Terahertz technology is a hotspot in the current academic research. In this study, a 400 GHz broadband multi‐branch waveguide hybrid coupler is designed, but it is very difficult to fabricate. In order to release the processing difficulty, a modified five‐branch hybrid coupler has also been designed, fabricated and measured. The hybrid coupler consists of five modified branches and has been optimised to a great performance, which increases the operation bandwidth. Compared to the traditional five‐branch hybrid coupler design, this structure has a wider operation bandwidth and the bandwidth is almost the same as traditional seven‐branch hybrid coupler. Based on this model, the performance of the coupler is optimised by HFSS software. The measurement results show good performance that >18 dB return loss (S11) and isolation (S23), 90° ± 2° phase difference and 0.3 dB amplitude imbalance are obtained in the frequency range of 380–460 GHz, agreeing well with the simulation results.
Frequency multipliers and mixers based on Schottky barrier diodes (SBDs) are widely used in terahertz (THz) imaging applications. However, they still face obstacles, such as poor performance consistency caused by discrete flip-chip diodes, as well as low efficiency and large receiving noise temperature. It is very hard to meet the requirement of multiple channels in THz imaging array. In order to solve this problem, 12-μm-thick gallium arsenide (GaAs) monolithic integrated technology was adopted. In the process, the diode chip shared the same GaAs substrate with the transmission line, and the diode’s pads were seamlessly connected to the transmission line without using silver glue. A three-dimensional (3D) electromagnetic (EM) model of the diode chip was established in Ansys High Frequency Structure Simulator (HFSS) to accurately characterize the parasitic parameters. Based on the model, by quantitatively analyzing the influence of the surface channel width and the diode anode junction area on the best efficiency, the final parameters and dimensions of the diode were further optimized and determined. Finally, three 0.34 THz triplers and subharmonic mixers (SHMs) were manufactured, assembled, and measured for demonstration, all of which comprised a waveguide housing, a GaAs circuit integrated with diodes, and other external connectors. Experimental results show that all the triplers and SHMs had great performance consistency. Typically, when the input power was 100 mW, the output power of the THz tripler was greater than 1 mW in the frequency range of 324 GHz to 352 GHz, and a peak efficiency of 6.8% was achieved at 338 GHz. The THz SHM exhibited quite a low double sideband (DSB) noise temperature of 900~1500 K and a DSB conversion loss of 6.9~9 dB over the frequency range of 325~352 GHz. It is indicated that the GaAs monolithic integrated process, diodes modeling, and circuits simulation method in this paper provide an effective way to design THz frequency multiplier and mixer circuits.
In this paper, the development of two high power 220 GHz frequency triplers is proposed. The GaAs Schottky diodes with six nodes are applied to realize high efficiency 220 GHz tripler, while the application of GaN Schottky diodes with eight nodes is another attempt to improve power handling of the 220 GHz tripler. To reduce thermal effect of high power multipliers, the AlN substrates with high thermal conductivity are applied to provide better heat dissipation at the diode areas. A combination of electrical and thermal model of the Schottky diodes is established while the optimization of 220 GHz triplers are realized with 3D electromagnetic (EM) simulation and harmonic balanced simulation. Good agreement is achieved between the simulated results based on electro-thermal model and measured performances of the triplers. At room temperature, peak efficiency of the tripler based on GaAs Schottky diodes is 17.8%, while the maximum output of the tripler is 38.2 mW with 300 mW input power. As for the 220 GHz GaN Schottky diode tripler, measured results show that the maximum power handling is beyond 400 mW. The peak efficiency and maximum output are 4.7% and 18.4 mW, respectively. The proposed methods of developing high power multipliers can be applied in higher frequency band in the future.
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