Fabricating an Ultra-Efficient Silicon Grating Coupler for Fiber Optics

Introduction

In the realm of silicon (Si) photonics, efficient coupling between on-chip waveguides and external optical fibers or lasers is a crucial requirement. While edge coupling via spot-size converters can achieve high efficiency, wafer-level testing becomes challenging. On the other hand, diffraction grating couplers facilitate wafer-level testing but typically suffer from low coupling efficiency, usually less than 50% (<3 dB loss), due to symmetric light emission toward the upper and lower directions.

Advanced grating structures have been proposed to improve the efficiency beyond 80%, but these typically require specialized processes incompatible with standard Si photonics foundry services or advanced processes with feature sizes smaller than 120 nm, resulting in higher costs.

The Proposed Solution

In this tutorial, we explore a novel grating coupler design proposed by researchers at Yokohama National University and the National Institute of Advanced Industrial Science and Technology (AIST) in Japan. This design achieves a theoretical coupling efficiency of over 85% while being compatible with standard Si photonics processes and a minimum feature size of 150 nm.

The key components of this grating coupler are a half-etched grating and an array of perforated rectangular holes, as illustrated in Fig. 1(a).

Fig. 1: (a) Schematic view of the grating coupler for fiber coupling. (b) Top view of an optimized grating coupler.

The perforated hole array relaxes the requirement for the smallest feature size, allowing the use of a 150 nm minimum feature size. This structure achieves high efficiency through a mechanism similar to L-shaped structures, where light scattering at the multi-step structure forms constructive and destructive interferences in the upper and lower directions, respectively.

Optimized Design

The researchers used Lumerical FDTD Solution to perform full 3D FDTD simulations of light propagation and estimate the coupling efficiency from the waveguide to a single-mode fiber. They modeled a 210 nm thick Si layer sandwiched between 3 μm thick upper and lower SiO2 claddings.

The number of grating rows was fixed at 17, while the waveguide width, the size and position of the gratings and holes, and fiber position were used as variable parameters. In total, 120 parameters were optimized using the CMA-ES algorithm [8] by repeatedly calculating the coupling efficiency and changing the parameters.

Fig. 1(b) shows an optimized structure, where the size and position of the gratings and holes are modulated to form a Gaussian-like emission profile. Fig. 2(a) demonstrates the selective emission from the waveguide to the upper direction, and Fig. 2(b) shows the spectrum of the coupling efficiency, exhibiting a maximum value of 85.1%.

Fig. 2: (a) Side view of light emitting from the waveguide. (b) Calculated spectrum of the coupling efficiency.

Fabrication and Experimental Results

The optimized structure was fabricated using AIST's 300 mm Si photonics process with ArF immersion exposure. Fig. 3(a) shows a top view of the device, which appears to be nearly as designed, except for slightly rounded edges on the rectangular holes.

Fig. 3: (a) SEM image of the fabricated grating coupler. (b) Near-field pattern and (c) far-field pattern of light output.

Fig. 3(b) and (c) show the near-field and far-field patterns of light emission observed from the top at a wavelength of 1540 nm. They exhibit semi-circular profiles with a 1/e^2 spot size of 12 μm × 10 μm and a corresponding beam divergence of 8.3° × 7.2°, respectively.

The researchers are currently investigating the upward emissivity, fiber coupling efficiency, and parameter dependencies of the fabricated device.

Conclusion

This tutorial presented a novel Si grating coupler design that achieves a theoretical coupling efficiency of over 85% from the waveguide to a single-mode optical fiber. The design can be fabricated using a standard Si photonics process with a minimum feature size of 150 nm, making it a cost-effective and practical solution.

The key features of this design include a half-etched grating and an array of perforated rectangular holes, which enable selective emission toward the upper direction through constructive and destructive interference mechanisms.

The researchers successfully fabricated the optimized structure and observed the expected vertical light emission, paving the way for further advancements in efficient fiber coupling for Si photonics applications.

Reference

[1] N. Tahara, R. Taira, Y. Maegami, T. Tsuchizawa, N. Yamamoto, K. Yamada, and T. Baba, "Fabrication of Efficient Si Fiber Coupler with a Meta Structure of 150 nm Minimum Feature Size," Department of Electrical and Computer Engineering, Yokohama National University, Yokohama, Japan, and National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan, 2024.