Introduction

Fabry-Perot interferometers have been widely used as tunable couplers and filters at millimeter and submillimeter wavelengths. These couplers consist of two reflectors and make use of interference between propagating waves to change their coupling coefficient. Consequently, the large and rapid change of the coupling coefficient with frequency results in a narrow bandwidth. A new metal mesh coupler using evanescent wave coupling has been proposed as a new quasi-optical component to overcome the trade-off between bandwidth and coupling coefficient of the Fabry-Perot couplers [1].

The new coupler consists of an inductive or a capacitive metal mesh and a flat dielectric plate placed close to the surface of the mesh (Fig. 1), and makes use of the coupling effect of evanescent waves to change reflectance and transmittance. The evanescent waves are induced by an incident wave on the mesh, and decay quickly away from the mesh, normally less than l/20 from the surface [2]. Therefore, in contrast to a Fabry-Perot interferometer, the coupling coefficient of the evanescent wave coupler can be significantly changed by small adjustments of spacing between the mesh and the dielectric plate. In principle, this type of coupler can also be wide band because the transmission properties of the couplers depend primarily on the mesh parameters (g and a in Fig. 1).

Fig.1. Configuration of an evanescent wave coupler.

Fig. 2 shows the transmittance of the couplers with and without the evanescent wave coupling effect by varying the spacing L from 0 to 4 mm at 56GHz. The coupling effect only happens when the silicon plate is very close to the mesh and the coupler behaves like a Fabry-Perot interferometer for larger spacing. Since the Fabry-Perot etalon effect appears together with the coupling effect around zero spacing, by comparing the part of the curve above l/2 and the part around the origin, the evanescent wave coupling effect alone can be isolated by subtracting these two parts.

In order to estimate the effective distance of coupling, transmittances of the couplers with g=2.12mm and a=0.43mm have been measured at 44 GHz (Fig. 3). Since the thickness of the silicon plate is about half wavelength in silicon at 44GHz, the Fabry-Perot etalon effect disappears at this frequency and we can estimate the effective distance of coupling effect more precisely. The transmittance of the capacitive coupler increases when the spacing increases while the one of the inductive coupler decreases.

Fig. 2 Transmittance of the couplers with g=1.7mm and a=0.345mm at 56 GHz. (a): With the coupling effect and (b): without the coupling effect.

where n_Si is the refractive index of the silicon plate, a is the field decay constant of an evanescent wave of the first order and k is a wave number of the incident wave in free space. First n_e is determined for various values of L, then the impedance Z_m(L) of a metal mesh, which is sandwiched by two dielectric plates with refractive indices of 2.12 and n_e, is calculated by using the method of moments. Infinite thickness of these dielectric plates is assumed to remove the Fabry-Perot etalon effect. Finally, reflectance and transmittance of the coupler are computed by using the simple equivalent circuit shown in Fig. 4.

Fig. 4. Equivalent circuit of an evanescent wave coupler.

Fig. 3. Transmittance of the couplers with g=2.12 mm and a=0.43 mm at 44 GHz.