Miniature heterodyne interferometer with reduced periodic nonlinearity and enhanced thermal stability for ultraprecision measurement

Heterodyne interferometers are widely used in ultraprecision measurement due to their high resolution and traceability. With the rapid development of extreme ultraviolet lithography, wafer alignment, and other fields, new requirements have emerged regarding the miniaturization and high thermal stability of interferometers. However, miniaturization is often constrained by periodic nonlinearity (PNL) and the thermal drift of optics (TDO). To address these issues, this work establishes a polarization–nonlinearity transfer model based on the Jones matrix formalism and optimizes the key polarization-dependent parameters of the optical components, thereby suppressing the “Type-I PNL” originating from polarization leakage at its source. In parallel, refractive index matching is employed to reduce parasitic interface reflections, and microwedged angles are introduced at the key optical interfaces to induce spatial mismatch of the ghost-reflection beams, effectively mitigating the “Type-II PNL” caused by their self-mixing interference. Furthermore, by implementing paired and symmetric microwedged angles, the optical-path imbalance introduced by single-sided wedge tuning is eliminated, enabling simultaneous realization of low nonlinearity, high stability, and high integration. Based on the proposed methods, a prototype interferometer was developed and experimentally evaluated. The experimental results show that the proposed interferometer achieves a PNL of approximately 59 pm, a TDO coefficient below 6 nm/K, and a temperature-drift-corrected measurement stability with a peak-to-peak variation better than 0.8 nm over 5 h. Furthermore, the direct comparison with a commercial interferometer operating in air shows a standard deviation of the difference of 5.1 nm.