The BS issue has been checked again today by switching off the oplev laser since I was suspecting that BS PSD maybe broken. Indeed, after switching off BS oplev laser, the BS oplev signal is as Fig.1 which is the same as what is taken last Thursday. So the strange signal we are seeing is from the broken BS PSD.

To confirm the health of BS suspension, we need to take an another PSD and use it to characterize BS suspension.