J. Driggers, S. Dwyer, J. Kissel, T. Shaffer TJ has been having issues locking ALS all day, and has traced down the problem *site* to be the ALS X Fiber PLL. He can lower the threshold on the ALS-X_FIBR_A_DEMOD_RFMON, and squeak by enough to move on with the lock acquisition, but the problem has been consistent all day and should be addressed by other means. Sheila suspects that these are similar issues that we've had in the past with the ALS laser mode hopping (most recently in May 2023 LHO:69773 and LHO:69712, but even further in years past as well). However, today's issues are a marked step-function drop in DEMOD power, rather than a slow gradual decay from +10 dBm-ish to below -10 dBm. In the past we've solved the slow gradual decay by lowering the beat note threshold from its nominal -10 dBm to unphysically low -30 dB threshold, left it there for a few days to get by, then the beat note would slow come back up, we'd re-raise the threshold back to -10 dB and move on. Jenne has, for now, dropped the threshold (H1:ALS-X_FIBR_LOCK_BEAT_RFMIN) to -20 dB. Earlier this morning, TJ and Jason tried increasing the power into the fiber coming from the PSL (see LHO:70944), but only was able to gain a little bit more power, and it didn't help the issue, so this further points to problems with the ALS laser, rather than what's coming in from the corner via the fiber. We have trended this issue in ALS-X_FIBR_A_DEMOD_RFMON against XVEA temperature metrics and we *do not* see any correlation. We tried flipping the laser's noise eater from ON to OFF then back ON again, and it had no effect. We can again squeak by, quickly lowering the threshold and making it past this portion of the greater IFO lock acquisition sequence, but this is a bandaid to the real problem that should be address ASAP. Sheila suggests a thing that *might* work is going to the X end station and adjusting the laser current and temperature. Meanwhile, the investigation continues. Attached are the last 24 hours, the last 24 days, and the last 24 months of behavior from this beat note channel compared against the laser crystal frequency (slow laser control via laser temperature), the laser head PZT frequency (fast laser control via PZT), and the ALS PLL Common Mode Board control signal prior to splitting between fast and slow control.
Opened FRS Ticket 28430 to track the issue.
One clarification, re: "...TJ and Jason tried increasing the power into the fiber coming from the PSL (see LHO:70944)..." The ALS lasers no longer get their PSL light for the PLL from the RefCav transmission, so improving the RefCav TPD will have no effect on the amount of power in the ALS fibers. The ALS fibers are now fed by a pickoff in the ALS path on the IOO side of the PSL table, after the PMC. From the as-built PSL layout, the fiber pickoff is ALS-PBS01, which directs the picked off beam into ALS-FC2; the IOO ALS path itself is from the transmission of IO_MB_M2, which is directly in front of the ISC EOM (IO_MB_EOM). This was installed in Oct 2019 in the break between O3a and O3b.
The beatnote strength stayed low overnight rather than coming back up. It seems like going to the end station to adjust the laser current and temperature before the long weekend is a good idea.
A note for the alog above: we looked at the fiber transmission and the fiber rejected polarization PDs, they do not show any problems. When the beatnote strength drops, there is also a jump in the crystal frequency needed to lock the PLL, which makes this look like mode hopping of the ALS laser.
