High precision free spectral range measurement using a phase modulated laser beam

A simple, new technique for high precision measurement of the free spectral range of an optical cavity is presented. The technique uses a laser beam which is phase modulated at a frequency near an integer multiple of the cavity free spectral range. When the modulation frequency is equal to the integer multiple of the free spectral range, a sharp dip in the component at the modulation frequency of the photocurrent generated by the transmitted beam is observed. Experimental measurements with a precision of 1.5 parts in 107 and theoretical discussion of the mechanism are given.