  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.
