After I and Evan relocated POP-X and POP-L2, Jenne increased the ASC gain of POP-X by a factor of 250 for PIT and 500 for YAW (alog 28666) just to get back to the old UGF, which sounded crazy to me as I didn't expect much change in Gouy phase.
Just to see if my assumption was wrong and we got super unlucky, I calculated the Gouy phase of the POPAIR path, and it seems like there shouldn't be much change.
In the first attachment, left column is the current configuration, right is the old one. Bottom is the entire POP path from ITM to the ISCT1, and top is the zoomed-in view from HAM1 to ISCT1.
POP-L2 was placed far enough from the waist originally. This lens was moved farther from POP-L1 by 4 inches later, which should have increased the Gouy phase, but it's only 9 degrees. The beam diameter is 2mm now instead of 4mm but that should be OK. These don't explain the crazy decrease of the optical gain.
Funny thing is that the POP-X spectrum itself looks almost the same before and after the relocation (second attachment), so I'm kind of dubious that the optical gain is lost.
It's not totally impossible, but very unlikely, that the Gouy phase was not great to start with, e.g. 81 degrees for the DOF we want to see, and after the change it became 90 degrees.
Today talking with Terra and Daniel, I remembered that we added a beamsplitter to the POP path to void saturating, and apparently I forgot about adding the factor of 6.7 to compensate for lost gain to the POP X RF loops. (alog 28324) So this explains a factor of 6.7 lost gain, but not all the gain that Jenne had to add to the loops.