C- and O-Band Dual-Polarization Fiber-to-Chip Grating Couplers for Silicon Nitride Photonics

Abstract

Highly efficient coupling of light from an optical fiber to silicon nitride (SiN) photonic integrated circuits (PICs) is experimentally demonstrated with simple and fabrication-tolerant grating couplers (GC). Fully etched amorphous silicon gratings are formed on top of foundry-produced SiN PICs in a back-end-of-theline (BEOL) process, which is compatible with 248 nm deep UV lithography. Metallic back reflectors are introduced to enhance the coupling efficiency (CE) from −1.11 to −0.44 dB in simulation and from −2.2 to −1.4 dB in experiments for the TE polarization in the C-band. Furthermore, these gratings can be optimized to couple both TE and TM polarizations with a CE below −3 dB and polarization-dependent losses under 1 dB over a wavelength range of 40 nm in the O-band. This elegant approach offers a simple solution for the realization of compact and, at the same time, highly efficient coupling schemes in SiN PICs.

High-Efficiency Metamaterial-Engineered Grating Couplers for Silicon Nitride Photonics

“…Recently, hybrid grating couplers with multi-layer configurations have emerged as promising candidates for efficient optical coupling, leveraging advanced fabrication processes applied to the chip's frontside [44][45][46][47][48][49][50][51][52][53][54][55][56]. Compared to a rather complex backside chip processing, implementing new material layers on the chip's frontside is advantageous as it provides an additional degree of freedom to improve the photonic chip functionalities and can be more readily integrated into a standard chip-scale fabrication process.…”

“…Compared to a rather complex backside chip processing, implementing new material layers on the chip's frontside is advantageous as it provides an additional degree of freedom to improve the photonic chip functionalities and can be more readily integrated into a standard chip-scale fabrication process. More specifically, such grating coupler designs utilize stacks of the same [44][45][46] or different [47][48][49][50][51][52][53][54][55][56] materials to enhance the coupling efficiency. Moreover, this also provides more opportunities for fiber-chip coupling, particularly in terms of multiband [55] or polarization-insensitive [56] operation.…”

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

“…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 .…”

“…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 . However, this approach is not suitable for thinner waveguides like the 400 nm SiN platform employed in our work.…”

“…The buried oxide (BOX) layer has a refractive index, n BOX , of 1.4467 50 and a thickness, h BOX , of 4.5 μm. The α-Si layer of thickness h α-Si is situated on top of SiN platform, separated by a buffer oxide layer of thickness h b Both the α-Si and oxide buffer layers provide an additional degree of design freedom to optimize the grating coupler performance 28 , 34 , 35 . The respective refractive indexes are n α-Si = 3.5187 50 and n b = 1.4502 49 .…”