Experimental Demonstration of sub-THz Wireless Communications Using Multiplexing of Laguerre-Gaussian Beams When Varying Two Different Modal Indices

The ability to multiplex multiple structured beams for sub-terahertz (sub-THz) wireless communications has been recently explored. The spatial orthogonality of structured beams enables mode-division multiplexing (MDM). In an MDM system, multiple data-carrying beams are transmitted and received simultaneously through a single aperture pair with low inherent crosstalk. This can increase data capacity and spectral efficiency of the system. Here, we experimentally demonstrate multiplexing orthogonal sub-THz Laguerre-Gaussian (LG_(l,p)) beams when varying two different modal indices (i.e., both l and p); this contrasts with prior reports when varying only one index (i.e., l) and could potentially provide a larger set of channels and beams in an MDM system. In our demonstration, specially designed phase patterns are used: (1) at the transmitter, to convert two THz Gaussian beams to two different LG beams, each carrying an independent data channel; and (2) at the receiver, to separate and convert the LG beams back to Gaussian-like THz beams. An 8-Gbit/s quadrature-phase-shift-keying (QPSK) link containing two multiplexed LG modes over 40 cm is experimentally demonstrated. For the above link, three different LG modal sets are chosen (i.e., {LG_(-2,0) and LG_(1,1)} or {LG_(3,0) and LG_(0,1)} or {LG_(2,1) and LG_(0,1)}). All channels are recovered with bit error rates (BERs) below the 7% forward error correction (FEC) limit. The experimental results indicate that: (a) higher-order LG modes experience more divergence, larger size, and lower conversion efficiency, contributing to higher power loss; and (b) the modal coupling and crosstalk for different LG modes is < -12 dB, which could be due to the transmitter/receiver misalignments as well as beam truncation by the limited-sized receiver aperture.