Coupled-Mode Theory For Anisotropic Optical Waveguides

The theory of dielectric optical waveguides deals with electromagnetic wave propagation in optical fibers and in the waveguides used for integrated optics. Wave propagation in these structures is described in terms of normal modes. 1-3 However, normal modes preserve their identity only in perfect waveguides without irregularities of either the refractive index distributions or the waveguide geometry. Electromagnetic wave propagation in waveguides with any kind of irregularities must be described by means of coupled-mode theory. 3,4 The electromagnetic waves in imperfect waveguides are expressed as superpositions of all the modes of a perfect waveguide. The mode amplitudes are coupled together by coupling parameters that depend on the nature of the waveguide imperfections. A description of wave propagation by means of coupled-mode theory allows calculation of radiation losses caused by intentional or unintentional fluctuations of the refractive index along the axis of the waveguide or by core-cladding boundary fluctuations.2,3 Coupling among guided modes is used to design modulators or distributed feedback circuits for lasers or to effect improvements in the multimode dispersion properties of overmoded waveguides. The coupled-mode theory is well developed for waveguides that consist of isotropic dielectric materials. 3,4 Some work has been done to extend this theory to waveguides consisting of anisotropic materials. 5-7 These waveguides are assuming increasing im985