Orbital angular momentum processing via diffractive networks for terahertz mode-division-multiplexed wireless links

The orbital angular momentum (OAM) mode processing method is crucial and highly desirable in terahertz (THz) mode-division-multiplexed (MDM) wireless links, to fulfill the requirement of flexible channel switching ability in the sixth-generation (6G) communications. However, most existing OAM manipulation methods cannot process multiplexed OAM modes with high flexibility in the THz band. To address this challenge, here we present a scheme for designing OAM processors for THz MDM wireless links, including OAM transformers and filters, based on diffractive optical neural networks (DONNs). The phase parameters of the DONNs are well optimized using artificial-intelligence-based algorithms to arbitrarily design OAM transformation and filtering functions in a desired manner. We fabricate the OAM processors and validate their functions through THz field scanning and wireless communication experiments, indicating excellent OAM processing performance with a high communication speed of 60 Gbps per channel. This design of the OAM processors exhibits excellent channel switching capabilities, making it suitablefor deployment in THz MDM wireless links, and potentially offering applications in 6G wireless communication systems.