Tunable Filtering of Orbital Angular Momentum Modes via Spatial Depth-Dependent Mode Transformation

Advancements in orbital angular momentum (OAM) mode-multiplexing communication networks require tunable mode filters for selective channel demultiplexing and downloading. In this study, we propose a spatial depth-dependent mode transformation strategy for the tunable filtering of OAM modes. By integrating the spiral phase and lens phase modulations, we achieved mode conversions that varied with the transmission depth, enabling selective demultiplexing in predetermined axial planes. This approach facilitates tunable mode filtering by adjusting spatial depths. As a proof of concept, we fabricated a mode filter using two-photon polymerization lithography (TPL) technology, successfully filtering five OAM modes with mode crosstalk below -10.9 dB. Additionally, the filter was applied in a mode-multiplexing communication link, achieving tunable demultiplexing of five mode channels with bit error rates below 10^-6. These results highlight the efficacy and flexibility of our strategy for OAM mode filtering and offer promising insights for the development of mode-multiplexing communication networks and channel interconnections.