(Kiwamu, Chris, Sheila, Daniel, Stefan) We tweaked both acoustic noise sources and feed-back scheme today. Result: 8Hz RMS down to 0.1Hz, 9Hz RMS down to 0.01Hz. Details: - turned off purge air escape noise and clean room HEPA fans (improved spectrum above 3Hz up to 6kHz. - lowered the CARM feed-back gain by 42dB (new UGF is probably around 150Hz) - added insane boosts at low frequency to squash arm motion - there was also a loose connection at the PFD input, which was the reason for a low beat node strengh - realigned the beat node beam splitter, we got 200mVpkk beat note Still to do: - there is some clipping on the doubler path and y-arm path - so we'll have to do a realignment. - we need to attenuated the signal power at the new IR trans LSC PD - need to do a noise characterization of the end station electronics, in particular we want the PFD noise and the PDH dark noise - finish setting up the IR REFL demod signal. In other news, we worked on the IR REFL path: installed a trigger PD and power source.
For the fun of it, we also measured the free spectral range by moving from one fringe to the next: FSR: 37455.5Hz +-15Hz (VCO uncertainty) Lengh: 4001.98m +-1.6m And BTW: we are now locked for 70minutes and countig
I forgot to mention that the above plots include Sheila's updated calibration for the detuned cavity pole. De factor this pushed all traced (including yesterday's) up at hight frequencies, but attenuated them somewhat around 100Hz.
This is a video showing how the infrared transmitted light looks like when we were stabilising and bringing the PSL frequency to a resonance of the Y arm cavity.
It is clear that ALS provides us with a smooth control of the PSL frequency !
We left the CARM at the half fringe of the arm. This stayed for approximately 5 hours from roughtly 4:00 to 9:00 in UTC. I am not sure why it lost the lock.
Josh Smith and other folks from DetChar are pointing out that the 65-73 Hz signal that limits the RMS above 2Hz is coherent with HAM3 GS13s:
https://wiki.ligo.org/DetChar/HIFOYDetcharChallenge
Attached is also a DTT plot of the 60Hz-80Hz reagion including the GS13s on HAM3. Interestingly the full coherent region corresponds to a dip in the GS13 power spectrum.