[Dan, Fujimoto, Komori, Sugimoto, Sugioka, Tanaka, Yokozawa, Ushiba]

Abstract:

Linear coupling from OMC stack vibration (OMC geophone) to DARM displacement was projected into DARM sensitivity.
Estimated noise level is much lower than the current sensitivity, so linear coupling from OMC stack vibration is not a problematic at this moment.

Detail:

We measured TF from OMC geophone to DARM displacement by shaker injection installed on the beam duct between OMC and OMMT chambers (klog31343).
Figure 1 shows the TF from OMC geophone to DARM displacement (IFO lost lock during the measurement, so the data below 30Hz could not be taken in this measurement but it should be fine because arm diaplacement noise would be a dominant noise at low frequency).

Figure 2 shows the noise projection into DARM with the sensitivity.
Note that, the geophone spectrum above 220 Hz seemed sensing noise according to geophone raw spectrum (fig3), so the noise projection above 220 Hz is an upper limit
So the linear coupling from OMC stack vibration to DARM displacement seems small enough at this moment as well as linear coupling from OMC error signals.

Note:

We are not so sure this linear coupling is reduced when increasing laser power (for example, this noise comes from the scattered light noise by the junk light at AS port, bule line won't be lowered even when increasing laser power.).
If the noise projection won't be lowered when increasing laser power, the blue line will be a sensitivity limit when injecting large power at some point.
Though this noise level is not a problematic for the next observing run, laser power dependence of OMC stack vibration noise should be checked again after increasing laser power.