[Jon, Gautam, Johannes]

Summary: In support of making a proof-of-concept RF measurement of the SRC Gouy phase, we've implemented a PLL of the aux. 700mW NPRO laser frequency to the PSL. The lock was demonstrated to hold for minutes time scales, at which point the slow (currently uncontrolled) thermal drift of the aux. laser appears to exceed the PZT dynamic range. New (temporary) hardware is set up on an analyzer cart beside the PSL launch table.

Next steps:

- Characterize PLL stability and noise performance (transfer functions).

- Align and mode-match aux. beam from the AS table into the interferometer.

- With the IFO locked in a signal-recycled Michelson configuration, inject broadband (swept) AM sidebands via the aux. laser AOM. Coherently measure the reflection of the driven AM from the SRC.

- Experiment with methods of creating higher-order modes (partially occluding the beam vs. misaligning into, e.g., the output Faraday isolator). The goal is identify a viable techinque that is also possible at the sites, where the squeezer laser serves as the aux. laser.

The full measurement idea is sketched in the attached PDF.