Numerical study of Two Photon Absorption effect on nonlinear directional couplers on coupled semiconductor waveguides

The nonlinear interactions of coupled semiconductor waveguides are important phenomena in ultrafast all-optical data processing of integrated photonic circuits. The coupling analysis of semiconductor couplers needs a careful study, because they produce large nonlinear optical responses such as Two Photon Absorption (TPA). In this work, the coupling process is studied by the evolution equations of the mode amplitude of coupled semiconductor waveguides. The formalism takes into account the effects of TPA by starting with Maxwell's equations and using the perturbation method. It is shown that free carriers produced by TPA lead to modification of refractive index. This, in turn, influences the coupling performance of semiconductor waveguides. Fast Fourier Beam Propagation Method (FFT-BPM) is used to investigate the problem, too. The inclusion of free carrier effects result in about one order of magnitude difference in the decoupling threshold.