andrew.lundgren@LIGO.ORG - posted 07:55, Wednesday 01 May 2019 - last comment - 17:19, Thursday 02 May 2019(48892)
Some Diagnostics of Squeezer Noise in h(t)
Andy, with input from Beverly, Laura, and Josh The strong wandering lines in h(t) caused by the squeezer are becoming more frequent and more severe. This seems like it could be quite a problem for the searches, so I'm putting some information here to hopefully push this to be diagnosed and resolved. The wandering line was noticed a week ago (alog 48772). It comes and goes, both in h(t) and in the squeezer channels. In the squeezer, it's an equally spaced comb of lines, with the first wandering from about 80 to 140 Hz. The line comes and goes quite suddenly. The first attachment shows what it looks like in one of the squeezer channels. This seems similar to the 200 Hz wandering line from the LLO squeezer. The solution (LLO alog 43837 was a highpass before the TTFSS. The attached PDF has spectra of several channels during the noise compared to a reference time. Nothing is visible on the fiber transmitted PD, but it is on the fiber PD, the IR laser LF, and a bunch of mixer channels. Also of note is that the CLF RF6 and REFL RF80 RFMONs both get much noisier when the lines appear. There's a very small bit of extra noise also seen on FIBR_SERVO_DEMOD_RFMON. I would speculate that this looks like a loop that's not quite stable, but everything is so coupled it's hard to tell where. Is it possible that the harmonics are generated by the noisy line going through the SHG and getting mixed with itself repeatedly?
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A couple of other things about this line. 1) When the line acts up, It is clearly visible in the OUTPUTOPTICS magnetometer channel which is the closest to ISCT6 2) Yesterday, the line came back as soon as got to NLN after maintenance. With Jenne's help, we closed the SQZ beam diverter - to use it as an opportunity to confirm that the squeezer was causing it. Closing it causes the line to go away in DARM, although the line is still visible in the magnetometer channel and some of the squeezer channels. The line came back when we turned it back on a minute later. Relevant spectrograms in slides 10 - 13 of the attached pdf
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The high pass filter in TTFSS was added at LHO, alog 47296.
The real solution at LLO was to disable the laser noise eater on the SQZ laser. The high-pass allowed us to make that switch without other consequences.
The lines aren't the only excess noise. LASER_IR_LF has a jump in the broadband noise floor when the lines appear. I've attached a spectrogram and a spectrum. The magnetometer sees the main line and its first two harmonics. They disappear instantly whenever the lines in the squeezer do - see third attachment. It seems like this is more likely to be pickup from a problem in the squeezer, because having all of the line harmonics be EM noise seems unlikely, and there would also likely need to be broadband noise. Was there any change to the outputoptics magnetometer during maintenance yesterday? The coupling of the squeezer lines into the magnetometer is much stronger starting from the first lock after maintenance.
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No, I don't think there was any change either with the squeezer or the magnetometer.
The noise eater could be toggled off and on again, using the switch on the controller mounted on top of ISCT6. It looks just like the noise eater switches on the PSL and ALS laser controllers. (Toggling it might not fix the problem, it might need to be off but it’s probably worth trying to toggle it first.)
Jenne and I went in to the LVEA and discovered the noise eater control on ISCT6 was hooked up to a remote system. Jenne toggled it off from the control room, and turned it back on ~ 30 seconds later at 16:14:55 UTC. Lets see if it works!
The noise came back, and doing an on/off test a few times indicated clearly that turning the SQZ laser noise eater off made these lines go away, so the noise eater is now off.