[Kiwamu, Stefan, Joe, Paul]

The first measurements for the IMC g-factor have been made, using the sideband sweep method described in e.g. LLO alog 4849 (https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=4849).

The broadband REFL AIR photodiode was used to observe light transmitted through the IMC while the 45MHz EOM input was driven from the HP network analyzer in the LVEA. We used the minicircuits RF amplifier which is currently clamped to the ISC rack to get enough drive voltage on the EOM: roughly 10Vpp at 30MHz. An iris was used to partially block the beam on ISCT1, breaking the symmetry on the PD such that the higher-order mode signals were not cancelled out due to their spatial orthogonality.

The attached plot shows the transfer function from network analyser drive signal to BB REFL AIR measured signal, over a range of about 1 FSR. The central peak can be attributed to the HG00 mode, but also displays the resonance of the EOM driver circuit at ~45.5MHz. The HG10/01 and HG20/02 mode peaks are clearly visible, and match up nicely with a Finesse model setup of the measurement. 

To measure the resonance frequencies of the higher-order modes more accurately, and hence to get an accurate value for the IMC g-factor, we will take zoomed in sweeps around these higher-order mode peaks soon.