keita.kawabe@LIGO.ORG - posted 11:16, Saturday 07 July 2012 - last comment - 18:17, Saturday 07 July 2012(3353)
Faraday replacement done, arm locking is still excellent, can actuate on the mass to manually relieve VCO DC (Jax, Elli, Alberto, Dick, Matt, Keita)
We're done with Faraday replacement. We might have to tune the optical paths on the ALS table a bit, but basically we're done.
We don't see any serious clipping in the PDH error signal, the arm locks, and the alignment is already reasonable (50% reflection visibility).
Now that we can do things without worrying too much about the clipping on the Faraday, we quickly tested to change the cavity length by pushing on the ETM while maintaining the PDH lock, and it worked. We were able to (manually) relieve VCO tuning signal by merely giving an offset to the ETM TEST L filter. We'll implement (or Matt has already implemented) a fully featured low frequency feed back to the ETM.
We haven't tuned WFS demod phase yet.
OPC database (database for variables used in Beckhoff world) seems to have been "reset" some time between Thursday and Friday afternoon. PDH settings, PLL settings and laser temperature control seemed to be gone. On Friday afternoon we recovered it manually (because we don't have an equivalent of burt restore for OPC). At least we thought we did.
We were able to lock to 00 mode, which was really tiny, and we were able to lock to some off-axis higher order modes, which were much brighter than the 00 mode. So we naturally thought that there was a massive alignment problem caused by either Faraday swap or SUS/SEI change in TMS, ETM and/or ITM.
We spent ridiculously long hours doing many things, we even started over from the baffle diode to center the input pointing on the ITM, and the 00 mode coupling was still orders of magnitude worse than before.
Finally it came to my mind that no matter what we did we're still in a close proximity of our old alignment, and that maybe we're locked to the 00 mode SB because I got the polarity of the CM board B wrong. I flipped the polarity and voila!
But at that point it was already late in the evening and I had to give up jogging with one of my kids.
Making the ETM off-load work was trickier than expected. Apparently the Fast Monitor channel does not montior the fast output on this CM board. (It may be AC coupled.) I cabled in the signal that we are sending to the VCO and then closing the loop was easy.
(It is running at the time of the entry.)
hmmm... AC coupled in a complicated way: the PDH and PHASE signals are swapped. The cables appear to match the drawing, so maybe the problem is in the model. Anyway, all cables are in their original places, and I've gone with a software "fix".