermal expansion coefficient (10 - 6 /K) 6.718.9 Young Modulus (GPa)28076 Yb:CaF 2 is another good candidate for disk laser Wide bandwidth Long fluorescent time Possibility of fabricating large size singe crystal and ceramics (Czochralski or Bridgman) Suitable for high-energy and high power laser operation Low dispersion behavior and limited nonlinear effects under intense laser irradiation ×Low emission cross section  Cryogenic environment ×Thermal conductivity decreases with increasing of Yb doping concentration

10. New Yb doped crystal candidates Gain test under cryogenic environment Vacuum chamber Cryogenic equipment Wavelength tunable laser Vacuum degree : 5.2×10 -6 Pa 4 K cryocooler Maximum output power: 20 mW Gain test schematic design (out of ratio)

11. New Yb doped crystal candidates Gain test under cryogenic environment

12. Summary New Yb doped crystals All candidates have been manufactured (bigger size Yb:CaF 2 will be booked) Gain test for Yb:YAG under cryogenic environment – Gain test for Yb:CaF 2 under cryogenic environment Cascade laser configuration mJ level 1035 nm Q switch laser has been achieved – Gain test for different crystals by use of high doping concentration samples The End

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