Coupled Optical Waveguides
Linear and nonlinear coupled optical waveguides are most useful devices in optical signal processing. Linear optical directional couplers can be used as optical switches and power splitters. A related useful effect is mode coupling within an individual multi-mode waveguide. The supermode or arraymode superposition technique is perhaps the most accurate and also the simplest method to characterise linear coupled parallel waveguides. The supermode superposition technique itself can equally be applied to any linear coupled parallel waveguides supporting any kind of guided modes. Coupled-mode theories for linear coupled waveguides have been well developed. The derivation of linear and then nonlinear coupled-mode equations, valid for anisotropic and lossy waveguides, is presented in the next subsection. Coupled-mode equations may also be derived from higher-order approximations and/or from scalar modal fields under the weak-guidance approximation to suit individual needs. The power-dependent property of nonlinear coupled optical waveguides has potential applications in ultra-fast all-optical signal processing.