J. Oberling, J. Driggers

As a result of yesterday's PMC work several beams downstream of the PMC were misaligned; one of these was the beam into the fiber pickoff for ALS and SQZ.  I went in this morning to tweak this alignment so there was sufficient light available for both the ALS PLL loops and SQZ.  In the past this has been a very quick tweak to the steering mirror that directs the pickoff beam into the fiber, but not today.  Adjusting the steering mirror only brought the fiber transmission signal from ~0.02 to ~0.035 (our past max has been around 0.09), and trending the external PD that monitors the light available to the fiber showed we had more light than before the PMC work.  Regardless, I used a power meter to check the amount of power incident on the fiber coupler and found it at 40 mW; when this path was first installed in 2019, and upon recovery after the PSL laser upgrade in 2021, we had 50 mW in this pickoff path, so I adjusted ALS-HWP2 to bring the power back to 50 mW (interesting that the external PD thought we had more light when we had less?).  Ultimately, I had to start adjusting the fiber coupler alignment as well as the steering mirror.  The coupler has 5 degrees of freedom to adjust: horizontal, vertical, longitudinal, pitch, and yaw; these adjustments move the fiber coupler's internal coupling lens (longitudinal adjustment is performed by moving the 3 screws that control pitch and yaw all in the same direction by the same amount).  Horizontal and vertical adjustment did nothing but quickly drive the beam off of the fiber; I accidentally did this when checking the horizontal alignment and had a very hard time finding it again, calling Jenne in for assistance (Thank You!!).  What ultimately worked was adjusting the lens away from the fiber (so towards the steering mirror) by a little bit (roughly 1/8 turn of the allen key; this always resulted in a loss of pickoff signal), tweaking the steering mirror to peak the signal (rarely got back to the starting value), then carefully tweaking each of the three pitch/yaw adjustments on the coupler to peak the signal again (it's at this point the signal would increase past is previous max).  At first the going was very slow, fighting for every 0.001 to 0.002 increase, but increases became larger around the middle of the 0.06 - 0.07 range.  In the end we stopped when the fiber transmission signal was reading ~0.101, which the highest it's been in over a year.

Why did we have to move the coupling lens?  The immediate answer is, "Because the beam size changed."  But why did the beam size change?  A couple of WAGs: The beam size out of this new PMC is different than the old, or the slight alignment change we saw post-PMC has the beam passing through the 2 lenses between the ALS/ISCT1 pickoff and the fiber pickoff (lenses ALS-L1 and ALS-L3; the fiber pickoff is a pickoff from the ALS/ISCT1 path on the IO side of the PSL table) in such a way that the beam size changed slightly (yeah, this one is pretty big WAG...).  Regardless, from what I saw this morning it's possible that the beam size output from the new PMC is slightly different than the old.