Marc, Michael, and Yuhang
Yesterday, we made some measurement of FDS with WFS based AA.
The measurement is flat until almost 30Hz. We have also seen more than 1dB squeezing from 30 to 60Hz. However, the high frequency squeezing level was only 2dB (can not be higher by changing LO phase). This is very different from what we understood.
Anyway, I tried to use the old code to fit FDS. The fit agrees well with some measurements, but not for all. As shown in the attached figure, especially the measurement which should have squeezing at high frequency couldn't be fit by the code.
We need to investigate more about this result.
Mode mismatch between filter cavity and LO was found to be relatively high. And this results in homodyne detection effeciency to drop by about 13.3%. Together with the bad mode matching inside filter cavity reported in elog2503, we could explain worse FDS measurement.
As shown in the attached two figures, the TEM00 peak is 1.19V while the LG01 peak is 0.104V. This corresponds to 8.0% mode mismatch. Note that this spectrum is after the optimization of alignment and filter cavity half-detuned.
We have tried to reduce this LG01 mode by moving the mode matching lenses. However, the mode matching can be barely improved.
To search for the reason of this mode mismatch, we checked the beam position on every in-air optics, we found no clipping issue.
We have also tried to measure the power loss, the total power loss from after the in-air Faraday to before homodyne is about 19%. This is in-agreement with the old measurments.
For the in-vacuum part, we could try to scan the injection steering mirror yaw or pitch slightly and see if there will be a clear power drop. We will try with this method to check if there is in-vacuum clipping.
If there is not clipping found, we need to first understand why this could happen. In principle, for our optical system, there should not be such large mode matching change. In the worst case, if we couldn't figure out what is causing this problem, we will need to measure the beam parameter again and redesign the telescope.