Two-frequency interferometer for a displacement measurement

We report on the construction of a two-frequency Michelson interferometer to measure small displacements based on the heterodyne principle. Unlike the common single-frequency interferometer, where relative displacements produce changes in output power, in the two-frequency device, displacements lead to phase shifts of the beating signal. The short- and long-term performance of the single- and two-frequency methods are compared. The heterodyne apparatus was also used to calibrate a piezoelectric transducer.