  Balanced homodyne interferometry is a well-known detection technique that
allows for sensitive characterization of light fields. Conventionally a
homodyne interferometer is operated by locking the relative phase of a
reference beam to the signal beam by means of an active feedback loop. A less
often used method is to perform a slow continuous modulation of the reference
beam arm length that corresponds to averaging all relative phases during the
measurement.Here we show theoretically and experimentally that this quadrature
averaging can be advantageous in estimating the parameters of a resonant
optical cavity. We demonstrate that the averaging turns the transduction
function, from cavity frequency fluctuations into the interferometer signal,
into a simple function of the laser detuning that, notably, does not depend on
the parameters of possible non-resonant channels present in the system. The
method needs no active feedback and gives results that are easy to interpret.
Moreover, the phase-averaged measurement allows to characterize the absolute
magnitude of a cavity frequency modulation.
