We took a look at several ALS PLLs to check if the ALS DIFF/COMM glitches we saw last night were associated with them. From Plot 1 we see that both the COMM and DIFF PLL control signals glitch strongly at the same time as the ETMY coils. Plot 2: We ran a test where we locked ALS COMM normally and looked for glitches, which we found (red). Then we shuttered the Y arm and the glitches in ALS_COMM went away. Seems that the COMM PLL was partially polluted by Y arm light, which is close in frequency to the X arm light. Solutions being considered are fixing the ISCT1 alignment/changing the polarization of the X vs Y transmission beam, or moving the VCO frequencies. Attachment 3 shows the beat frequencies of the entire ALS system. Seems that the X and Y arm beat frequencies are within 10s of kHz and the X and Y beatnotes could be getting confused on ALS COMM.
Georgia, Keita, Craig Georgia installed a 532 nm half wave plate at ISCT1 in the Y path while Keita and I installed a HWP at End Y to try and reject Y light from the ALS_COMM beatnote. We were unable to eliminate glitches in ALS COMM PLL CTRL.
Note: The half wave plate installed on ISCT1 is on the ALS-Y path, directly before the combining beamsplitter for the ALS-DIFF beatnote, i.e. between ALS-BS6 and ALS-BS7 on this handy layout of ISCT1.
After Keita and Craig installed the half waveplate at end-y, but before I installed the half waveplate on ISCT1, I noted that there was still a substantial beatnote on the DIFF PD. If all had gone to plan the X and Y path on ISCT1 should have been in orthogonal polarisations at this point. With the waveplate installed I could increase the beatnote pk-pk by a factor of 2.
Maybe somewhere along the line (steering optics, vacuum windows?) we are picking up some circular polarisation.
Investigations ongoing.
Just a note that LLO dealt with ALS-DIFF glitches by changing the beam interference on ISCT1, so that instead of interfering the ALS X beam with the ALS Y beam, each ALS arm beam is interfered with the PSL frequency-doubled beam. This is described in LLO log entry 28005.
Associated with FRS Ticket 11113.
Note for future reference:
This problem was caused by electrical cross coupling, not light..
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=43002