Optical cavity characterization with a mode-matched heterodyne sensing scheme

We describe a technique for measuring the complex reflectivity of an optical cavity with a resonant local oscillator laser and an auxiliary probe laser, each coupled via opposite ends of the cavity. A heterodyne sensing scheme is then used to observe the phase and amplitude of the interference beat-note between the promptly reflected field and the cavity-transmitted field injected through the far mirror. Since the local oscillator laser must pass through the cavity before interfering with the probe laser these measurements are not only independent of the spatial coupling of either laser to the cavity, but also obtained at the in-situ position of the cavity eigenmode. This technique was demonstrated on a 19 m cavity to measure the individual transmissivities of each of the mirrors as well as the round-trip optical losses to an accuracy of several parts per million.