[Shih-Hong Hsu, Chia-Jui Chou, Hirotaka Yuzurihara, Yi Yang]

    This report is about the recent analysis of the scattered light noise in MICH. First, following the method mentioned in klog-33042, I checked the relation of the seismic motion near BS (K1:PEM-SEIS_BS_GND_{X, Y, Z}_OUT_DQ) and the occurrence of scattered light noise in MICH. The result shows there is a positive correlation between the number of occurrences of scattered light noise in MICH and seismic motion in band 0.3 ~ 1 Hz near BS. (Fig 1)

    The second part is about two dates where the seismic motion is larger than the typical value. That is, 4/13 and 4/29. In these two dates, the amplitude of seismic motion in 0.3 ~ 1 Hz is around 0.2 units (from BLRMS time series: K1:PEM-SEIS_BS_GND_{X,Y,Z}_BLRMS_300MHZ1000.rms). (Fig. 2 / 3 / 4) On other dates in April, the average strength is about 0.1 unit. The number of scattered light noise labels obtained by the analysis is 372 in 4/13 and 352 in 4/29 in MICH. (Fig. 5 / 6 upper)

    Due to the large number of scattered light noise found in MICH, I further check if there is any coincidence or potential contamination of the scattered light noise into the strain channel (K1:CAL-CS_PROC_DARM_STRAIN_DBL_DQ). I used the data of MICH channel (K1:LSC-MICH_OUT_DQ) and strain channel to make a 30 s time-frequency map (Q-spectrogram) (Fig. 7). 

• We can see there is a clear arch-shaped pattern of scattered light noise around the 30 Hz region in MICH. As for strain channel signal, at the same time the arch-shape happened in MICH, there is signal excess in the strain channel also around the 30 Hz region. 

• By imposing a stronger constraint on the strain channel signal, which checks only 20 ~ 40 Hz of the signal. We can also make the scattered light noise labels for strain channel. (the concrete procedure of getting the labels: JGW-G2416344) As a result, the number of labels is 34 in 4/13 and 35 in 4/29 in STRAIN (Fig. 5 / 6 lower)

• To figure out the cause of scattered light noise being seen in the strain channel, we first checked the feed-forward control between MICH channel and strain channel, which is K1:LSC-MICHFF1_OUT_DQ. When there is the scattered light noise in the MICH channel, the feed-forward channel can also see the scattered light noise. When the seismic motion is normal, we can't see the scattered light noise in the strain channel. So, we guess that the scattered light noise in the strain channel comes from the residual of the feedforward control. We will continue to monitor it. (fig. 8)