I was investigating the beat note amplitude on the vertex PD again yesterday. The incident power on the PD was 150uW in the PSL green beam and 700uW in the X-ARM green beam. With perfect overlap and a transimpedance of 240, I expected to get a beat note signal of around 25mV or -19dBm. Instead, the size was -57dBm. Bryan and I adjusted the alignment of the green PSL beam to try and improve the mode overlap but we couldn't do much better than about -50dBm. (The noise floor of the PD is around -65dBm).
When we projected the beams to the wall of the enclosure, the xarm beam was 2 to 3x as large as the PSL green beam, indicating that the beam size and/or curvatures on the PD were less than ideal. There is a telescope that the XARM beam goes through just before it gets to the PD. I mounted the second lens in this telescope on a longitudinal translation stage. With some finagling of the position of that lens we were able to improve the beatnote signal strength to -41dBm.
Obviously the ideal solution would be to measure the beam size and RoC of the PSL beam and XARM beams and then design a telescope that would match them as precisely as possible because there's still another 20dB signal strength to be gained.