3D-printed demultiplexer circuits using suspended-in-air grating couplers for terahertz communications

Integrated photonic circuits are in great demand for the upcoming THz communications. This work explores 3D printing to realize high-quality, high-refractive-index-contrast integrated components and devices for demultiplexing terahertz channels within the Wavelength Division Multiplexing modality. Namely, by printing integrated circuits using Polypropylene filaments suspended in air, we profit from the high-refractive index contrast of such a material combination to realize relatively compact low-loss waveguides, bends, couplers, and fiber Bragg gratings. The two-nozzle FDM printer allows simultaneous printing with filaments of two distinct sizes of 800um and 400um, with the larger filament used to make waveguides and couplers, and the smaller one used to define high-quality fiber Bragg gratings containing as much as 100 periods and featuring stop bands as wide as 10 GHz. Furthermore, by employing judiciously designed mechanical supports we show how to integrate such subcomponents into functional components such as single-channel drop filters. Finally, we developed a low-loss splicing technique for joining several components into functional devices and demonstrated four-channel THz WDM demultiplexers with in-plane (horizontal) and a more compact out-of-plane (vertical) integration. Experimentally, three-channel demultiplexers of THz signals with individual data rates up to 6 Gbps were demonstrated. Using finite element numerical modeling, integrated circuits were optimized for operation in the 120-165 GHz frequency band featuring ~5 GHz individual channel bandwidths and ~3 GHz inter-channel spectral spacing, and good agreement with the experiments was observed. Additionally, the measured spectra closely resemble the simulated ones but exhibit a frequency shift of several GHz towards higher frequencies. Experimental results further reveal strong sidelobe suppression and a broader Drop port bandwidth (~6 GHz vs. ~4 GHz predicted). However, the measured Drop amplitudes (~0.5–0.6) are lower than the theoretical predictions (~0.8) due to ~10% scattering losses per supporting structure. We believe that the suspended-in-air integrated terahertz circuits hold strong potential for developing various linear optic transformers that will play a key role in energy-efficient analog processing of data streams for the upcoming terahertz communications. This is because of the high quality of the resultant circuits, ease of fabrication, and low infrastructure costs necessary for their manufacturing, thus allowing low-cost fast turnaround prototyping and development of terahertz signal processing devices even with the simplest 3D printing systems.