  The Kamioka Gravitational wave detector (KAGRA) cryogenic gravitational-wave
observatory has commenced joint observations with the worldwide gravitational
wave detector network. Precise calibration of the detector response is
essential for accurately estimating parameters of gravitational wave sources. A
photon calibrator is a crucial calibration tool used in laser interferometer
gravitational-wave observatory, Virgo, and KAGRA, and it was utilized in joint
observation 3 with GEO600 in Germany in April 2020. In this paper, KAGRA
implemented three key enhancements: a high-power laser, a power stabilization
system, and remote beam position control. KAGRA employs a 20 W laser divided
into two beams that are injected onto the mirror surface. By utilizing a
high-power laser, the response of the detector at kHz frequencies can be
calibrated. To independently control the power of each laser beam, an optical
follower servo was installed for power stabilization. The optical path of the
photon calibrator beam positions was controlled using pico-motors, allowing for
the characterization of the detector rotation response. Additionally, a
telephoto camera and quadrant photodetectors were installed to monitor beam
positions, and beam position control was implemented to optimize the mirror
response. In this paper, we discuss the statistical errors associated with the
measurement of relative power noise. We also address systematic errors related
to the power calibration model of the photon calibrator and the simulation of
elastic deformation effects using finite element analysis. Ultimately, we have
successfully reduced the total systematic error from the photon calibrator to
2.0 /%.
