Multilayer Platform for Ultra-Low-Loss Waveguide Applications

We investigate a multilayer vertical stacking scheme for use in the Silicon Nitride-based low-loss waveguide platform. In this letter, we use common fabrication techniques to produce test structures for probing the interactions between vertically separated parallel waveguide planes and the characteristics of vertical directional couplers. Single-mode and multimode waveguide geometries are investigated, with designs similar to our previously demonstrated ultra-low-loss technologies. Group index measurements via narrowband OFDR reveal an index mismatch between the vertically separated waveguides of 8.6E-3, generated by the stress of the deposited upper cladding. Vertical directional couplers with a 3-mu m vertical coupling gap and a 50 nm x 4 mu m geometry exhibited an excess coupling loss of 0.19 +/- 0.20 dB and cross-coupled power of 54%, limited by the aforementioned velocity mismatch. The 1.23-m spiral structures with perpendicular crossings on adjacent layers show a transmission loss of the crossings below 0.25 dB for a multimode geometry, while the single-mode design showed a minimum crossing loss of 1.0 dB.