@_thegeoff btw I've realized that heterodynic(sp?) interferometry is a thing, but haven't managed to dig up how it works exactly (in particular, what's the nonlinear mixer that can accept light and can emit RF, or how does it work without that).
@robryk Going well beyond my level now! From the sounds of it the idea is to superimpose the interferometry signal on a much stronger carrier wave, old-school AM radio style?
I've found a bit more on how that works and it's amusingly simple (tl;dr with a good timing source one can very precisely integrate frequency over time by just counting): https://spie.org/publications/spie-publication-resources/optipedia-free-optics-information/tt61_541_laser_interferometer
The thing I'm still missing is how one gets two closeby frequencies _at orthogonal polarisations_ out of a laser.
