Off-Axis Holography-Based Phase Retrieval and Linearly Polarized Mode Decomposition for Polarization-Resolved Transmission Matrix Characterization in Multimode Fibers

Abstract

Multimode fibers （MMFs） support high spatial degrees of freedom, which have enabled their wide application in optical communications and imaging systems. In particular, intrinsic imperfections and external perturbations induce coupling between the spatial and polarization degrees of freedom in MMFs, resulting in output polarization states that depend on the spatial distribution of the input field. In this study, techniques required for polarization-resolved transmission matrix characterization in multimode fibers were investigated. An off-axis holography-based phase retrieval method was implemented to retrieve the complex optical field, and linearly polarized （LP） mode decomposition was employed to represent the retrieved field in the LP mode basis. Since both the amplitude and phase of the field are required for TM construction, an off-axis Mach–Zehnder interferometer was employed. First, phase retrieval was performed for Gaussian and vortex beams to validate the off-axis holography-based phase retrieval method. The same method was then applied to retrieve the phase distribution of the MMF output field. To physically interpret the field and represent it on a modal basis, LP mode decomposition was performed. Based on the retrieved field, LP mode decomposition and reconstruction were performed to evaluate the suitability of LP modes as a basis for field representation. The reconstruction quality was quantitatively evaluated as a function of the number of LP modes in the basis and basis radius using the Complex Wavelet Structural Similarity Index （CW-SSIM）.