I tried some DRMI locking again tonight, but had no success. Here is the story.

• I started out by going to the AS table and measuring the light level on the REFL55 photodiode (with PRM aligned and the PRC flashing, but LSC disabled).
  • The Ophir power meter reads 13mW
  • The DC output of the photodiode shows ~500mV on an oscilloscope.
  • Both of these numbers line up well with measurements I made in April/May.
• Returned to the control room and aligned the IFO for DRMI locking - but LSC servos remained disabled.
  • At the nominal REFL55 whitening level of +18dB, the REFL 55 signals saturated the ADC (confirmed by looking at the traces on dataviewer).
  • But the signals still looked like PDH error signals.
  • Lowering the whitening gain to 6dB makes the PDH error signal horns peak around 20,000 counts.
  • Could this be indicative of problems with either the analog whitening gain switching or the LSC Demod Boards? To be investigated.
• Tried enabling LSC servos with same settings with which I had success right up till a couple of months ago, but had no success.
  • If it is true that the REFL55 signal is getting amplified because of some gain stage not being switched correctly, I should still have been able to lock the SRC with a lowered loop gain - but even lowering the gain by a factor of 10 had no effect on the locking success rate.

Looks like I will have to embark on the REFL55 LSC electronics investigation. I was able to successfully lock the PRC on carrier and sideband, and the Michelson lock also seems to work fine, all of which seem to point to a hardware problem with the REFL55 signal chain.

I did a quick check by switching the output of the REFL55 demod board to the inputs normally used by AS55 signals on the whitening board. Setting the whitening gain to +18dB for these channels had the same effect - ADC overflow galore. So looks like the whitening board isn't to blame. I will have to check the demod board out.