Thermal Drift of Optics in Separated-Beam Heterodyne Interferometers

Designs of next-generation interferometers tend to place most of their emphasis on periodic nonlinearities rather than on thermal drift of the optics, which can contribute more than the well-known periodic errors to the overall error. In this paper, a novel model for analyzing the mechanism of thermal drift in a separated-beam heterodyne interferometer (SBHI) is developed. To verify the proposed model, an SBHI with symmetric optical configuration is constructed, and an asymmetric SBHI is realized by inserting a glass plate into the measuring arm of the symmetric interferometer. Comparison tests are carried out between these two SBHIs. Experimental results show that the symmetric SBHI achieves a thermal stability of 1.2 nm/°C, and the thermal coefficient obtained from experiments, for the asymmetric SBHI, is close to the theoretical value calculated based on the proposed model.