The strength of optomechanical interactions in a cavity optomechanical system can be quantified by a vacuum coupling rate $vcr$ analogous to cavity quantum electrodynamics. This single figure of merit removes the ambiguity in the frequently quoted co
upling parameter defining the frequency shift for a given mechanical displacement, and the effective mass of the mechanical mode. Here we demonstrate and verify a straightforward experimental technique to derive the vacuum optomechanical coupling rate. It only requires applying a known frequency modulation of the employed electromagnetic probe field and knowledge of the mechanical oscillators occupation. The method is experimentally verified for a micromechanical mode in a toroidal whispering-gallery-resonator and a nanomechanical oscillator coupled to a toroidal cavity via its near field.
