lisa.barsotti@LIGO.ORG - posted 23:53, Wednesday 11 February 2015 - last comment - 09:55, Thursday 12 February 2015(16675)
2+ hour lock on DC readout, and beginning of the noise hunting season
Evan, Dan, Jeff, Peter, Daniel, Lisa2+ hours lock on DC readout, this time for real!
* Feb 12, 5:33 UTC lock on DC readout, engaged ISS loops shortly afterwards * Feb 12, 7:00 UTC improved low frequency noise by turning off ETMs optical lever damping * Feb 12, 7:35 UTC still lock on DC readout, starting alignment tests (intentionally changing BS alignment; also reduced the BS optical lever damping gain by a factor of 10 * Feb 12, 7:48 UTC unlocked by closing the AS beam diverter Positive news of the day - Thanks to the improved bounce mode damping , we can now reliably damp the bounce mode, and it is not a limiting problem anymore; - ETMX and ETMY optical lever PITCH damping loops have been reduced by a factor of 100; it turned out that these loops were responsible for the huge excess of noise that we saw yesterday below 40 Hz, and that was causing the OMC transmission to shake painfully. Now the OMC is quiet and happy (the Guardian has been updated to reflect this change). - We turned off the QPD alignment of the OMC, and replace it with dither alignment; - We turned on the ISS first and second loops: the positive news is that we improved the high frequency noise by a factor of 10. Negative news of the day - While we have improved noise at low and high frequency, we now have a new bump of noise around 100 Hz which was not there yesterday . - Our wonderful BS WFS loops, so effective yesterday, were not working today. However, today the BS alignment was not critical, and we have barely had to touch the BS anyway, but we need to figure out what happened. Right now we commented out the BS WFS in the locking sequence; - Yesterday the locking sequence was very reliable, and worked 5 times in a raw..today it has been less robust, despite similar environmental conditions.. - The RF of in-vac POP is somehow broken - no signal there. It is probably not a big deal right now at low power. - We also closed the OMC REFL beam diverter (level 2 of "Valera's levels of awesome": it works, but it doesn't do anything)
Attached are some plots of coherences and OMC noise.
Fig 1: Coherence between DARM and LSC, ISS - the excess noise at 70-400Hz, much worse than last night (grey), is coherent with the common DOFs (PRCL, SRCL, and CARM [not plotted]). Also the ISS 2nd loop out of loop PD.
Fig 2: DARM and ASC - there is coherence between IMC yaw WFS error signals and DARM, implying that the noise is due to some beam jitter coupling
Fig 3: OMC noise - note that we are shot noise / dark noise limited above ~1kHz
In figure 1, the grey trace is yesterday's almost-two-hour lock, the green is today before the oplev damping was turned downand without ISS second loop, the blue is at the end of the lock. There was a substantial improvement in many bands due to the ISS 1st loop and loosening up the oplevs. The second ISS loop didn't change much, although we did notice it reduced the intensity noise on, for example, ASC-POP_A_SUM.
The beam jitter into the IMC, as measured by DOF1_{P,Y}, hasn't changed from last night. The working hypothesis is that the alignment has drifted into a place where beam jitter couples more strongly into the length DOFs. We checked many PEM channels for coherence with DARM in case, for example, someone had left a fan on in the LVEA, but found no coherence with environmental channels.
A plot of POP vs. POPAIR demodulated signals is attached. Evidently, there is nothing meaningful in the in-vacuum signals, even though there is about 1.7 mW of dc power. This is roughly what we expect; with 2.8 W incident on the IMC, an IMC transmission of 90%, a power-recyling gain of 33 W/W, 250 ppm of transmission through PR2, and 10% transmission through mirror M12 on HAM1, we get 2.1 mW.
Also attached are some short videos of the AS port and the OMC transmission during full lock.
PeterF, Kiwamu
We found that the RF cables from POP_A were unplugged at the front panel of the demodulation board by the PSL enclosure. We hooked them back in. Confirmed that the demodulated signal showed some interferometer fringes.