Bend-size reduction on the SOI rib waveguide platform

The minimum bending radius of optical waveguides is typically the most important parameter that defines the footprint and cost of a photonic integrated circuit. In optical fibers and in planar waveguides with equally large mode fields (~10 μm) the bending radii are typically in the cm-scale. The main advantage of using a high index waveguide core with a thickness below 1 μm is the ability to realise single-mode bends with bending radii of just a few micrometers. In this paper we review the dependence of the minimum bending radius on the size and shape of waveguides with the main emphasis on silicon-on-insulator (SOI) waveguides. Then we present simulation and measurement results from advanced waveguide bends and mirrors that have been integrated with 4-10 μm thick single-mode SOI waveguides. We show that multi-step patterning and novel designs allow the reduction of the bending radius by up to three orders of magnitude while also reducing the bending losses by approximately one order of magnitude when compared to traditional rib waveguide bends on 4 μm SOI. This allows to use the μm-scale SOI waveguides for making almost as compact photonic integrated circuits as those based on sub-μm SOI waveguides.

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