Static birefringence sensing without polarization modulation using periodic nonlinearity in a quadrature interferometer

This work demonstrates weak-birefringence sensing using periodic nonlinearity (PNL) in a quadrature interferometer, where PNL is utilized as a sensing mechanism rather than treated solely as an error source. A weak birefringence-induced phase perturbation is converted into a measurable change in the PNL amplitude at a dedicated spatial frequency, enabling static retardance sensing without polarization modulation. Experimentally, the measured PNL response exhibited a highly linear region over the retardance range, with . The observed amplitude fluctuations corresponded to an equivalent retardance standard deviation of approximately , or about of phase retardation at . The measured response also revealed distinct effects of quadrature phase offset and gain imbalance, consistent with the predicted deformation of the PNL minimum. These results show that PNL-based phase-to-amplitude transduction provides a simple and sensitive approach for static weak-retardance measurements in quadrature interferometric systems.