As some of you may know, the oplev for IMMT2 is a folded type, so if the injected PEM signal was actually shaking the optical table inside the IFI chamber, that also may be shaking the oplev's folding mirror, and so there might be some responses in the relevant oplev signals.

Fig. 1 is for pitch, and Fig. 2 is for yaw. In Fig. 1 a linear (?) coupling between the PEM injection and pitch is seen clearly. But not for yaw for some reason; maybe I misused the pastavi?? Or, maybe the PEM injection would be done in vertically(?) the stack might be well shaken in pitch but not in yaw (to shaking it in yaw, it should be twisting, which might be difficult to happen...?)

Anyway, the optical table might be shaken lineary with respect to the PEM injection signals. If you have time, please check if, from the IMMT2 oplev spectra (without PEM injection), the optical table's fluctuation could be witnessed or not. Probably some whitening filtering for the relevant QPD would be required.

For your reference the same for MCO oplev (this is also a folded type), which would be the closest to IMMT2, are shown in Figs. 3 and 4; nothing special detected, so MCo might not be shaken due to the PEM injection, and that would mean the optical axis from IMC might not be shaken. (I assume the frequency region in concern would be well out of the ASC loop bandwidth.)

> But not for yaw for some reason; maybe I misused the pastavi??

I regenerated the plots for the same data using Pastavi and obtained the same plots. So, the usage will be correct.
For the double check, I attached the usual spectrogram (without whitening). We can't see the injected signal in the yaw signal.

Thank you for the confirmatoin.

Another way to observe the response of the optical table in the IFI chamber with respect to the PEM injection might be: without locking the MIF, but lock IOO, and let PRM aligned to reflect the IR beam toward REFL, and use this beam like an optical lever with some QPDs on the REFL table, and inject PEM.

This work is based on the suggestion in klog32035.

Abstract:

The peaks around 108 Hz and 128 Hz are likely to come from not entire stack structure but the individual optical components.

Detail:

I measured the transfer function from the shaker excitation signals to IMMT1T QPDA2 pitch (left top in fig1), the excitation signals to IMMT2 OpLev pitch (left middle in fig1), and IMMT2 OpLev pitch to IMMT1T QPDA2 pitch (left bottom in fig1).
To compare the TFs from the shaker excitation signals to IMMT1T QPDA2 pitch and to IMMT2 OpLev pitch, I also made a plot that both glaphs are drawn on the same plot (right top and right middle in fig1, red: IMMT1T QPDA2 pitch, blue: IMMT2 OpLev pitch multiplied 2e-3 to adjust the gain to the other TF).

Since white noise injection with entire measurement region cannot make a significant excess on the OpLev signals, I separated the measurmenets into 8 with 4Hz bandwidth (102-106, 106-110, 110-114, 114-118, 118-122, 122-126, 126-130, 130-134 Hz).
Though S/N of OpLev signals are small (~2), the signals can be seen in both signals at almost all frequency bands.

There are clear peaks at 108 and 128 Hz in the TF from IMMT2 OpLev pitch to IMMT1T QPDA2 pitch, which indicates that these peaks are not coming from the entire stack structure but the individual optical component on the IFI stack.
There are also several peaks such as at 118.5Hz but it is difficult to identify the peaks comes from the stack structure duue to the poor S/N of the measurement. 

I personally quickly checked what happend around 2022 Jan 17 15:10 JST, since when the 128 Hz peak appearing even in IMMT1T QPDs, according to the investigation in 24703. As far as I found in klog, only the post 19489 was directly related to this area.

What I find from this klog post are:

• According to the photo attached to 19489, there were no beam dumps attached to STM2. So the beam dump structure would not be related to this peak.
• The pedestal and the mirror holder should be identical ones we are now using. What they did are (1) moved the STM2 assembly a little bit, and (2) rotated the STM2 mirror holder.
• So, the pedestal structure itself might not be the main cause.
• So, the existence of the electric wires seen in the photo in concern might not be the main cause.
• So, the mirror holder structure itself might not be the maincause.
• The way to fix this foot to the optical table is bad, but this is still so bad. So I do not think this is the main cause.
• Assume beam clipping in IFI would happen due to the work regarding STM2. But it seemed they used some irises to preserve the input beam axis. They reported "...We put irises after MC, before IMMT1, after IMMT2 and at the POP table." and "After STM2 was moved, we adjusted the orientation of STM2 to put the beam through the irises." Reading this, if this is true, the STM2 at the new position can steer the beam passing through the iris before IMMT1 and after IMM2, and at the POP table (??). It would be natural to think, no NEW clipping in IFI happened if the beam path is so binded. They mentioned "We also checked the beam spots inside the Faraday and found no beam clipping.", but I think it is difficult to declare such a thing definetely with the real world. But anyway, this would not affect the my conclusion.
• They rotated the STM2 mirror holder. Looking at the pdf file in 19489, the photo in 19489 would be taken before rotating the STM2 mirror holder. Actually, in the recent photo in 29342, surely the STM2 mirror holder is rotated. But how can we think only rotating the mirror holder cause such peak generation??
• By the way, I found something in the photo in 19489. The the front face of the STM2 mirror holder has three "holes" near the mirror, and one of them (at the most right position) seems to have a stop screw-like part. I am not sure well what is this for. I can assume this part may come to the top of the mirror holder now. Anyway, this type of mirror holder has unnecessarily number of parts to support the mirror (like some PEEK? parts behind the mirror). These too much parts may lower the resonant frequency of the whold mirror holder.

FYI, the latest STM2 photo can be seen in klog29410.

Several concerns are:
1. Clamp is too short to clamp the post that has a significantly larger thickness compared with the clamp.
2. The post extends slightly beyond the edge of the optical table.

• https://klog.icrr.u-tokyo.ac.jp/osl/?r=22084
• https://klog.icrr.u-tokyo.ac.jp/osl/?r=24711
• https://klog.icrr.u-tokyo.ac.jp/osl/?r=29334 : Reinforced the foot flange of the STM2 pedestal.
• No sticking out stop screws on the STM1 holder's face. https://klog.icrr.u-tokyo.ac.jp/osl/?r=21655.

I found that the strange sticking out stop screw on the STM2 holder's face did not find in 2018 https://klog.icrr.u-tokyo.ac.jp/osl/?r=5943. And the STM2's mirror holder seems Newport/New Focus 8822-AC-UHV. Looking at the web site here, I think this part should not be sticking out. And also, some photos show that the corresponding screw's head seemingly sinks into the PEEK-like small part, while the design drawing would indicate that such sinking would not happen. Seems not so healthy... If the PEEK-like part was broken due to the "too-much" screwing, and the breaking flagments are in the holder, that would cause some resonances.