3D-Printed Multimode-Interference-Based Bessel Beamformer for Terahertz Dielectric Waveguides

We demonstrate a Bessel beamformer based on multimode-interference (MMI) for the terahertz (THz) wave range. Unlike traditional Bessel beam generation methods, which often rely on bulky free-space optical setups, our approach leverages a 3D-printed Cyclic-Olefin-Copolymer (COC) MMI structure that is contact-coupled to a dielectric waveguide (DW). The DW is interfaced with the MMI by employing a λ/4 slot-waveguide termination (SWT). Aided by the low reflection of this interface and the low loss of the constituent materials, the beamformer generates a Bessel beam with a measured transmission efficiency of 93.8% at 275 GHz at a propagating distance of 20 mm (∼20λ). Even at an extended distance of 35 mm (∼35λ), the beam maintains a notable 50% efficiency. The opposing end of the DW is likewise terminated with an SWT, optimized to couple to hollow metallic waveguides. This approach addresses key limitations of existing free-space methods, offering a workable guided-wave approach to launch a THz Bessel beam.