Present status of the laser system for KAGRA Univ. of Tokyo Mio Lab. Photon Science Center SUZUKI, Ken-ichiro

2014/06/20 6th KAGRA Workshop2 Outline Laser system –Fiber laser amplifier –Coherent addition Previous work Present status Summary & Future work

2014/06/20 6th KAGRA Workshop3 Laser System & Requirements MOPA (Fiber and Solid-state laser amplifiers) + Coherent Addition Nd:YAG NPRO 400mW 40W 80W 180W Fiber laser amplifiers Solid-state laser amplifiers Coherent addition on the beam splitter 40W Requirements ・ Laser power180W ・ Single frequency1064nm Single transverse modeTEM 00 ・ Linear polarization ・ Low intensity noise ・ Low frequency noise

Setup Widen dynamic range – Fiber stretchers Same path length – good spatial mode matching Change water chiller HWP Isolator 15MHz EOM Nd:YAG NPRO 400mW Fiber stretcher Collimator Fiber amplifier Photo detector Mixer Servo Optical fiber systemFree space optical system Beam splitter output +-

Previous work 78.9W was observed from two 40-W incident beams

2014/06/20 6th KAGRA Workshop6 Optical System – fiber system Fiber amplifiers Seed laser (NPRO 400mW) Fiber components Isolator coupler EOM stretcher Power supply for NPRO 19 inch rack mounted

2014/06/20 6th KAGRA Workshop7 Optical System – addition stage Collimators (Fiber amp. output) Isolators Beam splitter Bright port Dark port Dark port optical system

2014/06/20 6th KAGRA Workshop8 Experimental Result – long term operation Operation itself lasted 5 hours (Output power: 77 ～ 78W) The fiber stretchers had enough margin of control Large power fluctuation was observed for the first one hour Further brush-up is needed Stable operation output power of bright and dark port applied voltage of the fiber stretchers

Experiment Result – polarization fluctuation Power fluctuation results from polarization fluctuation Isolator is polarization dependent Polarization stability is important for power stability Collimator HWPIsolator Transmitted power Cut off power an example of power fluctuation

Experiment Result – fiber stretcher 2014/06/20 6th KAGRA Workshop10 NPRO Polarization rotation was seen at fiber stretcher. This may causes power fluctuation. Fiber stretcher Oscillo scope PD oscillator Signal: 10V Setup

2014/06/20 6th KAGRA Workshop11 Next Step – solid amplifier stage Use three Mitsubishi laser amplifiers Strong thermal lensing ( ～ 50cm/rod) Amplification of up to 180W is expected. 180W Three solid-state laser amplifiers 80W

2014/06/20 6th KAGRA Workshop12 Summary & Future work Coherent addition in new system was done Operation lasted 5 hours Longer operational term can be expected Control system worked well Noise evaluation of coherent addition Solid amplifier stage ( ～ 180W) Stabilization of intensity and frequency

2014/06/20 6th KAGRA Workshop13 Requirements of Laser System KAGRA is extremely high sensitive... Requirements Laser power180W Single frequency1064nm Single transverse modeTEM 00 Linear polarization Low intensity noise Low frequency noise

2014/06/20 6th KAGRA Workshop14 Fiber Laser Amplifier Advantages Good transverse mode High amplification efficiency Easy alignment Disadvantage Nonlinear effect in high power region –Stimulated Brillouin Scattering (SBS) –Stimulated Raman Scattering (SRS) Nufern 40W fiber laser amplifier

2014/06/20 6th KAGRA Workshop15 Coherent Addition Add two laser beams on a beam splitter by positive interference Interference fringe is kept constant using a feedback system Phase modulation is adopted for the fringe control Beam splitter Phase adjuster EOM Mixer Oscillator Photo detector Double-power output

2014/06/20 6th KAGRA Workshop16 Fiber Stretcher Wide control range 400V) Using 2 stretchers differentially –extend the dynamic range Mechanical resonance –limit the control bandwidth Fiber stretcher About 18 kHz

2014/06/20 6th KAGRA Workshop17 Experimental Result – control system Resonance peak disappeared by notch filter Unity gain frequency is about 3 kHz without notch filterwith notch filter Unity gain frequency

2014/06/20 6th KAGRA Workshop18 Experimental Result – noise measurement

2014/06/20 6th KAGRA Workshop19 Fiber laser amplifier & Coherent addition Good transverse mode High amplification efficiency Easy handling Output power is limited by SBS Using interference Phase is modulated for control Phase difference is kept by feedback system Yb-doped fiber 10mW40W Mixer Oscillator PD EOM Phase adjust Output

Previous Work Output power of 78W and efficiency of 95% have been obtained Continuous operation for 4 hours was realized Drift of the phase difference was observed Control range of the PZT limited the operation time Wider control range and better temperature stability are necessary for long term operation K.Oikawa (2013)