Optical technology is poised to revolutionize short-reach interconnects. The leading candidate technology is silicon photonics, and the workhorse of such an interconnect is the optical modulator. Modulators have been improved dramatically in recent years, with a notable increase in bandwidth from the megahertz to the multigigahertz regime in just over half a decade. However, the demands of optical interconnects are significant, and many questions remain unanswered as to whether silicon can meet the required performance metrics. Minimizing metrics such as the device footprint and energy requirement per bit, while also maximizing bandwidth and modulation depth, is non-trivial. All of this must be achieved within an acceptable thermal tolerance and optical spectral width using CMOS-compatible fabrication processes. This Review discusses the techniques that have been (and will continue to be) used to implement silicon optical modulators, as well as providing an outlook for these devices and the candidate solutions of the future.

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Silicon optical modulators

An overview is presented of the current state-of-the-art in silicon nanophotonic ring resonators. Basic theory of ring resonators is discussed, and applied to the peculiarities of submicron silicon photonic wire waveguides: the small dimensions and tight bend radii, sensitivity to perturbations and the boundary conditions of the fabrication processes. Theory is compared to quantitative measurements. Finally, several of the more promising applications of silicon ring resonators are discussed: filters and optical delay lines, label-free biosensors, and active rings for efficient modulators and even light sources.

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Silicon microring resonators

The past decade has seen rapid progress in research into high-performance Ge-on-Si photodetectors. Owing to their excellent optoelectronic properties, which include high responsivity from visible to near-infrared wavelengths, high bandwidths and compatibility with silicon complementary metal–oxide–semiconductor circuits, these devices can be monolithically integrated with silicon-based read-out circuits for applications such as high-performance photonic data links and infrared imaging at low cost and low power consumption. This Review summarizes the major developments in Ge-on-Si photodetectors, including epitaxial growth and strain engineering, free-space and waveguide-integrated devices, as well as recent progress in Ge-on-Si avalanche photodetectors. Owing to their excellent optoelectronic properties, Ge-on-Si photodetector can be monolithically integrated with silicon-based read-out circuits for applications such as high-performance photonic data links and low-cost infrared imaging at low power consumption. This Review covers the major developments in Ge-on-Si photodetectors, including epitaxial growth and strain engineering, free-space and waveguide-integrated devices, as well as recent progress in Ge-on-Si avalanche photodetectors.

High-performance Ge-on-Si photodetectors

Silicon photonics research can be dated back to the 1980s. However, the previous decade has witnessed an explosive growth in the field. Silicon photonics is a disruptive technology that is poised to revolutionize a number of application areas, for example, data centers, high-performance computing and sensing. The key driving force behind silicon photonics is the ability to use CMOS-like fabrication resulting in high-volume production at low cost. This is a key enabling factor for bringing photonics to a range of technology areas where the costs of implementation using traditional photonic elements such as those used for the telecommunications industry would be prohibitive. Silicon does however have a number of shortcomings as a photonic material. In its basic form it is not an ideal material in which to produce light sources, optical modulators or photodetectors for example. A wealth of research effort from both academia and industry in recent years has fueled the demonstration of multiple solutions to these and other problems, and as time progresses new approaches are increasingly being conceived. It is clear that silicon photonics has a bright future. However, with a growing number of approaches available, what will the silicon photonic integrated circuit of the future look like? This roadmap on silicon photonics delves into the different technology and application areas of the field giving an insight into the state-of-the-art as well as current and future challenges faced by researchers worldwide. Contributions authored by experts from both industry and academia provide an overview and outlook for the silicon waveguide platform, optical sources, optical modulators, photodetectors, integration approaches, packaging, applications of silicon photonics and approaches required to satisfy applications at mid-infrared wavelengths. Advances in science and technology required to meet challenges faced by the field in each of these areas are also addressed together with predictions of where the field is destined to reach.

https://iopscience.iop.org/article/10.1088/2040-8978/18/7/073003/pdf

Roadmap on silicon photonics

As silicon-based electronics approach the limit of improvements to performance and capacity through dimensional scaling, attention in the semiconductor field has turned to graphene, a single layer of carbon atoms arranged in a honeycomb lattice. Its high mobility of charge carriers (electrons and holes) could lead to its use in the next generation of high-performance devices. Graphene is unlikely to replace silicon completely, however, because of the poor on/off current ratio resulting from its zero bandgap. But it could be used to improve silicon-based devices, in particular in high-speed electronics and optical modulators.

A role for graphene in silicon-based semiconductor devices

We present the packaging of a large multi-chip energyefficient WDM silicon photonic interconnect prototype enabled by hybrid integration, high-accuracy optical alignment and thermal-mechanical aware design and assembly.

Hybrid integration and packaging of an energy-efficient WDM silicon photonic chip-to-chip interconnect

High-bandwidth, low-power and compact silicon electro-optical modulators are essential for future energy-efficient and densely integrated optical data communication circuits. The all-silicon plasma-dispersion-effect ring resonator modulator is an attractive prospect. However, its performance is currently limited by the trade-off between modulation depth and switching speed, dictated by its quality factor. Here we introduce a mechanism to leap beyond this limitation by harnessing the plasma absorption induced in a silicon metal–oxide–semiconductor waveguide to enhance the extinction ratio of a low-quality-factor, high-speed ring modulator. The fabricated devices demonstrate a modulation depth of ~27 dB for a bias of ~3.5 V. Modulation enhancement has been observed for operation frequencies ranging from kilohertz to gigahertz, with data modulation up to 100 Gbit s−1 on–off keying demonstrated, paving a way to the evolution of optical interconnects to 100 Gbaud and beyond per wavelength. The plasma absorption effect is demonstrated to enhance the performance of silicon ring modulators. 

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Harnessing plasma absorption in silicon MOS ring modulators

We present a silicon microring modulator with a 1 V drive voltage to achieve a 6.8 dB modulation depth at 10 Gbps. This drive voltage enables the modulator to be directly driven by CMOS electronics.

High-speed silicon microring modulator with a 1 V drive voltage

Electro-optic modulators capable of operating at cryogenic temperatures are of interest to a host of sensing, quantum, and supercomputing applications. Silicon photonics is compelling for its low cost and CMOS compatibility, but conventional tuning mechanisms are impacted at low temperatures. Bulk electro-absorption modulators are appealing since only the wavelength of the absorption band edge varies with temperature. Cryogenic effects on a fabricated high-speed modulator are shown, with consistent extinction ratio from 5-300K. (Abstract)

High-Speed SiGe EAMs at Cryogenic Temperatures

We demonstrate a grating coupler for use on a 3 µm device layer SOI platform by employing a two-stage taper. Our measured radiation efficiency is 75% and simulations show output as high as 86%.

Roshanak Shafiiha

Papers

Hybrid integration and packaging of an energy-efficient WDM silicon photonic chip-to-chip interconnect

Harnessing plasma absorption in silicon MOS ring modulators

High-speed silicon microring modulator with a 1 V drive voltage

High-Speed SiGe EAMs at Cryogenic Temperatures

High efficiency grating coupler for 3 µm SOI waveguides