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

We examine the current performance and future demands of interconnects to and on silicon chips. We compare electrical and optical interconnects and project the requirements for optoelectronic and optical devices if optics is to solve the major problems of interconnects for future high-performance silicon chips. Optics has potential benefits in interconnect density, energy, and timing. The necessity of low interconnect energy imposes low limits especially on the energy of the optical output devices, with a ~ 10 fJ/bit device energy target emerging. Some optical modulators and radical laser approaches may meet this requirement. Low (e.g., a few femtofarads or less) photodetector capacitance is important. Very compact wavelength splitters are essential for connecting the information to fibers. Dense waveguides are necessary on-chip or on boards for guided wave optical approaches, especially if very high clock rates or dense wavelength-division multiplexing (WDM) is to be avoided. Free-space optics potentially can handle the necessary bandwidths even without fast clocks or WDM. With such technology, however, optics may enable the continued scaling of interconnect capacity required by future chips.

Device Requirements for Optical Interconnects to Silicon Chips

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

We present a high-speed silicon optical modulator with a low Vpp (peak-to-peak driving voltage) and ultralow energy consumption based on a microring resonator, with the refractive index modulation achieved by electric-field-induced carrier depletion in a reverse-biased lateral pn diode embedded in the ring structure. With a Vpp of 2 V, we demonstrate a silicon modulator with a 3 dB bandwidth of 11 GHz, a modulation depth of 6.5 dB together with an insertion loss of 2 dB, ultralow energy consumption of 50 fJ per bit, and a small device active area of ~1000 µm2.

Low V_pp, ultralow-energy, compact, high-speed silicon electro-optic modulator

We present thermally tunable silicon racetrack resonators with an ultralow tuning power of 2.4 mW per free spectral range. The use of free-standing silicon racetrack resonators with undercut structures significantly enhances the tuning efficiency, with one order of magnitude improvement of that for previously demonstrated thermo-optic devices without undercuts. The 10%-90% switching time is demonstrated to be ~170 µs. Such low-power tunable micro-resonators are particularly useful as multiplexing devices and wavelength-tunable silicon microcavity modulators.

Thermally tunable silicon racetrack resonators with ultralow tuning power

We present thermally reconfigurable multiplexing devices based on silicon microring resonators with low tuning power and low thermal crosstalk. Micro-heaters on top of the rings are employed to tune the resonant wavelengths through the thermo-optic effect of silicon. We achieve a low tuning power of 21 mW per free spectral range for a single ring by exploiting thermal isolation trenches close to the ring waveguides. Negligible thermal crosstalk is demonstrated for rings spaced by 15 µm, enabling compact multiplexing devices. The tuning time constant is demonstrated to be less than 10 µs.

Low power and compact reconfigurable multiplexing devices based on silicon microring resonators

We present a wavelength-tunable, compact, high speed and low power silicon microring modulator. With a ring radius of 5 microm, we demonstrate a modulator with a high speed of 12.5 Gbps and a driving voltage of 3 V to achieve approximately 6 dB extinction ratio in high speed measurement. More importantly, tunability of the resonant wavelength is accomplished by means of a microheater on top of the ring, with an efficiency of 2.4 mW/nm (2.4 mW is needed to tune the resonant wavelength by 1 nm). This device aims to solve the narrow bandwidth problem of silicon microcavity modulators and increase the data bandwidth in optical interconnect systems.

Wavelength-tunable silicon microring modulator.

We demonstrate a compact, high speed Ge photodetector efficiently butt-coupled with a large cross-section silicon-on-insulate (SOI) waveguide in which the Ge p-i-n junction is placed in the horizontal direction to enable very high speed operation. The demonstrated photodetector has an active area of only 0.8×10 μm2, greater than 32 GHz optical bandwidth, and a responsivity of 1.1 A/W at a wavelength of 1550 nm. Very importantly the device can readily be integrated with high performance wavelength-division-multiplexing filters based on large cross-section SOI waveguide to form monolithic integrated silicon photonics receivers for multichannel terabit data transmission applications.

Hong Liang

Papers

Low V_pp, ultralow-energy, compact, high-speed silicon electro-optic modulator

Thermally tunable silicon racetrack resonators with ultralow tuning power

Low power and compact reconfigurable multiplexing devices based on silicon microring resonators

Wavelength-tunable silicon microring modulator.

High-speed Ge photodetector monolithically integrated with large cross-section silicon-on-insulator waveguide