[Mackenzie, Eleanor, Paul]

This afternoon we got the PLL aux laser measurement up and running, and fortunately this happened to coincide with a locked DRMI. 

We were initially able to observe the FSR dips, as we had seen them previously. We decided since we had the benefit of a locked DRMI we would try adding the clipping objects in the aux laser input path, and also in front of the REFL AIR B diode that was used to detect the beat note between the aux laser reflected from the PRM and the PSL beam reflected from the PRM. 

We observed new features in the transfer function, which are likely candidates for higher-order mode resonances. Attached are two preliminary plots of sweeps close to two FSR HG00 resonances.

A very quick calculation of the PRC one-way Gouy phase from these data is as follows:

HG00 peak resonance frequency - HG10 peak resonance frequency = 0.3MHz.

Fraction of FSR = 0.3/2.6 = 0.1154.

Convert to degrees one way Gouy phase: 0.1154*180 = 20.77degrees.

With the ITMs that are currently installed, we expect a one-way Gouy phase in the PRC of about 18 degrees (with no ITM thermal lensing). Design value accounting for thermal lens in ITMs from 18W input power and 0.5ppm absorption on the ITM HR surface is 25 degrees. 

Worth noting is that the DRMI was locked during this measurement, not the PRMI. It's not clear to me yet how this affects the results – maybe not much. Also, evidence for the additional peaks really being due to HOMs is the repeated resonance a further 3MHz away from the HG00 resonance, which I expect is the HG11+HG20+HG02 mode resonance. More precise analysis and measurements to follow.