Two-frequency laser interferometer for small displacement measurements in a low frequency range.

A two-frequency Michelson-type interferometer is studied for small displacement measurements. The interference term is composed of the two effects due to the optical homodyne and heterodyne detection processes. The small vibration signal of concern, which is included in the amplitude of a heterodyne beat component with no phase, is obtained with A.M. detection techniques. The interferometer allows measurements of any vibrational modes of a test mirror. For a sinusoidal vibration of 2 kHz, its amplitude is measured down to 1.2 A, whereas for a vibration of rectangular mode at 0.2 Hz its minimum displacement is 2.9 A. The small time-dependent refractive index of air caused by a process similar to the photoacoustic effect is detected as another application.