This afternoon, thanks to the 1 MHz channel PSTAB_HF_CORR that Alain just added, we were able to compare the glitches occurring just before fast unlocks (already mentioned in 44457).
We took the IMC unlock which occurred at 15:24:51 UTC, and zoomed on the first glitch, shown on figure 1.
Figure 2 shows 3 channels, all with 1 MHz sampling:
- INJ_EOM_CORR is the correction sent from the IMC rampeauto to the EOM (the 14 MHz on the LB, just before the slave laser)
- INJ_IMC_REFL_I_PRE is the IMC error signal (reflection from the IMC, demodulated at 22 MHz)
- PSTAB-HF_CORR is the correction sent from the PSTAB rampeauto, whose photodiode error signal is located on the lower part of SIB1 and monitors a pick off of the trasmitted beam. The correction is sent to the AOM just before the PMC.
From the 2nd plot seems quite clear that the glitch occurs at first on the EOM CORR signal (blue line), and we found it with 6 microseconds delay on the IMC error signal (orange line) and finally on the PSTAB correction (green line)