jeffrey.kissel@LIGO.ORG - posted 14:59, Friday 25 January 2019 - last comment - 15:28, Friday 25 January 2019(46644)
What It Looks Like in ALS When External Comm Fiber Bundle (Including ALS Optical Fiber) Is Tapped -- Source of 'Rainy' Day Glitches?
J. Kissel, reporting for F. Clara, R. McCarthy (Tappers), J. Driggers, S. Dwyer T. Shaffer (Witnesses), K. Kawabe (Photographer) After pictured recently by Keita (LHO:46607), Richard and Fil drive down to similar spots along the exposed Y-arm communication fiber bundle -- which includes the PSL to ALS laser optical fiber connection -- and tapped on it while we happened to be locked on ALS. The attached screenshot shows a time series of (among other things) the Y ARM power transmission in green laser power [Brightest Orange Trace], usually the canary in the coal mine when we're having "ALS glitches" and "it's raining." A T=16 sec are the beginnings of taps. At T=0 sec are the most invasive taps. At T=+11 sec there is more tapping. The times are all around 2019/01/24 19:05 UTC. These taps, and corresponding response of the Y arm transmitted power are representative of the range of impact that we've seen that impacts locking of ALS, which has cost us many an evening of frustration. This is likely the source of *some* (if not all) problems with ALS glitches, and was associated with rain (presumably the wind / rain would cause the same level of tapping Richard and Fil were inducing). This issue is related to the following FRS Tickets: FRS:11113, FRS:12209 Previous reports of ALS Y vs. Rain: LHO:46554, LHO:46302, LHO:45924, LHO:45891
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Just looking at the time series that Jeff plots, these don't look convincingly similar to the glitches which we have when no-one is tapping the fiber.
For some examples, see plots attached to 17576 22184 25523 36602 laser safety tag problem: 36645
Kara Merfeild is working on some code to try to look for corelation between glitches and weather.
Another step that we could take is to make a script or some other easy way to do the test where we lock the green laser to the arm bypassing the PLL (making sure that we have a reasonable loop), so that next time we have a rainy day we can easily switch the configuration to test if the glitches go away when the fiber is not involved. Setting this up would be a good Tuesday morning activity.