Analytical design of an achromatic double grating coupler for microwave and millimeter-wave regions of the spectrum

摘要

Diffractive elements have several distinct advantages over conventional refractive elements. They are lighter and more compact and can be manufactured easily and quickly. These elements are usually used as optical elements, but they can also be used in other regions of the spectrum. An example of a diffractive element which is used for millimeter waves is a grating coupler (GC), which couples a guided wave out of a waveguide into free space. In most applications, the preferred direction of the output wave is normal to the waveguide plane. Unfortunately, as the imaging properties of diffractive elements depend strongly upon the wavelength of the readout wave, it is difficult to use GCs when the input source of the system is polychromatic or when it suffers from wavelength variations. In this paper, we present a method to design a double grating coupler (DGC), which couples a polychromatic wave out of a waveguide with only negligible angular dispersion. The aim of the design is to construct two grating functions in such a way that the chromatic dispersions of the two gratings will mutually compensate each other. In order to achieve this, we use an iterative method, whereby the main ides is to design a double grating which deflects two different wavelengths, with no dispersion, resulting in negligible dispersion within the band.