In 2017 the PRC was measured to have a wrong length by 3-4mm: https://logbook.virgo-gw.eu/virgo/?r=36884
I have checked in Optickle that mistuning the sideband frequency by a 2 kHz (which should be equivalent to a 4mm cavity length change), the sideband behavior becomes asymetric with PRCL offset. WIth the sideband power increasing by 1-2% in one direction and decreasing by 1-2% in the other direction, when changing the PRCL offset by ~0.2nm. This is similar to the measurements done last Friday, but it disagrees with the PRCL calibration done a year ago: https://logbook.virgo-gw.eu/virgo/?r=36884. That calibration would say that the PRCL offset was changed by 1.5nm, which would make the sideband power change by a few tens of percent. Maybe that calibration is no longer valid, as some normalizing factors could have been changed when chaning PRCL error signal from 8MHz to 6MHz sideband, or when installing the RAMS on the 6MHz.
Figure 1 shows in red the normal situation, in blue with the frequency to amplitude noise coupling divided by ~2 by adding the PRCL offset and in purple by adding the PRCL offset which zeros the coupling. The power fluctuation on B2 at ~1.5Hz improve by a factor 10 when reducing the coupling, on B4 there is also an improvement by a factor ~3, while on B4 12MHz the situation becomes worse, with fluctuations increasing by a factor 2. That would make sense if we improve the resonance condition for the carrier and degrade for the 6MHz sideband.
Figure 2 are the same times but with colors not in the same order. It includes also the B7 and B8 powers, and while on B8 the improvement is monotonic when improving the frequency to amplitude noise conversion, on B7 it is not the case. Is that a consequence of one arm being 3cm shorter than the other?
Lets assume that the PRC length is wrong by ~3mm, there are several options to resolve this:
- Move PR by 3mm, which then requires retuning the input beam mode matching as it will change the input beam mode matching by one or a few percent
- Change the sideband frequency by ~2kHz, but that would require moving the IMC end mirror by ~5cm to follow, so it is not a viable option
- Move the input mirror by 3mm and maybe also the end mirrors to keep the arm length the same. It might be the easiest to do and is the most reversible. It will also change the SRC length, but we have no measurements whether the SRC length is right or wrong, and it should be less critical because of the low finesse of the SRC.