Backscattering in silicon photonic waveguides and circuits

The effects of roughness induced backscattering in optical waveguides and circuit realized on a silicon-on-insulator platform are investigated. A systematic experimental investigation on low-loss silicon nanowires, with a sidewall roughness rms around 1-2 nm, is presented, showing that a few hundreds of micrometers long waveguide exhibits a backscattering level that can hinder its exploitation in many applications. The effect is typically stronger for TE polarization and is significantly enhanced inside optical cavities, such as microring resonators, where backscattering is coherently enhanced according to the square of the finesse of the resonator and can modify dramatically the spectral response of the resonators, even at moderate quality factors. We found general relationships relating backscattering to the geometric and optical parameters of the waveguides, to polarization rotation effects, and to coupling with higher-order modes. On the basis of these results, design rules to mitigate backscattering effects are proposed. The main statistical properties of roughness induced backscattering were also experimentally derived, these results enabling an accurate modeling of realistic waveguides and the evaluation of the backscattering impact in integrated devices and circuits.