[Tanaka, Ushiba]

Abstract:

To avoid QPD saturation during 10W operation of IMC, we changed the gain switch from high gain mode to low gain mode.
This change reduced the QPD signals by a factor of 5, which is enough to avoid saturation for 10W operation.

Detail:

We planned to reduce the laser power by a factor of 2 by inserting BS in front of MCET and IMMT1T QPDs to reduce the beam power by a factor of 2 to avoid saturation during 10W IMCoperation.
However, we noticed that the gain switches for those QPDs are high gain mode, which can reducethe signals by a factor of 5.
So, we just changed the gain switches this time.

What we did:

1. Wait until IMC ASC made the alignment good.
2. Close laser shutters, measure the dark offsets of each segment, and subtract them.
3. Change the gain switches to low gain.
4. Measure the dark offset and subtract them
5. Lock IMC without ASC an confrirmed that QPD signals doesn't change.
6. Engage IMC ASC and confirm it works well.

After changing the gain of MCET and IMMT1T, we could lock PRFPMI with RF signals without any problems.
So, our work doesn't seem to change the alignment.
Now, the QPD signals are small enough, we can operate IMC with IMC ASC.