[Takano, Akutsu]

Continued from 11860

Abstract

We injected low-freq peaks to the ITMX/ETMX to investigate the strange coupling of P/Y at the IR QPDs of TMSX. So far, it seems the coupling coefficients are different at the same QPD in the condition that ITMX or ETMX is shaken even in the same DoF, which means it would be difficult to resolve this issue by simply inserting a diagonalization matrix for pitch and yaw.

What we did

To conform the situation further, we actuated the ITMX and ETMX and observed the responses from the two QPDs at the TMSX; the trials were all done with FPMI_LOCKED state today (though only we needed was Xarm only :) ) We firstly checked things one by one (the parameters are {E|I}TMX and {PITCH|YAW}), and took some data, but/and finally we shook all of them at once with different frequencies; see Fig 1. The excitations were injected from K1:VIS_{E|I}TMX_MN_LOCK_{P|Y}_EXC; for ITMX gain = 400, while for ETMX gain =80 at awggui. ITMX-PITCH: 0.10Hz, ITMX-YAW: 0.12Hz, ETMX-PICH: 0.09Hz, ETMX-YAW: 0.15Hz (see Fig 2).

Thanks to the stable lock of the interferometer, we could take a long data to distinguish those close frequency peaks (13:44 JST to 14:15 JST); ended by down of the interferometer.

Discussion

• Comparing the peaks, we can find the coulping coefficients are different at the same QPD in the condition that we actuated ITMX or ETMX in the same DoF!! (for exampl, even at the same QPD, you will obtain different coupling coefficient from yaw to pitch when you actuate ITMX and ETMX in pitch.) That means it will not be possible to resolve the coupling issue by a simple diagonalization matrix.When we did this one by one at 0.1Hz, we saw the coupling coefficients were consistent (later the numbers should be shown) with this all-at-once test, so I don't think the difference of the coefficients would be due to the difference of the frequencies at which each mirror was actuated...
• Crazy thing: looking at the 0.15Hz peaks (excited by ETMX-yaw), QPD2 shows even more pitch (green) than yaw (brown), while QPD1 shows an ordinary behavior (more yaw (blue) than pitch (red)).
• By the way, during the measurements, we found K1:VIS_BS_TM_OPLEV_PIT_DIAGMON showed very similar fluctuation w/ those QPDs, while the Yaw did not. So I checked their coherence. (Fig 3) What is this???? It is interesting that even the frequency at which ETMX was shaken, there is ~1 coh. (especially in pitch). We actually saw the flow of beat-ish signal in the BS oplev pitch channel shown always in the display at the control room, and that's why we could discover this.

We have no idea