As the semiconductor materials have strong nonlinear optical effects, they have a great deal of attention in ultrafast alloptical data processing. In semiconductors, the free carriers (FC) density change with field intensity in process of Two Photon Absorption (TPA), and leads to second order intensity dependant in refractive index and loss. In this paper, in addition to Kerr effect, the nonlinear optical effects (TPA) and free carriers is studied theoretically in nonlinear directional couplers by starting from Maxwell's equations and perturbation method. As results show that TPA and FC limit the field transmission between waveguides stronger than Kerr effect and the threshold of filed intensity decreases one order of magnitude. This phenomenon has important applications in switching operation when coupling and decoupling optical wave is needed.
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