We demonstrate a new form of interferometry which is independent of Maxwell’s equations and measures optical frequency differences rather than wavelength ratios. Stable laser and electro-optic FM sideband techniques make a direct measurement of the mode spacing of a high finesse cavity with subhertz precision (0.4 Hz rms) over a 50-THz region. Experiments indicate errors below ${10}^{\mathrm{-}10}$ over a 50-THz bandwidth and that improvements of several orders of magnitude are possible. New variables for interferometry simplify the use of multilayer dielectric mirrors and are directly measurable in situ. .AE

• Received 2 December 1987

DOI:https://doi.org/10.1103/PhysRevA.37.1802