Sheila, Jason, Evan G, Krishna
Mode hoping of the ETMY oplev has been showing up in hveto since April 19th, although the oplev damping is off. The glitches that show up are definitely glitches in the sum, and the oplev is well centered, so the issue is not that the optic is moving. There is a population of DARM glitches around 30 Hz that is present on days when the oplev is glitching but not on other days. We are curious about the coupling mechanism for these glitches and wonder if this coupling could be causing problems even when the oplev is not glitching loudly .
Evan, Jason and I connected the monitor on the oplev laser diode power to one of the PEM ADC channels used to monitor sus rack power (we used H1:PEM-EY_ADC0_14_OUT_DQ, which was monitoring the +24V power and is channel 7 on the field rack patch pannel. Jason can make the laser glitch by tapping it, with this test we saw clear glitches in the sum but no sign of anything in the monitor so this monitor might not be very useful. Plugging this in means that the lid of the cooler is slightly open.
We also unplugged the fiber, so that for the time being there is no light going into the chamber from the oplev. If these glitches are coupling to DARM electromagnetically, we expect to keep seeing them in DARM. If they were somehow coupling through the light (radiation pressure, something else), we would expect them to go away now. One glitch that we looked at is about a 75 uW drop in the laser power on the optic. (A=2P/(c*m*omega^2)= 3e-19 meters if all the power were at 20 Hz). We don't really know how centered the beam is on the optic, or what the reflectivity is for the oplev laser, but it seems like radiation pressure could be at the right level to explain this.
Using an ASD of the oplev sum during a time when the oplev is quiet, this noise is more than 3 orders of magnitude below DARM at 30 Hz.
The fiber was disconnected at ~19:05:00 25 April 2017 UTC. There will not be any Hveto correlations after this time because the OpLev QPD will not be receiving any light. We will be looking at Omicron histograms from the summary pages to determine whether this is the cause of noise.
With this test over, I have reverted the above changes; the ETMy oplev is now fully functional again, as of ~18:30 UTC (11:30 PDT). I also unplugged the cable we used for the laser diode monitor port and reconnected H1:PEM-EY_ADC0_14_OUT_DQ so it is now once again monitoring the +24V power on the SUS rack.
To address the glitching, I increased the oplev laser output power; using the Current Mon port on the back of the laser:
- Old: 0.865 V
- New: 0.875 V
The laser will need several hours to come to thermal equilibrium, as the cooler was left open overnight (as a result of the above test). Once this is done I can assess the need for further output power tweaks.
If the glitch happens tonight while Jason is unavailable, leave it until tomorrow when Jason can make another attempt to tune the temperature.
Even when H1 is observing, operators can go out of observing, let Jason work for a short while, and go back into observation as soon as he's done.
But it's clear that we need a long term solution that doesn't include intensive babysitting like this.