Optical fiber couplers-optimum solution for unequal cores

A concise scalar theory is developed for parallel fibers that are weakly coupled through their evanescent fields, providing the simplest description of optical crosstalk. The scalar theory is the most accurate framework possible because the weak coupling approximation is inconsistent unless restricted to couplers that are weakly guiding. The optimum solution is found using a standard variational method and this solution is shown to be identical to that of conventional perturbation or coupled-mode methods. Expressions are given for lossy couplers and for crosstalk between significantly different cores