Abstract

The heterodyne interferometer based on the two-frequency laser is widely used in metrology. This type of Michelson interferometer has a small nonlinearity which limits its accuracy. The nonlinearity is caused by the mixing between the phase shifted beat frequency from the measurement arm and secondary pure beat terms present in the optics. These pure beat terms arise from imperfect separation of the two frequency components at the beam splitter and secondary reflections from the optical surfaces. The nonlinearity has a periodicity of λ/2. The nonlinearity is directly observed by comparing the output of two independent interferometers which simultaneously monitor the displacement of a small oscillating mirror. An indirect but simple method of measuring the nonlinearity is available. In the presence of nonlinearity the average receiver power is modulated as the measurement arm mirror is moved. The fractional power modulation is approximately equal to the nonlinearity in units of λ/4π. The nonlinearity is typically λ/45.

© 1987 Optical Society of America