2ND-ORDER PERTURBATION-THEORY OF RECTANGULAR WAVE-GUIDES AND DIRECTIONAL-COUPLERS

Perturbation theory is applied to rectangular-core coupled waveguides, whose cladding can be homogeneous or not. Second-order corrections ar e evaluated for the phase constants of the guided modes and for the co upling coefficient. This requires the full spectrum of radiation modes , which are found within the same approximation as Marcatili's guided modes. It is found that the second-order terms are negligible in most cases of practical interest. However, there are cases for which these terms are so large that the usual first-order theory collapses.