I. Abouelfettouh, A. Jennings

Today during Tuesday maintenance, we decided to look into the oplev pd sum counts, which have been dropping steadily since Jason and I upgraded the laser with an armored fiber and cooling enclosure, done two weeks ago (alog here). We took the LVEA to LASER HAZARD then proceded to turn off the oplev laser in its enclosure. When doing this, I immediately noticed warm air coming out of the enclosure. The ice packs and the laser box itself were both warm to the touch. This is a sign that the laser itself is starting to go, which makes sense since this is the oldest oplev laser we have in the detector. In addition, if the laser was indeed dying, it would have been further exasperated when it was moved into a closed enclosure when we made the upgrade, trapping all the heat and degrading the laser at an even faster rate.

Once we turned off the laser and verified there was no feedback on the oplev overview, we then opened the housing to the laser to check to see if the fiber optic cable was potentially pinched or frayed somehow and we didn't notice it when Jason and I made the initial upgrade. These photos (1 / 2 / 3) are exactly what we saw when we first opened the housing. The optical fiber looks to be in the same position and we found no evidence of any fraying, pinching, or degradation anywhere on the fiber. We made sure that the zip tie we used to secure the fiber to the base plate was snug, but not too tight, and were able to confirm this by being able to feed the fiber both in and out of the zip tie with relative ease.

After verifying that the optical fiber checked out, we put the housing back on, which was a struggle. After moving and readjusting the housing, we saw that some of the screws used to secure the housing (looks like 0.25" x 5/32) appeared to be stripped, which made it hard to put back in. More importantly though, it seemed that some of the threads in the screw holes on the base plate might be stripped as well. Hard to get a picture of it, but have attached some photos here - 1 / 2. Even when we tried replacing the screws with fresh ones, it still would not thread into the hole (we verified that the issue was NOT the housing itself as it was lifted above when we tried this). After a lot of finnicking, we were able to get majority of the screws in, torqued, and secured with relative ease. However there was one screw, we were only able to get midway in (attached), and one hole we could not get a screw in at all. With 12/14 screws set in place, and one moderately secured, and double checking the housing was secured, we felt comfortable with leaving the housing in that state, but this should be fixed on another Tuesday (maybe use some helicoils to fix the screw holes?).

With the housing back on, we verified that the sum counts were back on the oplev overview. During this process, it appears that we slightly moved the alignment of the laser, with it now reading back at -15.5 P/-5 Y, when it was near 0 before we started. We went back out to grab the oplev controller and were able to get the alignment to sub 0.2 for both P/Y. We somehow were also able to get the oplev counts higher, ending up at ~4600 counts (up from 3600 before we started). Once we were satisfied with the alignment, we packed the controller, verfied the housing was secure a third time, and called it a day.

All this to say that we can confirm the fiber optic cable is in tact and not the cause of the sum dropping problem. Based on everything we found today, I believe that it is the oplev laser itself that is dying and has been for some time, and was just greatly expaserated by putting it into an enclosure when Jason and I did the upgrade. After talking with Fil, he thinks it also could be a possibility that the 5/10 V power block that feeds into the cooling enclosure could be faulty and therefore be an explanation for the dropping sum counts. However, he mentioned that if that were the case, we should have seen the counts drop with a "staircase" esque pattern, but the sum counts from these past two weeks seem to have followed a more linear degradation. Next steps would most likely be to remove the current laser and swap it, and if the problem still persists, look into the power block next.

This concludes the work in WP11445.