Related to the original report, some basic facts are

• IRX and IRY have strong coherence in some frequency range, but not all the frequency range; although for this type of trend-like or drift-like signals, taking coherence would not be useful due to the difference of time scale of those phenomena, coherence should be still the fisrt thing to do to be checked, so I put the memo here.
• Another point to be considered is that POP90I seems not having coherence with IRX or IRY.

So the alignment or lengthy degrees of freedom of {I or E}TMs might be related to this transmission power fluctuation. Another possibility might be drift of TMS-VIS tables. To see only inside power of the arms, NAB PDs might be useful.

A hint: IRX (and IRY) transmission seems to have strong correlation with ASC-POP_QPDA1_DC_SUM_OUT as shown in Fig. 1.

In the following, I put an assumption that the positive/negative assignment of this POP RF-QPD correspond to increase/decrease of the power illuminating this PD, respectively. Then, this correlation would mean light power fluctuation inside the PRC (or mode coupling of PRC with respect to the input beam mode) is the main cause of the arm transmission power fluctuation, because I guess this PD is looking at "coming-back" light beam from the arms, but the correlation between this PD signal and the arm transmission is not negative.

Currently the PRC ADS is tracking an error from POP90I maximum, but this might mean PRC WFS or ADS tracking to carrier power (like this POP QPD SUM) in the PRC would be better? -> Too fast for ADS?

Then I also checked the other channels in Fig. 2, where the first two are the same channels as in Fig. 1. The 3rd signal is of REFL RF QPD DC, which would have some correlation but not so clear comparing with POP RF-QPD DC SUM. The 4th signal is of POP_FORWARD, which has also strong correlation with POP RF-QPD DC SUM and so of the arm transmission power.

The 5th one is of POP90I for reference, which seems less correlation. In fact, this is currently used for ADS but the control bandwidth would be less (true?) than the arm transmission drift, so still it should be possible to compare this signal drift with the arm transmission drift. Anyway, to check the real correlation, it would be better to turn off the relevant ADS when comparing them.

The 6th is K1:LSC-POP_PDA2_DC_OUT, which is of DC of the same PD extracting the POP90I. This channel can be also usable as it has also strong correlation. Fig. 3 shows the correlation in the frequency space.

Then why IRX or IRY seems to have slight correlation with GRX or GRY sometimes? Occasional coincidence? I am not sure but the attached figure may imply that in the frequency region where IRX and IRY have strong correlation with each other, the correlation between IRX and {GRX or GRY} seems mostly low.

... do we have an optical filter to filter out IR from the Gr path in the TMS at each end?

Just for visual inspection; checked energy conservational viewpoint.

In terms of REFL power, the brighter is the better. Even while slight reduction of the REFL power, the arm transmission, PRC-inside carrier power, and AS contrast seem tolerable. But, if the REFL power reduced further, drastical reduction of the arm transmission, PRC-inside carrier power, and AS contrast quality simultaneously happens.

For example, if just only looking at the REFL power, and if it reduces like the one indicated as "2" in the attached figure, one may think it would be obvious that the contrast of the interferometer would get worse, but the fact is not so simple; seemingly not so "entangled". The phenomenon is not so linear. Where the energy went...? As if there would be two different causes of the REFL reduction. One cause worsen of the contrast, while the other would not do so.