The OMC scan was performed last night after the chamber was covered. While the scan showed noimnal resonant features of the slightly misaligned cavity, I saw negative trips of the DCPD outputs (both) when the photocurrent is of the order of ~0.1mA.
The attched plot shows the refl QPD sum (CH1), scanned PZT Voltage (CH2), and the DCPD A/B outputs (CH3/4). The output of the DCPDs are usually positve, while they trip to negative occasionally when the cavity hit the major resonances. The positive voltage segments show nice resonant features when they are plotted in a logscale.
The refl QPD sum shows that the light was nicely absorbed by (transmitted through) the OMC. So it seems it is not optical.
I quickly checked the power supply condition of the field rack and confirmed that the related chassis are on.
We want to figure out this feature by the time we close the dorrs next week as the DCPD preamps are located in HAM6.
First thing that comes to mind would be to check the status and operation of the OMC piezo shutter for anything spurious... The dips are perfectly common to both diodes, so I would also look at the Split Whitening for: Bias to the PDs, DC power as supplied by the OMC Split Whitening Interface, and give special scrutiny to the most recent changes to the split whitening involving AC coupling of the OMC DCPD Chain for PI use.
Robert, Richard, Koji, Stefan
Bottom line: We think that this is simply the due to acousting vibrations modulating the OMC transmission in air - leading to ADC saturations. We thus decided to go ahead with the install.
Details;
We noticed that the drop to negative voltages is due to ADC saturations. When reducing the light to avoid saturations, we find that the RIN on the light is ~0.25 RMS. It is dominated by a peak around 1 kHz, and is coherent with the microphone.
Note that the OMC fringe width is only on the order of 1 nm, and its body mode is around 1kHz. It is not surprising that the thing vibrates more than that when in the horrendous acoustic environment of an open HAM chamber.
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We verifyed with a light bulb that the electronics woks well up 0.3mA of current, which is higher than the previously observed problem. Case closed.