A paper recently published in the journal Npj Nanophotonics has reviewed recent advancements in waveguide grating couplers (WGCs) and high-speed modulators on the silicon photonics platform, highlighting their significance for high-speed communications.
Proposed setup for a single cell with an interference filter. The complete device consists of 36 or 40 such cells. Detailed explanations are in the text. Image Credit: https://www.mdpi.com/2313-433X/10/7/169
Background
Silicon’s optical properties, including its centrosymmetric crystal structure and indirect bandgap, make it less suitable for discrete photonic devices like optical modulators and lasers compared to materials such as III–V semiconductors and lithium niobate. Despite this, the scalable manufacturability of silicon photonics—allowing for the integration of multiple photonic devices with high yield within a single circuit—has established it as a leading technology in photonic integration.
Silicon photonics excels in integrating various functional elements for advanced photonic signal processing and multichannel optical transceivers. Its ability to scale for large-volume production makes it particularly advantageous for high-volume, low-cost applications like optical transceivers.
The expansion of generative artificial intelligence and the resulting increase in data traffic have heightened the demand for new integrated optical transceivers capable of supporting multichannel, high-capacity communications. In this context, the paper reviewed recent advancements in high-performance waveguide grating couplers (WGCs) and high-speed optical modulators on the silicon photonics platform, emphasizing their potential for meeting future high-capacity communication needs.
Silicon Photonic Modulators
Future optical transceivers will need to integrate multiple modulators on a single chip and handle an increased number of channels to address the rapidly growing data center traffic. Currently, the silicon photonics industry offers several types of modulators, including Electro-Absorption Modulators (EAM), Microring Modulators (MRM), and Mach-Zehnder Modulators (MZM).
Recent advancements in silicon photonics modulators have pushed performance close to the practical limits imposed by silicon's free carrier mobility and physical dimensions. High-performance MZMs, for instance, are now capable of operating beyond 3 or 6 dB bandwidths, achieving net bit rates exceeding 300 Gb/second and baud rates over 100 Gbaud.
To enhance MZM performance, segmented MZMs have been proposed to lower the driving voltage and increase bandwidth. This design connects several MZMs in series, with each segment modulating a single bit; for PAM-4, two segments are required. However, this design necessitates phase-matching between segments to ensure high signal quality.
Microring modulators (MRMs) offer excellent performance in terms of data rate and modulation area density due to their ultra-small size. Recent developments have achieved 280 Gb/second using PAM-4 and 330 Gb/second with PAM-8 modulation formats. Despite their impressive performance, conventional MRMs face challenges when used at wavelengths beyond their design range, leading to decreased performance.
The Adiabatic Microring Modulator (AMRM), a recent innovation, features an ultra-wide operational wavelength range. The AMRM maintains a coupling ratio within 0.5 dB variation across a 100 nm simulated range, resulting in over 80 nm of operational wavelength range with more than 20 dB extinction.
Additionally, the AMRM offers a high 3 dB electro-optic bandwidth exceeding 60 GHz. This AMRM achieved 250 Gb/second at 125 Gbaud with PAM-4 modulation. Its ultra-compact footprint, combined with wavelength selectivity similar to traditional MRMs and a broad operational wavelength range, positions it as a promising candidate for large-channel-count transceivers using wavelength division multiplexing (WDM) for future data center interconnects.
High Coupling Efficiency Silicon WGCs
To achieve high coupling efficiency in waveguide grating couplers, improvements in both mode matching and directionality are essential. Early approaches for enhancing coupling efficiency included using polysilicon overlay structures and apodization techniques. However, these methods required a 40 nm feature size, which posed challenges for integration with commercial foundry design rules.
Recently, a novel approach has been proposed that uses a polysilicon layer deposited on the silicon grating, with a minimum feature size of over 180 nm—compatible with commercial foundry specifications. In this design, the polysilicon layer's optimized pattern and shifted positioning relative to the lower silicon grating structure facilitate the engineering of constructive and destructive interference for upward and downward diffracted light, respectively, thereby improving directionality.
Additionally, the positional shift between the polysilicon and silicon layers creates four subwavelength structures, enabling a gradual, effective index change. This design achieves a grating's blazing effect, enhancing coupling efficiency for both vertical and off-vertical angled grating couplers.
In summary, this paper reviewed the latest advancements in silicon WGCs and photonic modulators, highlighting their potential for future high-capacity, multichannel communications applications.
Journal Reference
Zhou, X., Yi, D., Chan, D. W., Tsang, H. K. (2024). Silicon photonics for high-speed communications and photonic signal processing. Npj Nanophotonics, 1(1), 1-14. DOI: 10.1038/s44310-024-00024-7, https://www.nature.com/articles/s44310-024-00024-7
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