This study presents an analysis of the nonlinearity resulting from polarization crosstalk at a polarizing beam splitter (PBS) and a wave plate (WP) in a homodyne interferometer. From a theoretical approach, a new compensation method involving a realignment of the axes of WPs to some specific angles according to the characteristics of the PBS is introduced. This method suppresses the nonlinearity in a homodyne interferometer to 0.36 nm, which would be 3.75 nm with conventional alignment methods of WPs.

This note presents a digital signal processing module for the real-time nonlinearity compensation of a homodyne interferometer. The nonlinearity is corrected by using the parameter values describing two phase-quadrature signals, through simple arithmetic calculation of the quadrature signals at specific phases, which are multiples of π/4. A field-programmable gate array was employed for the real-time implementation of a processing module since it has reconfigurable input/output and high precision synchronization. The developed module has a minimum loop time of 4.4 µs and can compensate the nonlinearity error less than ±0.5 nm, which is comparable with the elliptical fitting method. We also proved the performance of the module by examining the convergence and the stability of parameter values under various operational conditions.