Influence of Surface Roughness on Microring-Based Phase Shifters

The dramatic phase shift and high transmission enable the over-coupled microring resonators for efficient pure phase modulator. Its compact size is especially valuable for large-scale coherent all-optical logic circuit and phase arrays on chip. In this letter, we examine the impact of fabrication imperfection-induced mode splitting for the microring-based phase shifters. We initially measure the phase shift in mode-splitting ring resonators, with the results fitted by coupled mode theory (CMT). Based on the extracted parameter space from the experimental data, we then studied the influence of surface roughness-induced photon scattering for the maximum phase shift and insertion loss of the microring resonators. Surface roughness pushes the ring-waveguide coupling condition from over-coupling toward under-coupling region. The maximum phase shift reduces from 2π to only 1 rad accordingly.