Low-loss, high-index-contrast Si_3N_4/SiO_2 optical waveguides for optical delay lines in microwave photonics signal processing

We report the design and characterization of Si₃N₄/SiO₂ optical waveguides which are specifically developed for optical delay lines in microwave photonics (MWP) signal processing applications. The waveguide structure consists of a stack of two Si₃N₄ stripes and SiO₂ as an intermediate layer. Characterization of the waveguide propagation loss was performed in race track-shaped optical ring resonators (ORRs) with a free-spectral range of 20 GHz and a bending radius varied from 50 μm to 125 μm. A waveguide propagation loss as low as 0.095 dB/cm was measured in the ORRs with bend radii ≥ 70 μm. Using the waveguide technology two types of RF-modulated optical sideband filters with high sideband suppression and small transition band consisting of an Mach-Zehnder interferometer and ORRs are also demonstrated. These results demonstrate the potential of the waveguide technology to be applied to construct compact on-chip MWP signal processors.