Polarization-dependent spectral broadening of femtosecond pulses in silicon waveguides

Abstract

We investigate the polarization dependence of the spectral broadening of femtosecond pulses inside silicon waveguides by using finite-difference time-domain (FDTD) simulations. Our FDTD model includes the anisotropic dependency of predominant nonlinear effects in silicon: Kerr effect, two-photon absorption, and Raman effect. In addition, free-carrier absorption and free-carrier dispersion effects are incorporated into the model. The anisotropic nature of the silicon nonlinearities leads to the polarization-dependent spectral broadening of optical pulses inside silicon waveguides. Our study unambiguously shows that the spectral broadening inside silicon waveguides can be enhanced by carefully selecting the polarization angle of the input optical pulse. Numerical calculations reveal nearly a 4.5-times increase in spectral broadening (inside a 0.1 mm long silicon waveguide) when the polarization angle of the input pulse is adjusted accordingly. The combined impact of silicon nonlinearities and output polarizer on spectral broadening is investigated for different input polarization angles. Finally we show numerically that, for a given waveguide length and input peak intensity, there is an optimum pulse width that corresponds to the maximum spectral broadening.