KAGRA is a Laser interferometer based gravitational wave telescope in Japan, joining the international gravitational wave observation run, O4 with other telescopes LIGO and Virgo.
In previous gravitational wave observation runs, O1, O2, and O3, 90 observation events have been reported, and many more are expected to be observed in this O4 period.
With the beginning of the gravitational wave astronomy, the calibration of the observed gravitational wave signal is becoming more important than ever before.
The main calibration system used in KAGRA, LIGO, and Virgo is Photon calibration system (Pcal) which injects reference signals into a telescope by radiation pressure of a Laser.
Based on the preparation and operation of KAGRA Pcals in O3, we made a upgrade plan for O4, which includes 3 main improvements: better Pcal laser beam alignment system, lower noise performance, and lower uncertainty.
Improvement work was carried out between the end of O3 and the start of O4 following this plan, with the following results.
First, precise remote alignment was made possible in the critical Pcal laser path region.
Second, Pcal noise sources were identified and improved by about 50 dB, reducing the Pcal noise below the design sensitivity of KAGRA.
Third, the calibration procedure of the Pcal was reviewed, and studies on temperature dependence and laser incidence state dependence were carried out to reduce the uncertainty on Pcal, which is directly related to the uncertainty of the telescope signal.
The details of KAGRA Pcal, and these improvements between O3 and O4 are reported here.