Efficient spot-size convertors are an essential component of modern integrated optoelectronics, providing a simple and reliable interface with fibers. One recently proposed design functions by forcing the field from a small spot rib waveguide into a large spot one by using a taper. Accurate models of these structures, and a clear understanding of the coupling mechanisms they rely upon, are essential if optimized designs are to be produced. Furthermore, it is necessary to develop analysis techniques which are accurate yet not computationally intensive, suitable for use within an iterative design environment. Here, the well known and proven spectral index method is extended to efficiently analyze this class of vertically coupled rib waveguide spot-size transformer, providing both the guided modes and for the first time, the substrate radiation modes of the structure. These modes can then subsequently be used to design and determine the performance of the full tapered structure.
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