J. Oberling, R. Short

Today we went in to primarily measure the beam powers between Amp2 and the PMC to ensure everything adds up (make sure we know what power is being lost where and is as expected), and to fix that stubborn EOM_Out calibration error.  It turned into a realignment of the ISS AOM and re-calibration of the ISS diffracted power percentage.

To start, we measured powers between the PMC output and the IOO EOM:

• PMC Trans: 101.7 W
• IO_MB_HWP1, PBS rejected beam: 1.03 W
• IO ALS path: 2.13 W
• EOM Trans: 99.5W

This mostly holds together, we had to use 2 different power meters due to space constraints (the big 300W water-cooled power meter for PMC Trans and EOM Trans, a smaller 10W power meter for the ALS and PBS rejected beam).

Next we measured the powers between Amp2 and the PMC:

• Amp2 Out: 137.8 W
• High-Power Attenuator Rejection 1: 0.6 W
• ISS AOM 1st-order diffracted beam: 6.3 W (!!!!!)
• PMC In: 123.1 W

We weren't able to get a power meter in to measure the 2nd rejected beam from the high-power attenuator (HPA).  The HPA consists of a HWP and 2 crossed thin-film polarizers (TFP) to provide power control and polarization cleanup; we could measure the beam rejected from the 1st TFP but could not fit a meter in to measure the 2nd.  As can be seen, we can't account for 7.8W of power between Amp2 and the PMC.  No clue where it could be, there are no beams of that strength in the area that we could find with either IR cards or an IR viewer; no stray beams, no leakage beams, everything looks as it should.  Now, in looking for this extra power, we noticed the ISS AOM was misaligned (hence the !!!!! after the ISS AOM power measurement).  When we set the ISS up we had ~4.4W in the 1st order diffracted beam with an ISS Offset slider value of 5; with that same offset slider value we now have 6.3W.  So we realigned the ISS AOM.

We used the HPA to drop the power through the ISS down to about 0.5 W and installed the AOM cover (which has rough alignment apertures on it), and sure enought, the alignment was off.  We roughly aligned the ISS AOM, then went back to full power through the HPA and went about finding the diffracted beams again.  With a slider value of 5, we slightly tweaked the AOM angle (moving clockwise) until we had ~4.4 W in the 1st order diffracted beam, returning it to where it was when we set the ISS up at the end of the laser upgrade.  The other diffracted beam (the -1 order) was barely visible as it should be (can't entirely get rid of it, and no there are not multiple watts hiding there).  We then took the data needed to check the Diffracted Power % calibration on the ISS MEDM screen.  This uses the AOM drive voltage and the power in the 1st order diffracted beam to calibrate the graph.  That done we set the ISS Offset slider back to its original value of 5 and measured the power input to the PMC at 127.9 W.  Now we are only "missing" ~5W, so we did recover a couple as expected with the ISS AOM realignment.  Still can't find where that 5W is, however.

After recovering and tweaking the PMC (ISS AOM realignment changes beam alignment into the PMC), we had 108.9W transmitted and 16.3W reflected.  At this point we were already ~30 minutes past the maintenance window, so we did not tweak the PMC mode matching as we were hoping to do.  We recalibrated the PMC Trans, PMC Refl, Amp2 Out, and EOM Out PDs based on our measurements, and left the enclosure.

Once back in the control room we relocked the PMC and tweaked its beam alignment for a final time (get better results with the enclosure environment controls OFF), and ended up with ~110.2W output.  Oddly enough, after re-calibrating the EOM Out PD and seeing it read less than the PMC Trans (as it should), once we left the enclosure and tweaked the PMC alignment it's back to reading higher than PMC Trans.  *sigh*  It only reads a couple tenths of a watt higher, not several watts, but it's still reading higher.  So we're going to have to go back in at some point and measure the EOM out again and recalibrate that PD, again.  Ugh.  When we go back in to do the EOM Out calibration measurement, I also want to try to fit a power meter in to measure the 2nd rejected beam from the HPA, and continue the search for this 5W we can't seem to find.

Finally, we re-locked the FSS RefCav and tweaked the beam alignment into it, improving the TPD from ~0.78V to ~0.94V.  I then performed the 2nd-order polynomial fit to the ISS AOM diffracted % data we took, and updated the polynomial (see attached picture); new values are: 254.56x^2 - 130.12x + 16.733.

Several SDF diffs were created and accepted, in both Safe and Observe.  Ryan has screenshots of those that he will attach as a comment to this alog.  Since we still have a few things to check on (like the EOM Out cal *sigh*), I'll keep WP 11164 open and we'll target the next maintenance window (5/9/2023) to get this wrapped up.