[Fujimoto, Yokozawa, Smith, Saito, Komori]

Abstract:

We successfully reduced the intensity noise on the OMC REFL PD.
The noise sources were found to be control noise from the SRY length and OMC length control loops, as well as residual 9 Hz noise in the OMC length due to insufficient gain.

Details:

As a continuation of the work reported in klog:37137, we investigated the origin of the intensity noise measured on the OMC REFL PD.

First, we measured the noise with the MCE feedback, as described in klog:37137.
We found that we had already tried this configuration previously (brown vs. red line in klog:37137), and that using only frequency feedback without mass feedback gives better performance.

Next, we tried to reduce the intensity noise by locking the OMC and allowing the carrier intensity noise to escape to the OMC transmission port, as proposed in klog:37139.
Although the intensity noise was reduced at high frequencies, the noise bump around 100 Hz did not change.
This suggests that the noise originates from intensity fluctuations of the RF sidebands.

The next step was to identify the origin of the RF sideband intensity noise.
In principle, the coupling from SRC length noise to intensity noise should be quadratic, but asymmetry can introduce linear coupling.

We turned on the whitening filter of the RF PD currently used to measure the SRY length signal (POP17-I), since sensing noise can introduce additional noise through the feedback control.
However, this did not change the noise, which means that the sensing noise is not dominated by ADC noise, but by other sources such as PD dark noise.

The hypothesis that sensing noise causes control noise in the SRY length loop seemed to be correct, because the shape of the noise spectrum reflects the transfer function of the SRCL filter, including the phase compensation and roll-off elliptic filters, as shown by the green line in Fig. 1.
When we changed the cutoff frequency of the elliptic filter from 300 Hz to 100 Hz, we immediately obtained an improved spectrum without the OMC locked, shown in orange, and a further improved spectrum with the OMC locked, shown in blue.

We also observed a 9 Hz peak, which is known to be a problematic resonance around the OMC, along with many peaks at high frequencies.
We successfully reduced these peaks by increasing the UGF of the OMC length control from 7 Hz to 30 Hz, as shown in brown.

In addition, we adjusted the filters to use a lower gain in order to further reduce the control noise.
We also found that the control noise of the OMC length loop was another dominant noise source around 100 Hz, so we reduced the filter gain while maintaining the same UGF by increasing the dither amplitude.

Finally, we obtained the lowest noise spectrum, shown in red, which is close to the dark noise level, shown in purple.
For reference, the coherence between the OMC REFL signal and the ISS/SRCL noise is shown in Fig. 2.

Using this configuration, we will perform the cavity scan with a better SNR.