M. Todd, S. Dwyer

Overall Summary: we cannot learn enough from the POP measurements in HAM1 to constrain the values of PR2 and PR3.

We double checked all the distances reported from alog 84307 and the distances of the POP lens in the model, and we found a 3(ish) cm discrepancy. We corrected this discrepancy before doing any fitting or analysis. We use the portion of the measurements in front of the POP lens (towards the POP LSC diode),  as they contain enough information at each Gouy phase region in the beam evolution, to fit for a q-parameter in front of the POP lens. We can propagate this to where the other measurements are behind the POP lens to get an understanding of the focal length of the POP lens. Figure 1 shows a plot of the fitted q from the portion of measurements compared to the ones not used.

We saw that this q-parameter was not well collimated behind the POP lens, making us think that the focal length may not be exactly what is in the model/reported. We adjusted the focal length by +2% (reducing lensing power) as is listed in the tolerance of the optic (see D1300313), and found much better agreement with the un-fitted measurements. This is shown in Figure 2, which shows that the overlap with the measurements before the POP lens is better, but the overlap with the model q-parameter is not as good as we would like.

We then increased the radius of curvature of ROM RH3 mirror (in HAM3) by +1%, and saw that the model beam becomes well overlapped with the fitted q-parameter fromt the measurements. This is shown in Figure 3.

Overall, I think this tells us that because we can tweak the parameters of the lenses in the POP path within their specs to give us the overlap we want with the Input mode through the PRC, we cannot use these measurements as a constraint on the values of the PRC telescope optics. It would be great if we could get a few beam profile measurements in transmission of PR2, without the POP lens or RH3 mirror but getting the IMC locked while HAM3 is vented seems unlikely.