Two-dimensional displacement measurement by quasi-common-optical-path heterodyne grating interferometer.

A method based on a specific quasi-common-optical-path (QCOP) configuration for two-dimensional displacement measurement is presented. The measurement system consists of a heterodyne light source, two-dimensional holographic grating, specially designed set of half wave plates, and lock-in amplifiers. Two measurement configurations, for single and differential detection, are designed. The sensitivity, resolution and nonlinear phase error of the differential detection type are better than those of the single detection type. The experimental results demonstrate that the QCOP interferometer has the ability to measure two-dimensional displacement while maintaining high system stability.