[Kiwamu, Sheila, Stefan, Paul]

Today we tried to get the Faraday isolation ratio measurement using the method Chris Mueller described in his LLO alog 8608, but were unsuccessful so far. 

First we had to restore the IMC and several other optics to their previous state after the power outage. We tried looking for the return beam with the viewer cards, but we didn't find any beams that were not present when the mode cleaner was unlocked (as the true PRM reflection beam would be). We expect this is mainly due to misalignment of the return beam from PRM with respect to the IMC, such that little of the return beam actually makes it back through the IMC, instead being reflected from MC3. At LLO when making this measurement this alignment state was already know well since the central IFO alignment can be used as a reference there.

We tried scanning the PRM alignment, but to no avail. We checked at what PRM alignment offsets the REFL beam on ISCT1 becomes clipped, because this gives a good indication of the alignment parameter space for PRM which we need to search within: if the FI rejected beam is not making it back through the Faraday correctly, then the beam we're looking for is certainly is not making it back through the IMC. The beam became clipped within ~±500 urad of our inital alignment values in pitch and yaw. As a rough comparison, IMC transmitted power can be expected to halve for a MC2 pitch misalignment of 60urad, so it looks like we still have some alignment parameter space to search before we would even expect to see the return beam

The other issue we were dealing with was the back reflection from the second mode matching lens, which was close to parallel (~1 deg) with the forward propagating beam. We aligned a PDA55 to this beam as reflected from the second pick off beam-splitter in the path, since it serves as a reasonable reference for the sought after PRM return beam. In order to help us locate the PRM return beam, as well as for the reason that it had to be done at some time anyway, we decided to rotate this lens (see Cheryl's post from yesterday). We were able to do this, apparently without causing too much change in the alignment into the IMC since the IMC transmitted power recovered to its previous value (~1240 counts on MC2 trans) with the WFS loops engaged. The back reflected beam from the second mode matching lens is now easily separated from the forward beam, and has been dumped on a razor blade dump. We estimated the power in this beam to be around 1mW in the absence of the pick off beamsplitters and with the power through the EOM at 1W.