Hybrid technique of FEM and Gaussian mode expansion method

A novel hybrid technique of finite element method (FEM) and Gaussian mode expansion method for the analysis of complex transition structures from waveguide modes to quasi-optical Gaussian beams in quasi-optical systems has been proposed in this paper. The electromagnetic fields in free space are expanded with Gaussian modes supported by dielectric lens. The transition region is discretized by finite element method. These two methods are then combined through the boundary condition on the interface. By comparing the results of reflection coefficients of an empty waveguide obtained by the proposed technique and that calculated by conventional mode matching method, the validity and the accuracy of the hybrid technique has been proved. Numerical results for a transition with complex tapered structure are presented.