The pulse coupling dynamics in a nonlinear directional coupler (NLDC) is analyzed by using the new normalized coupled nonlinear Schrödinger equations. The operation of an NLDC can be divided into three regions according to L<sub>D</sub> kappa: Region 1 (L<sub>D</sub> kappa ≤ 1), Region 2 (L<sub>D</sub> kappa > 50), and Region 3 (1 < L<sub>D</sub> kappa ≤ 50). In region 1, an NLDC can exhibit a good switching performance. In region 2, both the pulse coupling and continuous-wave (cw) coupling in an NLDC show the similar energy transfer characteristics. In the case of L<sub>D</sub> kappa ≥ 1000, even an ultrashort pulse coupling can be reduced to the cw coupling and follow Jensen's equations when certain criteria are met. Consequently, many of the applications of NLDC that are suggested under the condition of cw excitation can also be implemented under the condition of ultrashort pulse excitation. In region 3, pulses suffer from pulse compression and serious distortion. The conditions for a pulse following Jensen's equations are presented. A 0.8-ps pulse with a wavelength of 1.55 µm in a silica coupler with a half-beat length of 50 mm follows Jensen's equation. To get a good switching performance, the input optical pulsewidth should be smaller than a certain maximum value. In addition, the intermodal dispersion including the contribution of the material dispersion is derived, and zero intermodal dispersion is predicted.
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