J. Oberling, P. King
Quick and dirty summary: mode matching work is still ongoing.
All this work was done with the ISS OFF and the ISS AOM removed from the beam path. Using the beam propagation measurement we took yesterday, Peter generated a mode matching solution and implemented it this morning. Tweaking the lenses around, we saw as much as ~53W transmitted through the PMC, but couldn't get any more than that. At this point the visibility (as calculated by measuring the locked/unlocked voltage of the PMC locking PD) actually looks pretty good at ~91%, but there is something odd going on: the power transmitted through the PMC is lower than the visibility suggests it should be. For example, we currently have ~51.8W transmitted through the PMC and ~14.2W reflected, giving a total power at the PMC of ~66W; this means we are only transmitting ~78.5% of the PMC incident power. Ideally, with a visibility of ~90% we would expect to be transmitting closer to 59.4W. We are currently looking for things that could cause this behavior, as it indicates our PMC throughput issue is not caused by shoddy beam quality.
Another odd bit, the visibility gets lower as the power increases. I measured the visibility at several different input powers:
| Input Power (W) | Visibility (%) |
| 40.3 | 96.2 |
| 50.1 | 93.3 |
| 55.2 | 92.6 |
| 60.2 | 92.0 |
| 65.0 | 91.4 |
This also appears to indicate that something other than the beam quality is causing our PMC throughput issues. Inspecting some of the optics in the beam path we found that both mode matching lenses were dirty; I cleaned them but it didn't appear to help at all. Cheryl also pointed out that there appears to be some scatter issues with mirror AMP_M03. While we didn't do anything with this mirror today, this is another item we can check. We are going to think on this overnight and get back to it in the morning.