Im supposed to make model to demonstrate all-optical switching using a nonlinear effects of photonic crystals. And I will be using a directional coupler for this purpose. However Im having trouble in understanding the theory and concepts related to coupling / coupling length / use of dispersion diagrams. It would be a great help if someone could recommend me some books or resources to understand these concepts. Also a good book or an article which explains the actual use of optical switch will be also a great help, as I think Iv been misunderstood the concept.