jeffrey.kissel@LIGO.ORG - posted 13:34, Monday 14 November 2016 - last comment - 14:24, Friday 18 November 2016(31471)
1 Year Trend of Wrong Polarization in ALS Fiber Transmission
J. Kissel, P. Thomas, J. Warner We're having trouble get ALS locked after the corner station crashed this morning, so we took a look at the percentage of wrong polarization in the ALS fiber transmissions. Y was at an acceptable 2%, while X was a boardline 12%. Just to make ourselves feel better, Jim and Patrick adjusted and reduced the percentage to under 5% for both arms. Out of curiousity, to see how often this needs doing, I took a 1 year trend. I don't have much to say about the results, hopefully someone who knows more about this system can draw conclusions. Also, there is interest in moving the fiber polarization adjustment out of the CER.
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A. Staley (posted by J. Kissel on her behalf) Alexa -- still looking out for us -- had sent me an email about the above trend. Thanks Alexa (and Zsuzsa for prompting her to look)! I quote it below: The fiber polarization controller drifted quite a bit -- Sheila, Evan, and I would post alogs about it keep track of the drift. There are a couple of things worth noting: We discovered that the fiber polarization controller creates a peak in the green locking noise at 27 kHz (LHO aLOG 10275). So we decided to keep the polarization controller off. We didn't spend time characterizing how much the wave plates would drift with the controller off. We ensured the drift/hysteresis of the polarization wasn't drastic over a period of a day or so, but were still expecting some drift over time. We also know that the fiber polarization drifts with temperature in the MSR (LHO aLOG 7023, LHO aLOG 11509) and the temperature is not very well controlled in that room. A bit unrelated, but I had also seen funny behavior with the motorized polarization controller (MPC) upon turning it off and on (LHO aLOG 11505). I am not sure if anyone went into more detail than that to characterize this drift or mitigate it -- it was always pretty trivial to adjust. The DCC has the manual (T1200496), which states the expected rotational drift of the wave plates over time--this is very small. Between that variation, temperature drifts, and drifts in the laser out SOP, I would expect some drift in the polarization. Again, I don't think this has been characterized or quantified and compared with the trends we've seen.