Compact-size and high mode-scalability spatial-mode multiplexer using multiplexed volume holograms

A spatial mode multiplexer that uses multiplexed volume holograms provides a compact and high mode-scalability solution for multiplexing spatial modes in mode division multiplexing (MDM) transmission. In this approach, multiplexed volume holograms designed to match the electric field distributions of spatial modes are prerecorded on a recording medium such as a photopolymer or a photorefractive crystal. These recorded volume holograms are then used to multiplex and demultiplex spatial modes using the diffraction effect. In this study, we developed and evaluated volume holograms while simultaneously optimizing the exposure dose and compensating for the incident angles of the object and reference beams, ensuring the effective operation of the device as a mode multiplexer/demultiplexer. Additionally, we performed the first evaluation of a back-to-back configuration within a mode multiplexer/demultiplexer using multiplexed volume holograms. The configuration of the mode multiplexer based on volume holograms resulted in a compact optical system comprising two lenses, a single-mode fiber array, and a recording medium. Experimental results demonstrated that when the diffracted beam from the volume holograms was coupled to a three-mode fiber, an excitation ratio of 25 dB or more was achieved between LP01 and LP11, confirming the effective operation of the volume holograms as a spatial mode multiplexer. Furthermore, during the evaluation of the back-to-back characteristics, the maximum modal crosstalk was measured at approximately -20 dB, meeting the requirements for MDM transmission systems.