Scattering induced crosstalk in active directional couplers

Scattering from defects in an integrated optical device causes phase randomization and depolarization. Scattered light can be recaptured by the waveguides of the device and perturbs modal fields. This leads to crosstalk in directional coupler switches. A defect-scattering-induced crosstalk model is developed and crosstalk in one-, two-, and three-electrode directional couplers is investigated with the model. The number of independent electrode voltages needed to tune out crosstalk is studied. Simulations show that scattering-induced crosstalk can be tuned out completely in active directional couplers with two independent electrode voltages. When modal differential loss and unequal taper coupling are taken into account, two independent electrode voltages are insufficient to tune out the crosstalk, whereas three independent electrode voltages are sufficient. This agrees with the conclusion from previous three-electrode directional coupler experiments.