THz polarisation multiplexer for 6G
Researchers in Australia have developed the first ultra-wideband integrated terahertz polarisation (de)multiplexer implemented on a substrateless silicon base for 6G links.
The device developed at the University of Adelaide operates in the sub-terahertz J-band from 220GHz to 330 GHz for 6G communications and beyond. It is made using standard fabrication processes enabling cost-effective large-scale production and allows aggregated data rates up to 155 and 190 Gbit/s with bit error rates below hard- and soft-decision forward-error-correction limits for 6G links.
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.
The substrateless all-silicon polarization multiplexer uses tapered directional couplers and air-silicon effective media integrated monolithically on a compact footprint.
The device demonstrates a 37.8% fractional bandwidth, an average insertion loss of ≈1 dB, and a polarization extinction ratio above 20 dB over 225–330 GHz. This performance comes from the anisotropy of the effective cladding, affecting the two orthogonal guided modes differently.
“Our proposed polarisation multiplexer will allow multiple data streams to be transmitted simultaneously over the same frequency band, effectively doubling the data capacity,” said Professor Withawat Withayachumnankul from the School of Electrical and Mechanical Engineering who worked with Dr Weijie Gao, who is now a postdoctoral researcher working alongside Professor Masayuki Fujita at Osaka University.
“This large relative bandwidth is a record for any integrated multiplexers found in any frequency range. If it were to be scaled to the centre frequency of the optical communications bands, such a bandwidth could cover all the optical communications bands,” he said.
The polarization multiplexer was built using a deep reactive-ion etching (DRIE) process with a 250-μm high-resistivity intrinsic float-zone silicon wafer. Photoresist was spin-coated on the silicon substrate and patterned by photolithography. The patterned photoresist acted as an etching mask material for protection during the DRIE process. Then, a DRIE instrument removed the silicon perpendicularly from the front to the back side of the substrate through the ionized gas. Lastly, the remaining photoresist was removed using acetone and oxygen ashing.
The fabrication accuracy was closely related to the aspect ratio between the wafer thickness and hole diameter for the deep reactive ion etching process, with the required minimum diameter of the holes that could be etched through as ≈25 μm, given the wafer thickness of 250 μm. The hole size and tapered directional couplers could be precisely defined through photolithography, at least on the topmost part, as photolithography was good down to sub-micron accuracy.
However, the issue was the vertical tapering of the hole as the aspect ratio increased. Despite some imperfections like tapered holes and blind holes in the claddings, the device’s high performance remained, showing the robustness of effective medium.
The researchers set up a simultaneous two-channel real-time HD-video transmission at 300 GHz using the polariser. On the transmitter side, two tunable near-infrared laser sources are adopted to generate an optical beat signal with a frequency difference of 100 GHz. Amplified by the EDFA, the optical beat signal is divided into two channels by a 3-dB optical power divider, and modulated with a HD-video signal generated by a video player in each channel.
The amplified modulated beat signal is injected into a UTC-PD and downconverted to a signal at 100 GHz, which is amplified and upconverted to 300 GHz by an amplifier and a multiplier. This allows a reasonable signal-to-noise ratio at the receiver given the link budget. The terahertz signal in each channel is coupled to the multiplexer through a WR-3 hollow waveguide, while the polarization diversity is obtained by rotating the orientations of the hollow waveguides.
The two terahertz signals with orthogonal polarizations carrying two different data streams are combined by our polarization multiplexer and coupled to a terahertz hollow-core fibre, which can support two orthogonal polarizations over a broad bandwidth.
As the diameter of the hollow fibre (1 mm) much larger than the taper end, the taper structure can be easily inserted into the fibre without touching the walls. The reasons to adopt the fiber as an interconnect are to avoid the significant propagation path loss and to simplify the setup and alignment compared to the free-space transmission with lenses and/or mirrors introduced.
“This innovation not only enhances the efficiency of terahertz communication systems but also paves the way for more robust and reliable high-speed wireless networks,” said Dr Gao.
“As a result, the polarisation multiplexer is a key enabler in realising the full potential of terahertz communications, driving forward advancements in various fields such as high-definition video streaming, augmented reality, and next-generation mobile networks such as 6G.”
“We anticipate that within the next one to two years, researchers will begin to explore new applications and refine the technology,” said Professor Fujita who is a co-author of the research.
“Within a decade, we foresee widespread adoption and integration of these terahertz technologies across various industries, revolutionising fields such as telecommunications, imaging, radar, and the internet of things,” said Professor Withayachumnankul.
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