Hui Wang

The development of terahertz (THz) technology exploiting untapped bandwidth is essential for sensing and communications in 6G and beyond. Instrumentation systems for evaluating the performances of THz devices are of increasing interest. Currently, THz device testing with electronics-based vector network analyzers (VNAs) requires a series of frequency extender pairs to cover all individual bands in the THz range. This increases system complexity and cost. Here, we report on an ultrawideband (UWB) photonic THz source, which will enable simpler and cost-effective THz device testing. The source employs a modified unitraveling-carrier photodiode (MUTC-PD) operating from 100 to 600 GHz. The InP-based MUTC-PD is hybrid-integrated with a UWB high-resistivity Si waveguide interface to efficiently couple the optically generated THz waves to the device under test. By utilizing a UWB THz power detector, a waveguide-based scalar transmission measurement system is demonstrated, a crucial step toward a future waveguide-based photonic VNA.

Photonic technologies bring intrinsic advantages in terahertz generation and detection, making them key enablers for a wide range of applications. However, currrent photonic-based terahertz receivers are limited in conversion loss compared to state-of-the-art electronic solutions. Here, we present the design, fabrication, and characterisation of an optoelectronic 220–330 GHz receiver, based on photonically-pumped Schottky barrier diodes (SBDs). The receiver integrates antiparallel low-barrier SBDs with a uni-travelling-carrier photodiode (UTC-PD) acting as the local oscillator (LO) signal generator. By combining these two components, the high-performance down-conversion provided by SBD-based mixers is paired with the photonic advantages in LO signal generation and distribution. A compact quasi-optical subharmonic receiver was fabricated and tested, achieving a minimum conversion loss of 18.1 dB. These results confirm the modelling process, since experimental results are in good agreement with simulations. Moreover, the receiver shows near-optimum performance for a pumping photocurrent as low as 5 mA, which proves the viability of the approach. While this work is an initial step with room for performance improvements, it highlights the potential of this solution to be used in applications like high-speed wireless communications, spectroscopy and imaging, and radio-astronomy.

This work presents two manufacturing approaches for waveguide diplexers applicable to separating two of the G-band, 140-220 GHz, channels used in space borne radiometry of the Earth’s atmosphere. Waveguide diplexing is a lower volume alternative to a quasi-optical, i.e., frequency selective surface based, system. The two channels considered are 164-167 GHz and 175-191 GHz. The diplexer comprises a Y junction with two waveguide-cavity filters. Two high-precision fabrication technologies have been utilized: computer numerical control (CNC) machining and 3D printing. Two units were CNC machined as brass split-blocks and a third was 3D printed monolithically in stainless steel by a micro laser sintering process. The latter is an innovative structure that incorporates the diplexer with the waveguide flanges. All devices were gold coated to reduce loss. Measured insertion losses in the two channels were 0.6 and 0.34 dB for the CNC-machined diplexers and 1.8 and 0.8 dB for the 3D printed diplexer. The maximum frequency shifts from design were 0.695 GHz in the CNC-diplexers and 1.55 GHz in the 3D printed diplexer.

This paper presents a comprehensive evaluation of 3D-printed monolithic waveguide components fabricated by a high-precision micro laser sintering (MLS) process. The investigated devices are two 180 GHz bandpass filters and a straight G-band (140-220 GHz) waveguide section. All were made of stainless steel, which was later gold coated using an electroless process. One of the filter samples was characterized using X-ray micro-CT to inspect the printing quality as well as measure the internal dimensions. The sample was then sectioned to allow measurement of the surface roughness of the inner surfaces and inspect the gold coating quality. The as-manufactured stainless steel components showed high insertion losses: over 3 dB in the filter passbands and between 4.7 dB and 7 dB for the waveguide section, increasing with frequency over the G-band. This loss is due to the electrical conductivity of stainless steel as well as the surface roughness. Gold plating significantly reduced the insertion losses, to 0.5 dB for the filters and to between 0.6 dB and 1 dB for the waveguide section. The investigative study showed the high dimensional accuracy and good printing quality achieved by MLS, demonstrating the value of the technique in producing monolithic metal waveguide components with fine geometrics.

We report a $D$ -band waveguide diplexer, with two passbands of 130–134 and 151.5–155.5 GHz, fabricated using micro laser sintering (MLS) additive manufacturing with stainless steel. This is the first demonstration of metal 3-D printing technology for multiport filtering devices at a sub-terahertz (THz) frequency. For comparison, the same diplexer design has also been implemented using computer numerical controlled (CNC) milling. The diplexer, designed using coupling matrix theory, employs an all-resonator and $E$ -plane split-block structure. The two channels are folded for compactness. A staircase coupled structure is used in one channel to increase the isolation performance. The printed waveguide flanges are modified to adapt to the limited printing volume from the MLS. Effects of fabrication tolerance on the diplexer are investigated. An effective and unconventional electroless plating process is developed. The measured average insertion losses of the gold-coated diplexer are 1.31 and 1.37 dB, respectively. The respective frequency shifts from design values are 0.92% and 1.1%, and bandwidth variations are 4% and 15%. From a comprehensive treatment of the end-to-end manufacturing process, the work demonstrates MLS to be a promising fabrication technique for complex waveguide devices at a sub-THz frequency range.

The first Doppler spectra ever acquired by an atmospheric radar at 200 GHz (G‐band) are presented. The observations were taken during a light precipitation event in May (rain rates −1) at Chilbolton Observatory, UK, with coincident Ka‐band and W‐band Doppler radar measurements. The collected rain spectra agree with Mie theory predictions: at G‐band they show significant reductions in the spectral power return—as compared to theoretical Rayleigh scattering—corresponding to high Doppler velocities (i.e., large raindrops) with the presence of multiple peaks and “Mie notches” in correspondence to the maxima and minima of the raindrop backscattering cross sections. The first two G‐band Mie troughs correspond to smaller velocities/sizes than the first W‐band Mie notch. These features offered by G‐band radars pave the way toward applying, in rain, Mie notch vertical wind retrievals and multifrequency drop size distribution microphysical retrievals to smaller rain rates and smaller characteristic sizes than ever before.

This article explored the use of high-precision metal three-dimensional printing in subterahertz waveguide devices and demonstrated a 300 GHz waveguide bandpass filter made by micro laser sintering (MLS) process. The filter structure is composed of five rectangular waveguide cavities (fundamental TE ₁₀₁ mode), two back-to-back right-angle bends and WR-03 waveguide sections. It is made of two identical blocks of stainless steel and two brass plates were used to clamp them together and achieve secure contact in the E plane cut. The measured response of the as fabricated stainless-steel filter showed minimum passband insertion loss of 4.7 dB due to the degraded effective conductivity of the stainless steel and surface roughness. To reduce the insertion loss, the filter was gold plated using an electro-less process with nickel undercoat layer. Plating the filter significantly improved the passband insertion loss, measured to be between 1.1 and 2.7 dB. Inspection of the filter using an Alicona optical system showed that dimensional accuracy within ± 15 μm on average has been achieved by the MLS printer. The investigative study tested the boundary of the technology in subterahertz device applications.

This letter reports on the first Schottky diode frequency doubler with a split-block waveguide structure fabricated by a high-precision stereolithography (SLA) printing process. The printed polymer waveguide parts were plated with copper and a thin protective layer of gold. The surface roughness of the printed waveguide parts has been characterized and the critical dimensions measured, revealing good printing quality as well as a dimensional accuracy that meets the tight tolerance requirements for sub-terahertz active devices. The 62.5 GHz to 125 GHz frequency doubler circuit comprises a 20 m thick GaAs Schottky diode monolithic microwave integrated circuit (MMIC) in the waveguide. The measured doubler provides a maximum output power of 33 mW at 126 GHz for input power of 100 mW. The peak conversion efficiency was about 32% at input powers from 80 to 110 mW. This doubler performance is compared with and found to be nearly identical to the same MMIC housed in a CNC-machined metal package. This work demonstrates the capability of high-precision SLA techniques for producing sub-terahertz waveguide components.

This article presents a 135-150-GHz Schottky diode-based bias-less frequency tripler based on SU-8 micromachined WR-5 waveguides. The waveguides consist of five 432-μm-thick silver-plated SU-8 layers, which house the diode chip and form the output matching network. The input matching circuit is realized in a computer numerical control (CNC) milled waveguide filter, which also provides support and thermal sink to the SU-8 waveguides. Considering the low thermal conductivity of the SU-8 material, auxiliary metallic thermal paths are designed, and the impact of these is discussed through thermal modeling. The thermal simulations show that under 50-mW power dissipation in the diode anodes, the maximum temperature of the SU-8 tripler is predicted to be 346 K at the diode junction, only 7 K higher than in an entirely metal equivalent. The tripler was measured to have a conversion loss of 16-18 dB and the input return loss is better than 18 dB. This work demonstrates that SU-8 micromachined waveguides can be used to package high-frequency semiconductor components, which, like other photolithography-based processes such as silicon deep reactive ion etching (Si-DRIE), has the potential for submicrometer feature resolution.

A 290-310 GHz Schottky diode based sub-harmonic mixer with integrated, low loss, impedance matching waveguide cavity filters is presented in this paper. This mixer was designed for use in a 300 GHz communication system with a 20 GHz intermediate frequency (IF) band centred at 15 GHz. Image rejection of the 260-280 GHz lower sideband (LSB), as well as impedance matching, was achieved using an integrated 3rd order filter in the RF waveguide. The conventional coupling matrix was used to design the filter even though the impedance presented to the RF port was complex and frequency dependent. The mixer was measured to have: (1) Single sideband (SSB) conversion loss of 9-10 dB across the upper sideband (USB), with a mixer noise temperature of 2000-2600 K. (2) A return loss at the RF port better than 12 dB, with three filter reflection zeroes (poles) distinguishable. (3) A sideband rejection ratio (SBR) from 13 to 25 dB, demonstrating the RF filter's excellent performance in terms of impedance matching and filtering.