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 (
${{\bf L}}{{{\bf G}}_{\ell,{{\bf p}}}}$
) beams when varying two different modal indices (i.e., both
$\ell $
and
${{\bf p}}$
); this contrasts with prior reports when varying only one index (i.e.,
$\ell $
) 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., {
${{\bf L}}{{{\bf G}}_{ - 2,0}}$
and
${{\bf L}}{{{\bf G}}_{1,1}}$
} or {
${{\bf L}}{{{\bf G}}_{3,0}}$
and
${{\bf L}}{{{\bf G}}_{0,1}}$
} or {
${{\bf L}}{{{\bf G}}_{2,1}}$
and
${{\bf L}}{{{\bf G}}_{0,1}}$
}). All channels are recovered with bit errorrates (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.