Full 3D Modeling of Waveguide-Embedded Frequency Multiplier Arrays

The frequency region between 300 GHz - 10 THz has long been in interest of many scientific and industrial branches. However, the traditional ways to provide continuous wave (CW) signals are inapplicable at these frequencies due to fundamental limitations. There have been many approaches to bridge the lack of sources, commonly referred as the THz gap, both from the electronics and photonics side. To overcome the input power limitation of solid state electronics, quasi-optical grid arrays have been presented in order to spatially combine contributions from a large number of active devices. Originally they operate in free space environment, but to gain a compact component for i.e. satellite applications, the device can be enclosed in a waveguide. The interaction between the large number of nonlinear devices and the confined field in a waveguide cavity poses great modeling challenges. The thesis is investigating different modeling techniques for waveguide embedded varactor frequency multiplier arrays. A Heterostructure Barrier Varactor (HBV) frequency tripler array operating at 250 GHz output frequency was experimentally tested and modeled with the traditional unit cell method and a novel approach using full 3D electromagnetic simulation. The full 3D model follows the abrupt changes of the measured results both as a function of frequency and input power due to the interaction between all the nonlinear elements. Furthermore, it enables the quantitative characterisation of individual diodes and provides knowledge of the total field distribution. In the evaluated example, higher order mode excitation was found due to the interaction between the filter and antenna array. An analysis to account for diode failures using the full 3D model is presented. Due to the results obtained from the full 3D model, a new output filter was designed for the device. By matching the filter’s periodicity to the antenna array, simulations show a decreased higher order mode excitation and therefore an increase of 1 dB in peak output power. RF measurements on the fabricated filter are reported, showing an increased output power at the lower edge of the measured band with a maximum increase of 0.8 dB at 243 GHz. Altogether, the full 3D model provides a useful complementary to the present tools for waveguide-embedded grid array design. Taking into account the resource needs of the two models, the unit cell model is recommended for a quick design synthesis and the full 3D model can be employed for further improvements