High-Density and Low-Crosstalk Multilayer Silicon Nitride Waveguide Superlattices With Air Gaps

High-density and low-crosstalk waveguide arrays are critical components in optical phased arrays (OPAs) and are widely used in solid-state light detection and ranging (LiDAR). In this work, silicon nitride waveguide superlattices with air gaps are proposed and analyzed theoretically. Mode analysis shows that the introduced air gaps beside the waveguide help shorten the skin depth of the evanescent field and increase the effective index range of the waveguide under single mode conditions. Lower crosstalk is demonstrated between a pair of waveguides with air gaps compared with those without air gaps. On this basis, a waveguide superlattice with air gaps is designed by combining the eigenmode expansion method and particle swarm optimization. The crosstalk of the waveguide superlattice is optimized to −24.3 dB when the waveguide pitch is 0.9
$\mu \text{m}$
, the propagation length is 1 mm, and the wavelength is 905 nm. The crosstalk is still below −22.4 dB under the typical process variations and over the wavelength range of 890 nm–920 nm. Therefore, the proposed air-gap waveguide superlattices with high density and low crosstalk offer opportunities to improve the beam steering performance of OPA chips in the near-infrared (NIR) waveband.