Development and Characterization of Optical Follower Servo for Photon Calibrator for Kagra Gravitational Wave Observation
Bin-Hua Hsieh1*, Yu-Kuang Chu2, Sadakazu Haino3, Yuki Inoue3,4, Takaaki Kajita1, Nobuyuki Kanda5, Takaharu Shishido6, Takayuki Tomaru4,6, Satoshi Tsuchida5, Takahiro Yamamoto1, Takaaki Yokozawa5
1Institute for Cosmic Ray Research, The University of Tokyo, Kashiwa, Chiba, Japan
2Department of Physics, National Taiwan Normal University, Taipei, Taiwan
3Institute of Physics, Academia Sinica, Taipei, Taiwan
4High Energy Accelerator Research Organization, Tsukuba, Ibaraki, Japan
5Graduate School of Science, Osaka City University, Osaka, Japan
6SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki, Japan
* Presenter:Bin-Hua Hsieh,
KAGRA is a 3km Large-scale Cryogenic Gravitational Wave Telescope located in Kamioka mine. In order to detect microscopic space-time perturbations and to observe the event, the detector needs to be accurately calibrated while it is operating. Therefore, calibration system plays an important role in this Gravitational Wave detector.
The principal tools used in the advanced GW detectors is Photon Calibrator based on radiation pressure. Photon Calibrator is used in order to reduce the calibration error. By emitting laser beams from its transmitter module, which are reflected by the mirror, and then are received by the receiver module, Photon Calibrator can calibrate the mirror displacement.
Photon calibrator has some issues related to uncertainty. We use 20 watts laser in our Photon Calibrator. To reduce the calibration error, the laser beam needs to be controlled accurately. Since the acoustic optic modulator in the transmitter module has large nonlinearity, we introduce an active controller called Optical Follower Servo to improve the waveform shape of Photon Calibrator. The photodetector catches the laser beam signal, transferring the signal to Optical Follower Servo which reduces the noise. The signal then be feed backed into acoustic optic modulator, modulating the laser power, forming a feedback system to reduce the relative power noise of the laser. The Photon calibrator is now being assembled and developed in KEK (High Energy Acceleration Research Organization). After we finish the characterization, we will install it at the Y-end at KAGRA site in May 2018. Before 2019, the two photon calibrators will be installed, and we can calibrate the interferometer and reconstruct the gravitational wave.
In my talk, I will introduce the development and characterization of the Optical Follower Servo in Photon Calibrator, and the performance of this Optical Follower Servo will be discussed.

Keywords: Gravitational Wave, KAGRA, Photon Calibrator, Optical Follower Servo