Selection of proper modes of an NRD guide using a perturbing boundary

A novel method to remove unwanted modes in the analysis of nonradiative dielectric (NRD) guide is proposed. Those modes are originated by the compulsory truncation of an infinite boundary to restrict finite-element domain. It will be shown that unwanted modes can be filtered out by a special boundary concept named a perturbing boundary condition. This method is applied to obtaining the propagation constant of inhomogeneous NRD guides and NRD guides with a trapezoidal cross section of which analytic solutions do not exist.