In order to better measure the effect of this nonlinear current to frequency modulation, we'll need to do Zach's measurement but with much higher drive frequencies. (His measurement was 1kHz).
We'd like to do a full TF of the nonlinear current amplitude modulation path to the laser frequency. There are two effects in Zach's setup that limit the bandwidth of the measurement.
First, is the modulation input of the Marconi, which only reaches 30kHz. We plan to use a mixer to do higher frequency AM of the RF carrier.
The second is the frequency readout. We potentially could PLL the two lasers together and have a pretty high bandwidth readout. or, instead we decided to add some additional PDH sidebands to the light using the fiber modulator. This was then sensed in reflection of the PMC and demodulated. We used 30MHz at 0.5Vpp into the fiber modulator.
With this setup, we were able to measure some amount of nonlinear current to frequency modulation, and when we unlocked the cavity the transfer function was reduced by at least 20dB, which rules out some other coupling path.
Next step is to set up high bandwidth AM of the 500MHz marconi output (driving the current).