Photonic Waveguide Directional Coupler

Integrated photonic chips route light through etched waveguides only a few hundred nanometres across, and when two of them run close and parallel their evanescent fields overlap — light launched into one waveguide leaks continuously into its neighbour and back again as it propagates. This simulator renders that evanescent coupling in 3D: two waveguides on a chip, with a stream of light visibly sloshing between them as it travels down the coupler, following the exact coupled-mode-theory solution for the power in each guide as a function of distance. Adjusting the gap changes the coupling strength exponentially, adding a phase mismatch between the waveguides caps how much power can ever cross over, and the chip-length control lets you read the output split at any propagation distance — the same physics that makes directional couplers the standard on-chip beamsplitter for photonic quantum circuits.