Loss compensation symmetry in a multimode waveguide coupler

Abstract

Abstract We investigate the loss compensation phenomenon in an asymmetric coupler composed of a pair of unequally sized dielectric cylinders with gain and loss. A multimode analytical approach is applied to solve the eigenvalue problem for the coupler. This approach relies on the field expansion in terms of two sets of orthogonal azimuthal modes, which are coupled due to finite distance between the cylinders. To achieve the loss compensation symmetry (LC-symmetry), a design strategy is developed for the coupler operated in a pair of distinct transverse magnetic modes. It is shown that the LC-symmetry can be achieved for a certain distance between the cylinders, which is associated with the exceptional point of the system. For smaller distances, the adverse impact of high-order azimuthal (hybrid) modes are found to be essential. The results obtained are validated against full-wave simulations.