As I mentioned in my last aLOG entry [8087], when performing the IMC sideband sweep measurements, the broad peaks when the sideband frequency is around an integer multiple of the IMC FSR are only expected to appear in the presence of a length/frequency lock offset. As such, we can try to compensate any offset in the length error signal by applying a digital offset in the IMC-L feedback path until these peaks are minimized*.

I recorded sweeps across the 45.5MHz FSR peak while adjusting the offset in the IMC-L path. The results are shown in the attached plot. As the offset is increased towards 20k counts, the peak reduces in magnitude.

Beyond 20k counts, the IMC would not stay locked long enough to make the measurement. I suspect this is caused by one of the outputs railing due to the additional offset, but I wanted to move on to higher-order mode peak measurements so I didn't investigate further. Maybe there is a better location to add the offset, for example on the analog common mode servo board itself. Apart from the lock losses above 20k counts, I didn't notice any drop in the IMC transmitted power with added offsets. This eliminates the possibility that the offset was driving the cavity further from carrier resonance and thus causing a large drop in the carrier light power, in turn causing the peak to drop in magnitude.

On a side note, since a bigger locking offset makes the FSR peaks easier to observe, it might be worth deliberately making this offset worse (i.e. adding -20k counts) for future measurements of the IMC FSR/length. This could help improve the SNR around e.g. the 18.2MHz, 27.7MHz and 54.5MHz peaks to make those measurements useful.

*At low frequencies the IMC-L path dominates over the IMC-F path, so applying an offset in the IMC-L path should suffice and eliminate the need to directly cancel any offset at the analog servo board input.