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

Waveguide grating couplers with bandwidth beyond 200 nm

We propose and validate a new approach for wideband waveguide grating couplers (GC). The wideband operation is achieved using a slot waveguide grating structure above the conventional channel waveguide. With this slot waveguide grating structure, both the grating strength, mode effective index and dispersion in the grating region can be flexibly tuned to enable high coupling efficiency and wideband operation. 3D FDTD simulations predicted coupling efficiency of -4.08 dB with unprecedented 1 dB bandwidth of 229 nm. The experimental result in coupling with standard single mode fiber in the C band to a lithium niobate waveguide achieved -4.47 dB coupling efficiency with 1 dB bandwidth of 171 nm and 3 dB bandwidth of over 200 nm. The unprecedented wide optical bandwidth is achieved without using bottom metal reflectors or the etching of grating structures on the lithium niobate material.

Broadband and low-reflection mid-infrared grating coupler for a perfectly vertical fiber-chip interface

Short-wavelength mid-infrared (SWMIR) silicon photonics has gained significant attention due to its applications in sensing, spectroscopy, and communications. A perfectly vertical grating coupler is a valuable packaging technique that is convenient for chip-to-chip optical interconnects and has low risks of mechanical failure during testing. However, SWMIR grating couplers have fewer periods to tailor the diffracted light, hindering the improvement of bandwidths and backreflections. Herein, we demonstrate a perfectly vertical subwavelength grating coupler by using a modified inverse design approach. The device exhibits a coupling efficiency of -5.9 dB with a 1-dB bandwidth of ∼122 nm and a low backreflection of -19.2 dB at 2200 nm wavelengths. Besides, the device also exhibits exceptional spatial fiber misalignment tolerance. The study underscores the effectiveness of the inverse design strategy in subwavelength grating couplers, charting a path to advance the mid-infrared silicon photonic packaging.

O-band and C-band dual-polarization SMF-28 edge coupler with SiON taper cladding based on silicon nitride platform

Optical communication is progressing towards low power consumption and lightweight solutions, necessitating the integration of multispectral output capabilities within a single optical module. We demonstrate a SiN photonic platform-based edge coupler for a standard single mode fiber (SMF) that enables operation in both the O-band and C-band simultaneously. The device is composed of a multi-segment SiN inverse taper and SiON taper cladding with specific refractive index. The measured edge coupler exhibits a coupling loss of less than 1 dB/facet, 0.5 dB bandwidth exceeding 100 nm from 1260 nm to 1360 nm; and a coupling loss of less than 2 dB/facet, 1 dB bandwidth exceeding 100 nm from 1500 nm to 1600 nm. Furthermore, the polarization dependent loss (PDL) remained less than 0.3 dB throughout the measurement range.

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

Efficient fiber-chip coupling interfaces are critically important for integrated photonics. Since surface gratings diffract optical signals vertically out of the chip, these couplers can be placed anywhere in the circuit allowing for wafer-scale testing. While state-of-the-art grating couplers have been developed for silicon-on-insulator (SOI) waveguides, the moderate index contrast of silicon nitride (SiN) presents an outstanding challenge for implementing efficient surface grating couplers on this platform. Due to the reduced grating strength, a longer structure is required to radiate the light from the chip which produces a diffracted field that is too wide to couple into the fiber. In this work, we present a novel grating coupler architecture for silicon nitride photonic integrated circuits that utilizes an amorphous silicon (α-Si) overlay. The high refractive index of the α-Si overlay breaks the coupler's vertical symmetry which increases the directionality. We implement subwavelength metamaterial apodization to optimize the overlap of the diffracted field with the optical fiber Gaussian mode profile. Furthermore, the phase of the diffracted beam is engineered to focalize the field into an SMF-28 optical fiber placed 55 µm above the surface of the chip. The coupler was designed using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations supported by genetic algorithm optimization. Our grating coupler has a footprint of 26.8 × 32.7 µm2 and operates in the O-band centered at 1.31 μm. It achieves a high directionality of 85% and a field overlap of 90% with a target fiber mode size of 9.2 µm at the focal plane. Our simulations predict a peak coupling efficiency of - 1.3 dB with a 1-dB bandwidth of 31 nm. The α-Si/SiN grating architecture presented in this work enables the development of compact and efficient optical interfaces for SiN integrated photonics circuits with applications including optical communications, sensing, and quantum photonics.

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

Silicon nitride (Si3N4) is an ideal candidate for the development of low-loss photonic integrated circuits. However, efficient light coupling between standard optical fibers and Si3N4 chips remains a significant challenge. For vertical grating couplers, the lower index contrast yields a weak grating strength, which translates to long diffractive structures, limiting the coupling performance. In response to the rise of hybrid photonic platforms, the adoption of multi-layer grating arrangements has emerged as a promising strategy to enhance the performance of Si3N4 couplers. In this work, we present the design of high-efficiency surface grating couplers for the Si3N4 platform with an amorphous silicon (α-Si) overlay. The surface grating, fully formed in an α-Si waveguide layer, utilizes subwavelength grating (SWG)-engineered metamaterials, enabling simple realization through single-step patterning. This not only provides an extra degree of freedom for controlling the fiber-chip coupling but also facilitates portability to existing foundry fabrication processes. Using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations, a metamaterial-engineered grating coupler is designed with a coupling efficiency of -1.7 dB at an operating wavelength of 1.31 µm, with a 1 dB bandwidth of 31 nm. Our proposed design presents a novel approach to developing high-efficiency fiber-chip interfaces for the silicon nitride integration platform for a wide range of applications, including datacom and quantum photonics.