Phase-modulated interferometer for evaluating phase displacement resulting from charges in path length

摘要

A phase-modulated interferometer with novel control and signal processing utilizes superimposition signals capable of evaluation in a phase-modulated interferometer without complicated control of the phase modulator. A sinusoidal control signal with a modulation frequency (. omega..sub.0) having an amplitude (.sub.0) is supplied to a known phase modulator. Multiplicative mixing of the superimposition signal produced in the interferometer from the measuring and reference arm with a sinusoidal signal of a determined mixing frequency (.sub.M) which is rigidly coupled with respect to phase and frequency with the control signal is effected. When the amplitude (.sub.0) of the control signal satisfies the condition for a suitable operating point of the phase modulator, a cosine signal conventionally used for evaluating the phase displacement is filtered out in an electronic bandpass filter at whose filter frequency (.sub.F) the sum and difference frequencies of two harmonics of the modulation frequency (.sub.0) and the mixing frequency (.sub.M) assume the same value. The invention is applied in phase-modulated interferometers, in particular for precision distance measuring devices, preferably by the heterodyne evaluating method.