[Ushiba, T.Yamamoto, Komori]

We have estimated the contributions of shot noise and dark noise as reported in klog:25782.
For the shot noise estimation, we used only the information of the 15 mW power on the OMC DC PD instead of complex shot noise calculations, ensuring a robust estimation.
The results are presented in the figure.
The current sensitivity (blue) can be explained by the presence of dark (cyan) and shot (red) noises between 400 Hz and 1 kHz.

We selected the DARM displacement spectrum recorded at 17:00 UTC on June 14th.
To calibrate the dark noise and shot noise, we required the calibration factor H, which represents the ratio between the error signal (K1:OMC-TRANS_DC_SUM_OUT_DQ) and DARM displacement (K1:CAL-CS_PROC_DARM_DISPLACEMENT_DQ), as well as the open-loop transfer function 1+G.
We obtained H by comparing the spectra of the measured error signal and the DARM displacement.
By multiplying the measured dark noise and estimated shot noise with H / |1 + G|, we projected their contributions as shown in the figure.

Even below 400 Hz, the contribution of dark noise is not significantly smaller than the measured noise floor.
Therefore, efforts should be made to reduce the dark noise.