Ultra-Wideband Terahertz Integrated Polarization Multiplexer

Polarization-division multiplexing is crucial for increasing channel capacity and spectral efficiency in communications. A key component is the polarization multiplexer, which combines or separates signals with orthogonal polarizations. Existing planar multiplexers lack an ultrawide band for terahertz communications. While microwave-inspired orthomode transducers (OMTs) provide broadband operation, they suffer from high ohmic loss and bulkiness at terahertz frequencies. Optical integrated polarization beam splitters (PBSs) offer low loss and good integrability but have narrow bandwidths. This work presents a substrateless all-silicon polarization multiplexer based on tapered directional couplers and air-silicon effective mediums, integrated monolithically on a compact footprint. The device demonstrates a 37.8% fractional bandwidth, an average insertion loss of $\approx$1 dB, and a polarization extinction ratio above 20 dB over 225–330 GHz. This superior performance is enabled by the anisotropy of the effective medium claddings, affecting the two orthogonal guided modes differently. The multiplexer enables simultaneous two-channel terahertz fiber communications with polarization diversity. It supports aggregated data rates up to 155 and 190 Gbit ${\mathrm{s}}^{-1}$ with bit error rates below hard- and soft-decision forward-error-correction limits, respectively. This advancement holds promise for 6G terahertz integrated systems with enhanced channel capacities.