M. Todd, E. Capote, J. Wright, S. Dwyer

We are attempting to understand discrepancies between model estimates and measurements of various parameters in the corner cavities. In particular, we have measurements of the PRC and SRC Gouy phase [alog 66215, alog 66211], along with input mode (IMC) overlap with the OMC mode from OMC scans [alog 87342], and beam profile measurments at the REFL and POP port in single bounce, with ITMY misaligned [alog 84307].

Using the LHO O4 yaml file in the modeling repo, we can model what we expect these values to be given our understanding of the optics geometries and lengths between optics. We've already identified a few issues with the model as several estimates disagree with the measurements. The first example of this was the SR2 position relative to the SRM and SR3. It was discovered that the model had not accounted for a 5mm shift of the SR2 towards the BS, adding roughly 5mm to each of the corrresponding lengths in the model.

This does not explain all of the discrepancies with measurements, however, so we are exploring how uncertainty in optic geometries can affect the model. Below is a table summarizing this effort so far.

The PR3/SR3 uncertainty in the RoC is +/- 6mm. The PR3 LIGO reported RoC is 36.021, the vendor reports 36.006 mm. The SR3 LIGO reported RoC is 36.013, the vendor reports 36.00. At each bound of these uncertainties, we analyze the beam overlap between the Input mode propagated to the POP port with the q-parameter fit from the measurements. We also analyze the mode overlap between the Input mode and the OMC mode, with ITMX misaligned (this was measured to be 91.7%).