Noticed SDF was showing a difference for EY ECAT PLC2. The fiber temperature control had switched off at 12:50. Called Sheila. While talking to Sheila noticed two other channels related to fiber temperature were cycling on an off. Sheila said this would not be a problem maintaining lock but could be a problem for relocking.
Sheila noticed that the Fiber Wrong Polarization percentage was close to its limit of 40. It was between 37 and 39 percent. This should not be a problem while we are locked, but should be addressed at the next lockloss.
This is all related to the lock of the auxillary laser at the end station to PSL light transmitted through the fibers. The beckhoff automation tries to keep this locked at all times, which saves a small amount of relocking time and also some hastle compared to just letting this laser drift while the IFO is locked. This laser is shuttered while in full lock, and should have no impact on the IFO.
What Jeff noticed is that the automation stops trying to lock this and sends errors if the polarization of light coming out of the fiber has drifted by 40% or more. This is a problem that should probably be addressed next time we unlock by following the instructions here : https://lhocds.ligo-wa.caltech.edu/wiki/Troubleshooting%20the%20IFO#ALS_Fiber_Polarization
While everything is behaving as intended here, and it seems good that our automation gave Jeff an early warning about the polarization drift which could prevent relocking if not fixed, this would be a problem if Beckhoff SDF were integrated into the intent bit.