Spot-size conversion of guided optical wave with Gaussian approximation

We propose an approach to optical wave field spot-size conversion that uses a waveguide structure. An optical wave field is decomposed with a Hermite-Gaussian basis, and we describe its motion with partial differential equations for the field radius and center parameters derived from the propagation of the lower order components. We use the equations to design a spot-size conversion waveguide structure for expanding and reducing a field spot as an example in a quasi-single-mode optical waveguide.