Polarization differential self-mixing interference of a Zeeman laser with second-order phase modulation for retardation determination

This study presents a dual-frequency laser self-mixing interferometry system that integrates polarization differential interference within the resonator with phase modulation in the feedback cavity. The dynamic response of a Zeeman laser to the reinjected optical field, with varying orders of phase modulation facilitated by dual acousto-optic modulators (AOMs), is systematically investigated. By leveraging a transverse magnetic field and intracavity stress-induced birefringence to generate orthogonally polarized dual-frequency light, the orthogonally polarized modes undergo secondary phase modulation via dual acousto-optic modulators (AOMs), thereby yielding composite modulated signals with prominent higher-order components. Utilizing Bessel functions and Fourier expansion, the amplitude ratio and phase delay of the two modulated self-mixing interference fringes are quantitatively compared. To validate the system's polarization differential interference, a liquid crystal variable phase retarder is employed to induce external-cavity phase retardation, revealing nonlinear correlations between its driving voltage and self-mixing interference parameters.