title: Second-order perturbation theory of rectangular waveguides and directional couplers.
abstract: Perturbation theory is applied to rectangular-core coupled waveguides, whose cladding can be homogeneous or not. Second-order corrections are evaluated for the phase constants of the guided modes and for the coupling 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.
authors.forename: B
authors.initials: B
authors.lastname: Costantini
authors.forename: C
authors.initials: C
authors.lastname: De Angelis
authors.forename: A
authors.initials: A
authors.lastname: Galtarossa
authors.forename: C G
authors.initials: CG
authors.lastname: Someda