Demonstration of Large Coupling-Induced Phase Delay in Silicon Directional Cross-Couplers

We investigate directional couplers in silicon-on-insulator photonic technology. We theoretically and experimentally demonstrate a large coupling-induced phase delay that occurs when nearly all light is coupled from one waveguide to the other, i.e., when the coupler operates as a cross-coupler. We show that even a tiny asymmetry in the two waveguides of the coupler causes a significant additional phase delay. The observed change in the free-spectral-range of ring resonators from 5.0 nm to 6.4 nm is explained by a small 0.1% difference in the propagation constants of the two waveguides. Such a difference can be caused by for example a 1 nm difference in the widths of the two waveguides.