Polarization dependence is a major factor of consideration in the design of optical waveguide devices for application in optical fiber transmission systems. In this paper, we apply the recently proposed effective-index method with built-in perturbation correction to the study of the polarization dependence in the coupling coefficient of a directional coupler that consists of two parallel rectangular-core waveguides. Numerical results are found to agree with those calculated by the vector version of the Marcatili method with perturbation correction. With numerical illustrations, we show that the polarization dependence in the splitting ratio of a directional coupler can be eliminated by a proper choice of the physical parameters of the coupler. Our results provide useful guidelines for the design of polarization-insensitive integrated-optic directional couplers.
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