Analysis of Modal Reflectivity of Optical Waveguide End-Facets by the Characteristic Green’s Function Technique

A novel method for computation of guided mode reflectivity from optical waveguide end-facet is presented. The method is based on the characteristic Green's function (CGF) technique formulation combined with the complex images (CIs) method for dielectric planar waveguides. By separability assumption of the structure, a uniform and closed-form expression of spatial Green's function is obtained. Derived expression consists of discrete and continuous spectrum contributions which denote guided and radiation modes effects, respectively. Having a full-wave solution, efficient optimization procedure is then used to calculate the exact reflection coefficients of guided modes at the end-facets. To find mode coupling coefficients, CGF-CI formulation is modified to incorporate coupling matrix of abruptly terminated waveguides. The main advantage of this method lies in its simple implementation as well as accuracy for any refractive index contrast. Excellent agreements with rigorous methods are illustrated in several examples.