Abstract:

The OLTF of CARM loop when PRFPMI is locked withRF signals has peaks and notches around 50kHz and 100kHz while there is no clear peak and notch aaround 25kHz and 75kHz.
So, it is necessary to careful gain adjustment to keep UGF less than 50kHz during increasing laser power.

Detail:

I checked similar peak and notch stractures reported in klog35455 can be seen in CARM OLTF when PRFPMI was locked with RF signals.
Figure 1 shows the OLTF of CARM from 10kHz to 300kHz, which is the usually used measurement band for CARM loop.
Figure 2 to 6 show the OLTF of CARM around 25kHz, 50kHz, 74kHz, 76kHz, and 100kHz, respectively.
There are peak/notch pairs around 50kHz and 100kHz while there are no clear peak and notch around 25kHz and 75kHz.
So, the stracture of CARM OLTF is somehow different between single X arm lock and PRFPMI RF lock.

Anyway, due to the stracture around 50kHz, if the CARM UGF becomes greater then 50kHz, CARM loop would become unstable.
So, it is necessary to careful treatment of CARM UGF during increasing laser power to avoid instability.
Since current guardian increases UGF upto 60kHz, we need to modify the guardian.

Note:

To confirm the stracture around 25kHz and 75kHz come from the X-arm or optical components, it would be nice to perform similar measurement with single Y-arm lock.