Understanding, mitigating, and exploiting surface effects in optical waveguides

The interface between the core and the cladding of optical waveguides is a critical surface where a number of physical effects may arise. Not only roughness-induced scattering is responsible for radiative loss and backscattering, but also enhances optical crosstalk between adjacent waveguides. In this work, we point out, through theoretical models and experiments, key relationships between scattering effects and geometrical and physical parameters of the waveguides. A unified model is presented demonstrating that, given the standard deviation and correlation length of sidewall roughness, radiation loss and backscattering depend only on the sensitivity of the mode effective index to the waveguide width, independently of the waveguide technology and shape. We also show that, when the distance between adjacent waveguides increases, radiative coupling dominates the exponentially decaying evanescent coupling, leading to the excitation of higher order modes and phase decorrelation effects. In semiconductor waveguides, free carriers are locally generated at the waveguide boundaries due to surface-state absorption. The possibility to exploit this phenomenon to realize in-line transparent light detectors, neither tapping photons from the waveguide nor perturbing the optical field, is shown, and applications to circuit monitoring, reconfiguration and feedback stabilization are presented.