This paper discusses an improved coupled mode theory model for the nonlinear directional coupler in the strong-coupling range. A new single-line matrix equation is derived for the multiwaveguide case. For the two-guide case, a new critical power formula and two exact identities are derived. Interrelation between the overlap integral and nonlinear cross-coefficients is demonstrated. It is shown that inconsistent inclusion of these terms in previous approaches leads to inaccuracies. Results obtained by consistent inclusion of these terms and our critical power formula show a great improvement in accuracy over previous analyses in strong-coupling. [References: 54]
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