[Sheila, Jenne]
Although the squeezer seems to not have gone into oscillation since last Thursday when we lowered the green power (alog 55339), we are looking to see if we can better understand the cause and ameliorate it (spoiler alert: so far we don't know, although we have further tests to try today).
Here's what we've done so far:
- Noted that the OPO_PZT_1_VOLTS was 57.4V and had been drifting downward lately. Recall that we don't tend to see oscillations unless the OPO locks with the PZT above 60V.
- While on the phone with Sheila, I moved the green ISS setpoint (H1:SQZ-OPO_ISS_SETPOINT) to bring the OPO_TRANS_DC_NORMALIZED power up until we started to see oscillations.
- The oscillations started when the TRANS_DC was just shy of 1.7.
- This brought the SHG_LAUNCH_DC_POWERMON to about 17.7mW, which is under the threshold of 20mW (to prevent fiber degredation we prefer not to launch too much power in).
- Interestingly, the OPO_REFL_DC_POWER was slowly increasing as I increased the green power, but then it jumped up a lot when the oscillations started.
- In the first attachment (IncrPowerToStartOsc.png) you can see that the TransDC and ReflDC are increasing as I change the ISS setpoint, but around -4490sec the REFL power jumps up and we start oscillating.
- We turned off the green ISS using the Servo/Ramp button. The oscillations were gone, but the power had gone down since we were no longer servoing to that ISS setpoint. Turning the ISS back on brought the power back up and the oscillations back.
- Turning off the ISS is shown at the very end of the first attachment, around -4455sec.
- I went out to the squeezer rack and changed which of the 2 PZTs that can be used to lock the OPO we were driving.
- We have access to both via the squeezer interface chassis in the squeezer rack's slot 12, but only one driver.
- Sheila had brought the squeezer manager guardian to DOWN, and ensured that the PZT voltage out was near zero (~1V). Richard confirmed that it should be totally fine to switch the cables on the back of the interface chassis.
- The driver had been plugged in to PZT1, and a terminator was on the PZT2 SHV connector. The second attachment (PZT_connectors_before_change.jpg) shows their configuration before my change.
- I unplugged both the cable and the terminator, and put the terminator onto the PZT1 connector and the cable from the drivere onto the PZT2 connector. The third attachment (PZT_connectors_after_change.jpg) shows their configuration after my change.
- Sheila requested the squeezer manager to relock. Using PZT2 (although the channels are still called PZT1) the OPO locked at around 26.5V.
- The oscillations were really bad, although that was partly because the ISS was railed. (The voltage limiter at the output of the ISS had a red box around it.)
- We turned off the ISS and the oscillations went away.
- We returned the ISS setpoint to the value we'd been using that gave green power of 1.2, then reclosed the ISS and the oscillations were gone.
- We moved the ISS setpoint to re-increase the green power, and the oscillations showed up just shy of 1.6 (a little lower than when we were using the original PZT).
- This must have been very close to the threshold where the oscillations start, because they seemed to breathe, and then went away on their own.
- In the fourth attachment (PZT2_oscGoAwayByThemselves.png), the oscillations start near the left side of the plot, and decay away by about -2540sec.
- I increased the green power a small amount, and the oscillations came back.
- I decreased the green power and the oscillations went away, so this was definitely on the threshold of where the oscillations are happening.
- The increase and decrease of power happened around -2500sec on the fourth attachment.
- This must have been very close to the threshold where the oscillations start, because they seemed to breathe, and then went away on their own.
- Sheila changed the locking parameters for the squeezer guardian (the voltage at which it starts the scan for locking the OPO), and relocked the OPO at a higher PZT voltage. It ended up being about 58.5V.
- The oscillations were present. I reduced the green power a small amount around -2385seconds in the fifth attachment (PZT2_higherVoltage.png).
- The oscillations in the TransDC and ReflDC got a bit smaller, but the oscillations on the launch power completely went away. This is new behavior.
- The oscillations were present. I reduced the green power a small amount around -2385seconds in the fifth attachment (PZT2_higherVoltage.png).
- At this point, we are thinking about other tests, so we leave the PZT driver connected to PZT2 (we'll want to revert this at the end of our tests today). The squeezer manager guardian is in DOWN.
Jenne cabled the LO PZT driver channel (used only for locking with the diagnostic homodyne, not during runing) to the OPO PZT2. The attached screenshot shows that with the OPO locking using PZT1 (blue line, around 50V), we can move the offset on PZT2 (LO channel, blue line around 0-20V) to reduce the voltage on PZT1. You can see that the reflected power increases when there is a higher voltage on PZT2, which is on one of the curved mirrors, because the cavity gets more misaligned from driving PZT2 than PZT1.
We tried but weren't able to recreate the oscillation a second time, so we don't know if moving around the offset on the second PZT will have any impact on the instability. We can try to move this offset if the instability does reappear, although it might not help if the problem is related to the misalignment of the OPO that the high PZT1 voltage causes.
Keita was wondering if part of the difference (that prevented us from recreating the oscillation) was the fact that both PZTs were connected to cables, neither of them was connected to a terminator. So, I reverted PZT2 back to being a terminator (and re-plugged in the LO cable to the LO PZT output of the driver). Now everything cable-wise is back to how it was this morning. However, I still wasn't able to cause oscillations, even if I brought the green power up above 1.8.
So, for now I'll leave all of the cables as they are, which is fully reverted from the tests today. If we see the oscillations come back, then we'll reconsider again borrowing the LO driver channel for PZT2.
When I left things at a green power of 1.3, I had also reverted the squeezer angle, but had forgotten to revert the OPO crystal temperature, H1:SQZ-OPO_TEC_SETTEMP. I have now reverted it to its value from before last Thursday's green power change (33.014 C). I did not measure the nonlinear gain. For this, we were out of Observing for ~1 minute.