Sheila, Dan Brown, Alexei, TVo
Because we were a bit distrusting of last night's OMC mode scan measurements, we looked at the beam quality going into HAM5 from the squeezer and it showed a pretty nice symmetric Gaussian beam.
However, when we looked at the beam quality coming into HAM6, we saw that the vertical direction was 1500um and horizontal was 2100um at about 1.36 meters upstream from OM1. Which means something in HAM5 is causing this odd astigmatism. Dan roughly calculated that the differential pressure between the air side and the vacuum side wil result in a ~hundreds of meter lensing effect and if we're going to the lens an offset+misaligned angle then it's possible we could see some astigmatism, but not to the level that we're seeing here. A thought could be the Faraday, but it's not clear how to definitively determine that.
We were able to see clipping with the Nanoscan on the IFO side and tried to get rid of this effect by moving around ZM1 and ZM2. We were able to bring the astigmatism down from [1500um, 2100um] to [1500um,1800um], which is still a lot but we decided to give this configuration another OMC mode scan with various lens positions, which Dan will post as a comment. After the fact, we measured some more beam profiles to try to fit more q-parameters on the IFO side as well as OPO in the further field on SQZT6 with the beam diverter.
Attached are the OMC scans using the OPO beam after reducing the astigmatism mentioned above, these are looking much cleaner compared to last night. Far, close, and middle refer to the lens translation stage position relative to the OPO. Taking the ratio of the 2nd order and 0th order peaks we can estimate the mismatch as: close = 89%, middle = 90%, and far = 87%, which isn't as high as we'd hoped. Analysis of the beam profiling of the new lens setup to come.