Prism coupling can be simulated in OptiFDTD just as it would be done in real life:
1. Create a waveguide geometry and calculate associated modes (this can be done using OptiMode, that comes along with OptiFDTD)
2. Position a high index prism (where n_prism > n_waveguide) directly above the waveguide. Here, we are only simulating the near-field, so a prism is approximated as a rectangle of semi-infinite extent.
3. Launch a plane wave into the prism with carefully chosen angle (we can use a gaussian beam in order to avoid interferences at the boundary edges)
See the example attached.