Embedded Silicon Gratings for High-efficiency Light-Chip Coupling to Thin Film Silicon Nitride Waveguides

Abstract

Thin film silicon nitride (<150 nm) waveguide has emerged as a dominant ultra-low-loss platform for many loss-critical applications. While thin-film silicon nitride propagation loss is a crucial characteristic, coupling light between an optical fiber and the waveguide is still challenging. While the larger mode size of the decoupled thin waveguide offers better coupling than a highly-confined waveguide, the coupling efficiency is still sub-optimal. The poor diffraction efficiency of such thin films limits the scope of implementing standalone surface gratings. We demonstrate an efficient way to couple into thin film silicon nitride waveguides using amorphous silicon strip gratings. The high contrast gratings provide an efficient means to boost the directionality from thin films leading to an enhanced coupling performance. In addition, we incorporate a bottom reflector to further improve the coupling. We present an optimal design for uniform strip gratings with a maximum coupling efficiency of -1.7 dB/coupler. We achieved a maximum coupling efficiency of -0.28 dB/coupler by engineering the scattering strength along the grating through apodization. We experimentally demonstrate the highest coupling efficiency reported yet of -2.22 dB/coupler and -1.84 dB/coupler for uniform and apodized grating couplers in the C-L band, offering $>75 nm$ of 3dB bandwidth. We present a detailed design strategy, simulation, fabrication and characterization data on the effect of various parameters on the coupling.

High-efficiency self-focusing metamaterial grating coupler in silicon nitride with amorphous silicon overlay

“…Alternatively, blazed SiN grating couplers with intrinsically high directionality have been reported, both theoretically and experimentally 25,26 . These gratings leverage bi-level etching topologies or multi-layer gratings formed by SiN 27 , Si 28,29 , or SOI 30,31 material stacks. To reach the sub-dB loss level, both near-field apodization and the selfimagining effect have been utilized to improve the fiber-to-grating field overlap 25,26,32 .…”

“…These demand precise alignment between multiple patterning layers 25,26 or a customized fabrication flow not typically available in public photonic foundries 32 . Recently, the use of high-index overlays has been shown to improve the radiation characteristics of SiN couplers and enhance the overall fiber-chip coupling efficiency 28,[33][34][35][36][37] . To date, a variety of design strategies for hybrid α-Si/SiN grating couplers has been developed.…”

“…To date, a variety of design strategies for hybrid α-Si/SiN grating couplers has been developed. This includes multi-layer grating configurations 28,29,33 , mirror-based couplers 34,36 , or structures based on an inter-layer mode interference effect 35 . For 800 nm thick SiN waveguides, a technique has been shown to produce a nearly Gaussian radiation profile through multi-mode excitation 34 .…”