*acousto-optic, of course. (I swear, auto-incorrect is getting worse.) To expound slightly on your (correct) explanation: The phonons impart (or remove) energy

*acousto-optic, of course. (I swear, auto-incorrect is getting worse.) To expound slightly on your (correct) explanation: The phonons impart (or remove) energy to (from) the laser light, the same way a moving mirror does: via doppler shift. I made a differential heterodyne interferometer in undergrad using the method you describe.