Embedded silicon gratings for high-efficiency light-chip coupling to thin film silicon nitride waveguides.

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 have experimentally shown 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. We present a detailed design strategy, simulation, fabrication and characterization data on the effect of various parameters on the coupling efficiency.

Comprehensive grating enabled silicon nitride fiber-chip couplers in the SNIR wavelength band.

We present silicon nitride grating enabled fiber-chip coupling in the sub-near-infrared band. We present a comprehensive design and simulation and experimental demonstration of uniform and apodized grating couplers, with and without bottom reflectors. The mode engineering yields a best efficiency of -1.6 dB for apodized grating design, which is further improved to -0.66 dB with a bottom reflector. Experimentally, we demonstrate a coupling efficiency of -2.2 dB for the optimized design. Furthermore, we present a detailed simulation and measurement comparison of various grating parameters and the effect of fabrication tolerances on the grating performance.

High-efficiency vertical fibre-to-polymer waveguide coupling scheme for scalable polymer photonic circuits.

Polymer photonic circuits offer a versatile platform for various applications, including communication, sensing and optical signal processing. Though polymers offer broadband, linear and nonlinear optical properties, the coupling between an optical fibre and a polymer waveguide has been a challenge. In this work, we propose and demonstrate a wafer-scale vertical coupling scheme for polymer waveguides. The scheme uses a silicon nitride grating coupler with an inverse taper to couple between an optical fibre and a SU8 polymer waveguide. We demonstrate a maximum coupling efficiency of -3.55 dB in the C-band and -2.92 dB in the L-band with a 3-dB bandwidth of 74 and 80 nm, respectively. A detailed design and simulation, fabrication, and characterisation results are presented. The scheme demonstrates a scalable and efficient surface grating approach for polymer photonic integrated circuits.

High efficiency DBR assisted grating chirp generators for silicon nitride fiber-chip coupling.

Silicon Nitride (SiN) is emerging as a promising material for a variety of integrated photonic applications. Given its low index contrast however, a key challenge remains to design efficient couplers for the numerous platforms in SiN photonics portfolio. Using a combination of bottom reflector and a chirp generating algorithm, we propose and demonstrate high efficiency, grating couplers on two distinct SiN platforms. For a partially etched grating on 500 nm thick SiN, a calculated peak efficiency of -0.5 dB/coupler is predicted, while for a fully etched grating on 400 nm thick SiN, an efficiency of -0.4 dB/coupler is predicted. Experimentally measured coupling efficiencies are observed to be -1.17 and -1.24 dB/coupler for the partial and fully etched grating couplers respectively in the C-L band region. Furthermore, through numerical simulations, it is shown that the chirping algorithm can be implemented in eight additional combinations comprising SiN film thickness between 300-700 nm as well as alternate claddings, to achieve a per coupler loss between -0.33 to -0.65 dB.

MNOS stack for reliable, low optical loss, Cu based CMOS plasmonic devices.

We study the electro optical properties of a Metal-Nitride-Oxide-Silicon (MNOS) stack for a use in CMOS compatible plasmonic active devices. We show that the insertion of an ultrathin stoichiometric Si(3)N(4) layer in a MOS stack lead to an increase in the electrical reliability of a copper gate MNOS capacitance from 50 to 95% thanks to a diffusion barrier effect, while preserving the low optical losses brought by the use of copper as the plasmon supporting metal. An experimental investigation is undertaken at a wafer scale using some CMOS standard processes of the LETI foundry. Optical transmission measurments conducted in a MNOS channel waveguide configuration coupled to standard silicon photonics circuitry confirms the very low optical losses (0.39 dB.µm(-1)), in good agreement with predictions using ellipsometric optical constants of Cu.