[Saito, Takano, Dan]

Background

In the previous investigation, two beams were observed on the beam profiler. One of them was suspected to be a beam coming from ITMX. Therefore, we decided to largely misalign ITMX in order to remove this beam and then try the PLL control using the remaining beam.

Recovery of the previous condition

First, we restored the setup to the same condition as last Friday. The situation was successfully reproduced, and two beam spots were observed again on the beam profiler.

Misalignment of ITMX

ITMX was set to MISALIGNED_BF. After this operation, the beam spot located in the lower part of the beam profiler image moved and disappeared from the beam profiler screen. A screenshot was taken at this point.

This result suggests that the disappeared beam was likely associated with ITMX, and that the remaining visible beam was mainly the beam from ITMY.

PLL control trial

We then tried to control the auxiliary laser frequency with a PLL. The auxiliary laser beam and the interferometer laser beam were overlapped on a PD, and the beat signal was detected. The beat signal was mixed with a 75 MHz LO signal. After passing through low-pass filters of 100 kHz and 1 MHz, the signal was used as the error signal for the PLL control.

The beat frequency was fluctuating significantly, by several MHz to more than 10 MHz. The control filter was implemented using MokuLab. At this stage, the filter was just a flat gain.

When the control loop was closed, the beat signal became much broader in the frequency domain, and its sharpness was significantly degraded. This suggests that the feedback control was probably not working as intended. Instead, noise was likely injected into the auxiliary laser frequency through the PZT actuator, which contaminated the beat signal.

Control loop configuration

The control loop used in this trial was:

RFPD  -> Mixer with 75 MHz LO  -> 100 kHz LPF  -> 1 MHz LPF  -> MokuLab, gain only  -> SR560, for range expansion  -> Auxiliary laser PZT

Summary

• The previous two-beam condition was reproduced.
• After setting ITMX to MISALIGNED_BF, one beam spot disappeared.
• PLL control was attempted using the beat signal, but we could not close the loop.
• The loop likely injected noise into the auxiliary laser frequency rather than stabilizing it.