  Length and g-factor are fundamental parameters that characterize optical
cavities. We developed a technique to measure these parameters in-situ by
determining the frequency spacing between the resonances of fundamental and
spatial modes of an optical cavity. Two laser beams are injected into the
cavity, and their relative frequency is scanned by a phase-lock loop, while the
cavity is locked to either laser. The measurement of the amplitude of their
beat note in transmission reveals the resonances of the longitudinal and the
transverse modes of the cavity and their spacing. This method proves
particularly useful to characterize complex optical systems, including very
long and/or coupled optical cavities, as in gravitational wave interferometers.
This technique and the results of its application to the coupled cavities of a
40-meter-long gravitational wave interferometer prototype are here presented.
