[Tanaka, Ushiba, Komori]

Abstract:

We started investigating a mysterious coupling between the IR flashing and the green PDH signals.
Our initial tests suggest that the coupling is likely mediated through the RF signals of the IR RFPDs.

Detail:

The issue of the mysterious IR–GR coupling has persisted for a long time.
Due to this issue, we cannot open the shutter in front of the REFL QPDs or increase the power at the out-of-loop CARM PD, and we frequently suffer from lock losses during the IR handover stage.
If this issue can be resolved, a significant improvement in several operational aspects is expected.

To investigate the origin of this issue, we compared the power spectrum of the ALS DARM signal under several configurations, as shown in the figure.
The reference spectra are the blue and green lines, where the PRM is misaligned and aligned after the IR finds resonance with the conventional cable connections, respectively.
IR flashing clearly produced a noisy DARM spectrum, consistent with the noisy behavior observed in the time-series data.

First, we turned off all REFL PDs by switching off the PD power supply board, and we did not observe the additional noise (brown).
Next, we focused on one of the problematic PDs, REFL PDA3, with the PD power supplies for PDA1 and PDA2 turned off and only PDA3 powered on, while the RF output signal was terminated (magenta).
We did not observe additional noise in either case.
This result suggests that the issue is not caused by a simple cable connection without the power supply, but rather by the connection between an operating PD and the demodulator.

Next, we measured the noise with PDA3 connected using a different cable instead of the conventional one.
In this case, the noise reappeared (green), indicating that the issue does not depend on the cable routing.

Tomorrow, we will test the effect of inserting an RF transformer (T1CA made by R&K), which isolates the ground and blocks the DC signal between the input and the output.