Improving Alignment of Free-Space Coupling of Multi-Mode Fibres using Off-Axis Digital Holography

Off-axis digital holography is employed to align multi-mode fibers in a free-space optical setup that can be used for space-division multiplexing (SDM) transmission. We show that alignment based on power coupling measurements alone does not guarantee a low mode-dependent loss, limiting the system capacity. The alignment method we proposed previously enables reliable fiber coupling with low mode-dependent loss and cross-talk for few-mode to multimode fiber alignment, by using digital holography to capture the full complex optical field at the output of the fiber of interest. After capturing the full complex field, by means of digital demultiplexing, we can calculate relevant
parameters such as mode-dependent loss and cross-talk. Here we extend these results with few-mode to few-mode fiber alignment measurements and look at alternative optimization metrics such as the cross-talk between the mode groups of interest and all guided modes. The proposed method allows for precise (automated) alignment of space-division multiplexing components, devices and subsystems.