Rib waveguide spot-size transformers: Modal properties

Efficient spot-size convertors are an essential component of modern integra ted optoelectronics, providing a simple and reliable interface with fibers, One recently proposed design functions by forcing the field from a small s pot rib waveguide into a large spot one by using a taper. Accurate models o f these structures, and a clear understanding of the coupling mechanisms th ey rely upon, are essential if optimized designs are to be produced, Furthe rmore, it is necessary to develop analysis techniques which are accurate ye t not computationally intensive, suitable for use within an iterative desig n environment. Here, the well known and proven spectral index method is ext ended to efficiently analyze this class of vertically coupled rib waveguide spot-size transformer, providing both the guided modes and for the first t ime, the substrate radiation modes of the structure, These modes can then s ubsequently be used to design and determine the performance of the full tap ered structure.