Low-loss waveguide crossing using a multimode interference structure

A novel design technique for low-loss waveguide crossing using the self-imaging properties of multimode interference (MMI) structures is presented. The technique exploits the observation that optical fields with a small lie width relative to that of the MMI structure in the image reformed region experience negligible propagation loss even in the absence of lateral guiding mechanism around the region. This allows the introduction of a slab in the MMI region. The technique could also be used to design planar light-wave circuits with small wavelength-dependent loss which is important for DWDM devices. Using two-dimensional wide-angle BPM simulations, taper width technology and MMI structure technology is compared for low-loss slab propagation in orthogonal intersection. We also demonstrate that low-loss crossing waveguides with intersection angle as small as 23° can be achieved with the aid of MMI structures.