J. Oberling, C. Vorvick

Today I began tuning up alignment and mode matching into the PMC and RefCav.  I began by tweaking alignment remotely from the control room before entering the PSL enclosure; all work was done with the ISS OFF.  The changes are summarized in the below table:

I then attempted to adjust the operating current and temperature for the pump laser diodes, but did not see any improvement so I left them as found (documented here).  As can be seen, the RefCav transmission was completely recovered with a simple alignment tweak, therefore no adjustment to the FSS will be needed.

Upon entering the enclosure, we began by taking a beam propagation measurement of the 70W amp output.  This is done by picking off the beam to the bullseye PD and directing it down a rail and into a WinCam beam profiler.  I've attached a picture of the beam profile, which can be compared to the last profile taken in November 2018 (documented here); the picture in the linked alog is rotated by +90° w.r.t. the image attached to this alog, due to a different orientation of the WinCam).  The halo now looks a little more pronounced and the beam isn't quite as round as it was (attenuation settings are different (different ND filters, don't remember what we used last November), therefore I can't really say how much of a change this is).  I'll analyze the beam propagation data and attach the fit as a comment to this alog.

This done, we began adjusting the position of the PMC mode matching (MM) lenses.  We started by tweaking the 2nd MM lens in both directions, looking for improvement.  Once a better location was found, we then adjusted the 1st MM lens, then returned to tweaking the position of the 2nd, once again looking for improvement (if it doesn't get better, adjust the 1st MM lens in the opposite direction, then go back to adjusting the 2nd).  Lather, rinse, repeat.  At first, improvements were small (a few mW here, a few mW there, but nothing approaching where we left things last November).  This lasted until translation stage adjustment range on the 1st MM lens ran out.  To recenter the adjustment range I had to move the entire mount 8mm closer to the 70W amp.  Once I had done the gross mount move, I was recentering the translation stage in stages (so as to not lose the PMC alignment) and noticed that when the lens was 4.5mm away from the center of the translation stage (i.e. 4.5mm closer to the 70W amp than before I moved the entire mount) there was a large improvement (relative to previously observed changes) in the PMC trans and refl.  Since it was after 4pm at this point we called it a day; will start Round 2 tomorrow morning.  When we left the enclosure the PMC was transmitting 53.1W and reflecting 11.0W.  I've attached pictures of the before and after locations of the PMC MM lenses for future reference (yes, PMC MM Lens 01 looks like it hasn't moved.  It has, the mount is in a different location, the translation stage being in the same spot is a coincidence).