Contrast Enhancement Using Silicon Photonic Nonlinearities

Abstract

We explore how to take advantage of nonlinear optical effects in silicon to perform contrast enhancement on pulsed photonic signals. Improving the contrast ratio of two-level photonic signals with low power contrast has implications for developing a cryogenic modulation system to be used in quantum computing applications. Integrated resonator-enhanced interferometers are promising candidates for implementing such functions because they operate by converting small nonlinear phase switching effects to large amplitude switching effects. Using both simulation and experiment, we study the resonator-enhanced Mach-Zehnder interferometer (REMZ) and the dual resonator enhanced asymmetric MZI (DREAM) structures on a silicon-on-insulator platform. The first-order optical effects of free carrier dispersion and free carrier absorption (influenced by the nonlinear carrier rate equation) and the third-order effects of Kerr and two-photon absorption are the critical physical phenomena underlying the structures’ nonlinear behavior. Coupled mode theory is used to analyze the dynamics of the microring circulating field, and simulations are carried out for the ring-enhanced Mach-Zehnder structures. An experiment is conducted on the DREAM device that confirms the qualitative accuracy of the model. Unlike in previous theoretical descriptions of nonlinear resonators, we discover that free carrier dispersion can dominate over the Kerr effect in silicon, and therefore cannot be neglected in the design of nonlinear silicon photonic devices.