[Yuzurihara, Komori]

Abstract:

We performed a noise budget analysis for the sensitivity measured on March 3rd, which is likely the best sensitivity achieved so far, albeit for a very short duration.
The most critical noise sources are considered to be input jitter noise and coil driver (actuator) noise.

Details:

One of the best sensitivity levels was achieved on March 3rd.
We extracted the DARM spectrum from 11:13:10 UTC for approximately 40 seconds (8-second timespan, 75% overlap, 20 averages).
At that time, the optical gain in the DGS was under-estimated by 2 dB.
To correct for this, instead of using the calibrated DARM signal, we used the error signal (OMC-TRANS_DC_SUM_OUT) and recalibrated it using the optical gain compensated and the open-loop transfer function by 2 dB.
Therefore, it should be noted that this best sensitivity result is still preliminary.

We performed a noise budget analysis considering only suspension thermal noise, actuator noise, input jitter (REFL acoustic) noise, and shot and dark noise.
It should be noted that the input jitter noise was measured with an input power of 1 W.
These noise sources are the primary limitations of the current DARM sensitivity.
If we assume the pessimistic actuator noise scenario (represented by the dark green line in the attached figure), as described in klog:32132, the total estimated noise aligns more closely with the measured data.

To further improve sensitivity, we need to mitigate input jitter noise and actuator noise.
Potential solutions include addressing STM2 jitter and replacing the high-power coil driver with a low-power alternative, respectively.