Abstract:

I measured transfer function from OMC error signals to DARM by exciting OMC length by PZTs.
According to the measurement results, linear coupling from OMC error signals to DARM is more than 10 times smaller than current sensitivity.

Detail:

Since acoustic injection data is difficult to separate the effect of OMC length fluctuation from the other effect suc as vibration of chambers, stacks, snd so on, I measured the transfer function from OMC error signals to DARM by injecting the signals into OMC LSC loop.
In this method, we can excite OMC length without affecting the other components, os we can purely estimate the linear coupling of OMC length fluctuation.

Figure 1 shows the TF and coerence from OMC error signals to DARM.
Since the coherence is low, this TF doesn't seem correct but at least this result would give us an upper limit of TF fromOMC error signals to DARM.

Figure 2 shows the noise projection from the OMC error signals to DARM by using measured TF.
Though this noise projection is an upper limit, the projection line is more than 10 times smaller than the current sensitivity.
So, linear coupling from OMC error signals to DARM seems already small enough for the next observing run.

Note:

During the measurement, we can clearly see the non-linear coupling of OMC error signals, so this method can be also used for investigating non-linear coupling.
So, it's worth trying to inject lines or broadband white noise into OMC LSC loops and see what happens.