[Fujimoto, Komori]

Abstract:

We will try to reduce the intensity noise on the OMC REFL PD in order to perform cavity scans with a better signal-to-noise ratio.
We measured the intensity noise of the injected light and confirmed that it is sufficiently small.

Details:

As reported in klog:37124, we are suffering from intensity noise on the OMC REFL PD, which degrades the signal-to-noise ratio of the cavity scan.
Figure 1 shows the recently measured intensity noise, where the red and orange lines represent the best-case measurements with good alignment and intensity stabilization turned on.
If we can reduce this noise, we will be able to estimate the FSR more precisely.

First of all, we suspected that the intensity stabilization might have degraded because its performance had not been checked for some time.
Figure 2 shows the intensity noise measured by the out-of-loop PD with the ISS turned off and on, shown in red and blue, respectively.
This result indicates that the ISS is working properly, and that the other noise sources should be investigated as possible origins of the intensity noise on the OMC REFL PD.

Other noise candidates are as follows:

• Frequency noise
  Currently, the IMC is locked using a combination of frequency feedback and MCE mass feedback.
  We do not know the crossover frequency at the moment, but if it is too high, the frequency noise may not be sufficiently suppressed.
• Suspension noise
  Some suspensions related to the SRY cavity may be noisy enough to imprint displacement and angular motion onto the intensity noise.
• Beam clipping
  Beam clipping may occur at the OMC REFL PD.