Corey, Sheila, Daniel, Keita

Summary: We are back to squeezing and in observing after having introduced some yaw offsets in the 42MHz WFS, for now this has addressed the problem that Jim describes in 53956.  It seems that the original problem is that the nonlinear gain has slowly degraded since the crystal move yesterday.  This issue cuased us to be out of observing for about 4 hours this morning, and in preventative maintence for a little while. 

• There was a race condition in the squeezer guardian, where when the LO loop looses lock, the beam diverter opens and the guardain goes to SQZ_READY_IFO, and checks if all the other squeezer loops are OK.  If so, it tries to go back to squeezing and open the beam diverter.  In our most common squeezer failure mode (SHG or OPO PZT railing) it takes a few seconds for all the other squeezer loops to relock, so that the guardian will not try to re-open the beam diverter until a few seconds after it finishes closing.  In this case, only the LO and ASC loops came unlocked, so the guardian would issue the command to re-open the beam diverter before the beam diverter was "ready".  Based on notes in the guardian, it seems like this has been encountered before and a busy flag was added to the beam diverter library for this reason, but the guardian code hadn't been updated to check it. I added a check that the beam diverter be ready.  This should mean that if a similar situation occurs again the squeezer would be able to re-lock itself.
• We relocked the squeezer, and saw that the 3MHz power through the OMC was degrading as the ASC control signals moved away, repeating the pattern that happened during Jim's shift.  We disabled the ASC (using the gain slider on the SQZ ASC overview) and saw that the transmitted 3MHz power stabilized.  I then manually walked the alignment of the ZMs, and found that ZM2 yaw was able to restore the 3MHz power to nearly -30dBm, about 1dBm lower than the level before yesterday's crystal move (all of this is shown in the first attachment).  I adjusted the offsets in the AS42 yaw signals, and we reclosed the ASC loops.  The second screenshot shows the offsets that we accepted in SDF to go back to observe.  
• 3rd screenshot:  It seems that the nonlinear gain was started to slowly degrade after we adjusted the crystal position yesterday (the time of the crystal adjustment is just after -80000 seconds in the 3rd attachment) (53943).  The OPO reflected power was increasing, while the fiber launch power also slowly increased, (OPO green trans is servo'd).  The 6MHz power dropped, which indicated that the nonlinear gain was dropping, as well as the 3MHz power on the OMC.  
  • It seems likely that the crystal green absorption has increased quickly in this new position.  I seem to remember hearing about a fast degradation at LLO after they moved the crystal (48897) although I'm not finding the right alogs at the moment.  If this is the case, the crystal temperature would have changed due to a change in the local heating which isn't sensed by the thermistor.  This means we can probably restore our nonlinear gain by adjusting the thermistor temperature setting.  
  • We didn't attempt to optimize the crystal position yesterday to overlap the co-resonance with the phase matching temperature.  Doing this could make us a bit less sensitve to changes in temperature, which would be good if the absorption is gonig to be changing.