Keita, Sheila
We had another example of the squeezer oscillations that were seen last weekend: 54864 The first screenshot shows the signals related to gree power in the OPO getting very noisy over ~ 6 hours this morning. We went out of observing earlier than our planned commisioning time to try to address this. We turned off the green ISS, which brough all the signals back to normal. Turning that loop back on brought the noise back. We also tried reducing the gain in that loop, which did reduce the peak to peak noise seen in the ndscope. Based on loop measurements taken on Tuesday, 54888, the shape of this loop at its nominal 31dB gain setting should be fine.
Next I went to the floor, while Keita took spectra and changed the gain from the control room. On the floor I measured a spectrum of IN1 (the error point) on the green ISS, and I saw large lines at 15kHz and 30kHz. Keita saw that there is an aliased line at 1249Hz in the spectra in the control room, which would correspond to a line at 15135 Hz. The height of the 15kHz and 30kHz lines didn't change with gain changes, although the overall noise floor did change. Ketia disabled the loop, and I could no longer see the lines on the error point, and the low frequency intensity noise increased (as you would expect).
We then unlocked the squeezer by requesting no squeezing, and relocked it. We saw no evidence of the 15kHz and 30kHz lines on the analyzer, although the red trace in the second attachment shows that the 1kHz line is still present. We reset the ISS gain to 31Hz, which is nomal, and everything seems fine for the time being.
Photos attached (after screenshots):
- 1st spectrum of ISS error point with gain change (reference is lower gain, I think this was 20dB and 26dB)
- 2nd same reference as 1st photo, ISS error point with loop open
- 3rd same reference as above, after OPO unlocked and relocked no sign of 15kHz and 30kHz lines.
For operators:
The first attached screenshot is from the ndscope that is accessible from the upper right corner of the SQZ_OVERVIEW screen, called locking signals. If you see the range dropping, open this ndscope and see if there is trend similar to what is shown in the attached screenshot that lines up in time with the range drop. In the left column of the screenshot you can see that the OPO REFL power is changing and the max-min difference gets larger, the max-min difference of OPO_IR_RESONANCE trigger gets larger, and the SQZ_SHG_LAUNCH power max-min difference also gets larger. If this is happening and corresponds to a range drop, please unlock the squeezer by requesting "NO_SQUEEZING" from SQZ_MANAGER, then once it arrives, request SQZ_READY_IFO. Once it arrives there hopefully the signals in the ndscope will have returned to normal, if so you can request "SQUEEZING" again. If not you can try again or call for help.
Tagging OpsInfo to propagate the message
During today's calibration time, this oscillation happened again, and Patrick called me so I could make a few more observations.
- There are two PZT positions in which the OPO can lock, when the PZT voltages is towards the high end of the range the reflected power is higher because of a misalignment in the OPO (and the trans power is servo'd). Looking at the trend over the last couple of weeks, the oscillation has only happened when the PZT is in this position and the reflected power is high. The two cursors show examples of this in the attachment, but there are several more examples without cursors. This suggests that we could avoid this problem with a guardian or setting modification that only allows us to use the lower PZT position.
- The state of the CLF loop and CLF ISS seem to have no impact on the oscillation.
- The oscillation stops and starts depending on the OPO green power level, not the AOM drive level:
- I could stop the oscillation and see it repeatably return if I turned the green ISS set point down (more positive set point meaning less green light in OPO trans). In the second attachment, the OUTPUTRAMP channel is 0 when the servo is on and 1 when the servo is off. You can see that at three different times, I increased the ISS_SETPOINT while the servo was on and each time that the setpoint reached -2.582 the oscillation stopped, as you can see by the OPO_TRANS peak to peak dropping. The OPO trans is also noisy when the servo is off, which might be confusing when looking at this plot.
- I tried moving the green power control half waveplate in small steps (which didn't unlock the OPO), to change the operating point of the AOM. The oscillation happens repeatably at the same OPO trans power, even when the AOM set point has changed. In the second attachment you can see that when I repeated the set point stepping with the picomotor moved, the ISS CONTROLMON signal is at a sligthly different value each time we get to the ISS setpoint of -2.852, but this is still the point where the oscillation stops.
