Coupled Mode Theory of Round Optical Fibers

Dielectric optical waveguides support a finite number of guided modes and an infinite number of radiation modes.1 Even if the number of guided modes that can be supported by the waveguide is reduced to one, the presence of the infinite number of radiation modes forces us to be concerned about mode conversion phenomena. Coupling among the guided modes of a multimode optical waveguide (multimode waveguide refers to the guided modes) is caused by imperfections in the refractive index distribution or the geometry of the optical waveguide. Its effects may be beneficial for reducing the delay distortion that results from uncoupled multimode operation.2-3 Coupling between guided modes and the continuum of radiation modes is usually not desired unless the waveguide is intended to serve as an antenna. However, a certain amount of coupling is unavoidable and results in scattering losses.1>4'5 E. G. Rawson has calculated light scattering from fiber waveguides with irregular core surfaces by an approximate technique.6 However, his method is not suitable to calculate coupling between guide modes. 817