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 is deemed to be the solution for dense on-chip optical networks. Now, by using cascaded silicon microring resonators, scientists demonstrate an ultracompact switch that is insensitive to wavelength and temperature. The switch also has fast error-free operation in multiple 40-Gbit s−1 optical channels and is suitable for scalable networks.

High-throughput silicon nanophotonic wavelength-insensitive switch for on-chip optical networks

A compact pin Ge photodetector is integrated in submicron SOI rib waveguide. The detector length is reduced down to 15 μm using butt coupling configuration which is sufficient to totally absorb light at the wavelength of 1.55 μm. A -3 dB bandwidth of 42 GHz has been measured at a 4V reverse bias with a responsivity as high as 1 A/W at the wavelength of 1.55 μm and a low dark current density of 60 mA/cm2. At a wavelength of 1.52 μm, a responsivity of 1 A/W is obtained under -0.5 V bias. The process is fully compatible with CMOS technology.

42 GHz p.i.n Germanium photodetector integrated in a silicon-on-insulator waveguide

We report the experimental demonstration of a germanium metal-semiconductor-metal (MSM) photodetector integrated in a SOI rib waveguide. Femtosecond pulse and frequency experiments have been used to characterize such photodetectors. The measured bandwidth under 6V bias is about 25 GHz at 1.55 µm wavelength with a responsivity as high as 1 A/W. The used technological processes are compatible with complementary-metal-oxide-semiconductor (CMOS) technology.

High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide

Experimental results for refractive index variation induced by depletion in a silicon structure integrated in a PIN diode are reported. Thermal effect has been dissociated from the electrical contribution due to carrier density variation induced by a reverse bias voltage. A figure of merit VπLπ of 3.1 V.cm has been obtained at 1.55μm. Numerical simulations show a good agreement between experimental and theoretical index variations.

Jean François Damlencourt

Papers

High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide

Optical modulation by carrier depletion in a silicon PIN diode.

High speed all-silicon optical modulator