Dielectric Rod Waveguide Couplers as Harmonic Filters for Millimeter and SubMillimeter Wave Frequencies

The paper describes the design and the realization of bandpass filters for suppression of harmonic frequencies based on coupled dielectric rod waveguides at 75 GHz, 300 GHz and 450 GHz [1]. The theoretical values determined by Marcatili’s method of approximation [2] were verified by broadband measurements on geometrically scaled coupler models at Eband frequencies. Afterwards, coupler filters were realized and experimentally characterized in the original frequency range at 300 GHz and 450 GHz. The filter setup was optimized with respect to low weight and compact dimensions for use on the mechanical field scanning unit of a modular vector field measurement system [3]. I. I NTRODUCTION A modular vector field measurement system (VFMS) for scanning spatially extended Gaussian beam fields at 150 GHz, 300 GHz and 450 GHz [3] was recently developed within the framework of a research project with contributions from the Technical Universities of Hamburg-Harburg and Darmstadt and from the University of Erlangen-Nuremberg called MEMSTIC (Multi Elements Multi Substrates Terahertz Integrated Circuits) [4]. The matter thereby is a novel quasioptical circuitry for efficient power combining of multiple solid-state components at 150 GHz and its harmonics. These elements are arranged in two-dimensional arrays and solely connected by a spatially extended quasi-optical beam ([5], [6]). For controlling or optimizing the quasi-optical frequency multiplication, field measurements have to be done at the harmonics of interest. In this context, the presence of unwa nted harmonics as well as the strong fundamental frequency has to be anticipated. Consequently, bandpass filters at 300 GHz resp. 450 GHz to suppress the unwanted harmonics (in particular 450 GHz resp. 600 GHz) had to be developed and realized. Different concepts were evaluated. To integrate the harmonic filter on the mechanical field scanning unit of the vector field measuremen t system, the best suitable concept should fulfill the followi ng requirements in equal measure: Low transmission losses in passband, high harmonic suppression in stopband ( 20 dB), required width of the passband n (150 GHz 5) GHz, n 2f2;3g (as a result of the maximum LO bandwidth), low weight of the filter setup as well as compact dimensions. A combination of coupled dielectric rod waveguides (DWGs) and metal hollow waveguides was chosen as base for the harmonic filter. At frequencies lower than the passband, it reflects signals by the cutoff of the hollow waveguide. Moreover, the frequency-dependency of the dielectric coup ler separates the higher harmonics. Though other investigated concepts like quasioptical filters with a frequency selecti v surface on an (electrically) thick silicon substrate [7] al so exhibit very good electrical properties, the coupler filter was preferred because of its low weight and the small filter setup . In addition, dielectric SMMW rod waveguides were used as main components of the VFMS anyway so the coupler filters do not cause any extra dielectric losses. II. CALCULATION AND LAYOUT OF DIELECTRIC ROD WAVEGUIDE COUPLERS A considerable part of the electromagnetic field is guided outside the (core) dielectric of the DWG which is also named ”fiber” because of its tiny dimensions. Two parallel DWGs are therefore coupled, if the distance is low enough so that the electromagnetic wave is guided by both waveguides. This wave can formally be divided into two waves, namely the so-called even mode with the phase constant e and the socalled odd mode with the phase constant o. If e and o are different, a coupling lengthL0, the so-called zero-dB coupling length, exists as follows: ( e o) L0 = : (1) This is illustrated by Fig. 1. The coupler is excited at port 1 . After L0, there is a complete power transfer from one guide to the other because of destructive interference at guide 1 a nd constructive interference at guide 2. As the power of the signal is concentrating more and more in