K = ∫∫ E₁(x,y)E₂(x,y) dxdy
where E is the electric field, ω is the frequency, c is the speed of light, and n is the refractive index.
In integrated optics, optical components such as waveguides, couplers, and resonators are designed to interact with each other. The coupling between components is described by the overlap integral of the electric fields.