Takano, Washimi

We roughly estimated the sensitivity of OMC TRANS in terms of RIN and tried to find the noise sources.

Figure1 shows the current sensitivity of OMC TRANS normalized by DC power of 27 mW with different settings. During these measurements ISS was engaged.
The black curve is the dark noise of PDs. All the other curves are well above the black curve, therefore these curves were not limited by the PD dark noise.
The red curve is the sensitivity with both OMC QPD and OMT2 TRANS QPD centering loops are closed. Above 200 Hz, the sensitivity is limited by some white noise to ~ 2e-6 /√Hz.
The blue curve is that with only OMMT2 TRANS QPD centering loop is closed. In this case, DC power was degraded to ~ 13 mW as shown in Figure2, but the normalization factor was not changed, therefore the true RIN is about 2 times larger than this plot. The curve is limited by a white noise above 40 Hz to ~ 5e-7 /√Hz, that means the true sensitivity considering the difference of DC power is ~ 1e-6 /√Hz, which is still 2 times better than the red curve.
The green curve is the sensitivity with only OMC QPD centering loop. Unlike the case of the blue curve, the OMC TRANS power was ~ 27 mW, almost the same as the power with both centering loop closed. Between 2 Hz and 100 Hz, the sensitivity without OMMT2 QPD centering loop was better than with the loop closed. It seems that we have to optimize the control loop, or not to engage this loop during the observation mode. Above 200 Hz, the sensitivity is still limited by some white noise and almost the same as the red curve.

We also took coherence with other channels. Figure3 shows the coherence between OMC TRANS and other channels. First, it seems that so far the intensity fluctuation does not limit the sensitivity linearly. Second, there is coherence between the OMC TRANS and many witness channels of OMMT1/2, OSTM and OMC QPD below 100 Hz. This implies that the beam jitter couples to OMC TRANS and reduction of beam jitter is necessary. Third, none of these channels has coherence with OMC TRANS above 200 Hz. Therefore, we are still not sure the source of the white noise in OMC TRANS above 200 Hz.

Note

The current sensitivity is more than 10 times larger than the acoustic noise level estimated in O4a (Figure4). It seems hard to test PEM injection with the current sensitivity. It could happen that FPMI configuration improve the sensitivity, but I'm not sure.