P. King, J. Oberling
While recovering the 70W amplifier after last week's FE NPRO swap, we noticed the beam going into the 70W looked strange; not how we would expect it to look given where we left things on Friday. So we moved the WinCam to view the FE beam before the 70W amp mode matching lenses and saw that beam is now smeared diagonally. The first attachment compares the FE beam from last Friday (on the left) to what we see today (on the right). It should be noted that to fit the WinCam for this measurement it needs to be rotated by 90°, so horizontal on the image is actually vertical on PSL table, and vertical on the image is horizontal on the PSL table. After seeing this we tried a couple things. First, we attempted to tweak the alignment into the MOPA, thinking that maybe the mounts had "relaxed" and misaligned the beam into the MOPA. This did not help. We then tried rotating the pump fibers for the FE (very slightly loosen the connector, rotate the fiber without removing it, tighten the connector). This did nothing for the beam profile, but we did gain roughly 1.5W of output power from the FE. The 2nd attachment shows the same comparison as the first, just after our tweaking; as can be seen, no real change.
Based on past experience at LLO, this looks like a possible break or crack in one of the MOPA crystals. I'm sending an email to the lsc-psl mailing list to discuss this with the broader PSL folks within the LVC. In the interim, Peter and I decided to recover as much of the PSL as we can so we can hopefully provide a usable beam for commissioning. In light of this, the beam was aligned through the 70W amplifier (we did not change the mode matching lenses yet). The 3rd attachment shows the output beam from the 70W with the "bad" FE beam seeding it. It looks better than we expected, so tomorrow morning we will put this beam through the PMC and see what we get. The hope is we can get enough out of the PMC to facilitate some more commissioning work while we figure out the FE beam quality problem. Stay tuned.
Attached is a 4 day second trend of the 4 power monitor channels within the FE. H1:PSL-AMP_PWR1-3 are all uncalibrated (but can still show changes in the light level read by the PDs, which could indicate an issue with the crystal); H1:PSL-AMP_PWR4 was re-calibrated last Friday.
The mess at the beginning of the trends is our alignment and mode matching work with the MOPA on Friday. After an extended OFF period, we re-calibrated H1:PSL-AMP_PWR4 to 35.1 W, as read from our roving 300W power meter. We then shut the MOPA down to remove the power meter from the beam. Upon turning it back on, H1:PSL-AMP_PWR4 jumps up to over 36W, then slowly levels off at 33.8W, where it stays until we started our tweaking this afternoon (after noticing the beam quality change).
