[Miyakawa, Ushiba (remote)]

Abstract:

We found that the signal generators for IMC LO (13.77 MHz, 7.4dBm) was OFF.
After turning on it, IMC can be locked.
However, IP1 PIT output was saturated, so it might be necessary to offload it at some point.

Detail:

Miyakawa-san checked the circuits related to the IMC lock (detail will be posted later by Miyakawa-san) and found that signal generators for IMC LO was turned off.
After recovering the LO, we tried to lock IMC and it can be locked without any problems.

After that, IMC ASC was engaged and IO guardian reaches PROVIDING_STABLE_LIGHT state.
However, one of the PZTs (IP1 PIT) is saturating.
Since all workers has already been out from the mine, we keep it as it is.
So, it might be necessary to offload it at some points.

Following the procedure described in klog:35425, I set the frequencies and amplitudes of the LOs for IMC, PLLX, PLLY, Gr X PHD, and Gr Y PHD around the IOO rack to the specified values, and turned on their RF outputs.

• Only the PLLY LO had kept its settings from before the power outage. All the other signal generators had been reset when the power was lost.
• The PLLX LO had been set to remote-control mode, so I first disconnected the LAN cable, switched it to the local mode, configured the settings, and then reconnected the LAN cable.

I then entered the PSL room and checked each circuit.

• Most of the electronics automatically returned to their previous states after the power was restored, and there did not seem to be any particular problem. I also confirmed that the HV amp was actually providing approximately +/-191 V.
• I monitored the output of the SR560 just before the signal goes to the laser PZT with an oscilloscope. I confirmed that some signal appeared when the common-mode servo attempted to acquire lock.
• I also monitored the output of the HV amp just before the signal goes to the wide-band EOM with an oscilloscope. Again, I confirmed that some signal appeared when the common-mode servo attempted to acquire lock.

Since the IMC still did not lock properly, I left the PSL room and checked the signal generators at the IOO rack again. I then realized that I had turned on the LO for the main interferometer instead of the one for the IMC. After correctly configuring and turning on the IMC LO, the IMC locked successfully.

In the end, simply looking at the feedback signals does not allow us to distinguish whether they are the correct signals or spurious ones. If necessary, it would be better to check the error signals as well.

Hirose, Ushiba(remote), Miyakawa

1. IMC ASC

The PZT for the IMC ASC was saturated. It is sometimes possible to avoid ASC saturation by changing the IMC LSC offset, but this time the required correction was outside the available range. We entered the PSL room and offloaded the PZT.

The pitch of PZT1 was saturated at +75 V. Hirose-san turned the adjustment screw clockwise and, with the control-loop gain reduced so that the feedback signal was almost zero, adjusted it until the error signal was close to zero.

After restoring the gain and closing the feedback loop again, the output signal to the PZT became close to 0 V, and the ASC control started working properly.

2. No light visible from the FNC

Since the green light was not visible on the POP or POS tables, we checked the green Fiber Noise Cancellation (FNC) paths on the PSL table.

The behavior was similar for both X and Y. In both cases, when we blocked the path to the fiber in one arm of the Michelson interferometer, no returning light was observed from that arm. The light from the other, short arm was returning, but its power was too low to be detected by the Michelson PD.

In fact, the optical power just before the Michelson BS itself appeared to be quite low. Sufficient optical power was present farther upstream, closer to the light source.

Since the same behavior was observed for both X and Y, the cause was expected to be something common to both paths. Ushiba-kun noticed that there might be a problem with the AOM, so we left the PSL room and checked the VCO signal generator for the AOM at the very bottom of the ALS2 rack.

Actually, the signal generator settings had been reset due to the power outage. After setting it to 70.00 MHz and 13.0 dBm and turning on the RF output, light became visible on the Michelson PD, and the FNC could also be locked successfully.