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

Slow light with a remarkably low group velocity is a promising solution for buffering and time-domain processing of optical signals. It also offers the possibility for spatial compression of optical energy and the enhancement of linear and nonlinear optical effects. Photonic-crystal devices are especially attractive for generating slow light, as they are compatible with on-chip integration and room-temperature operation, and can offer wide-bandwidth and dispersion-free propagation. Here the background theory, recent experimental demonstrations and progress towards tunable slow-light structures based on photonic-band engineering are reviewed. Practical issues related to real devices and their applications are also discussed. The unique properties of wide-bandwidth and dispersion-free propagation in photonic-crystal devices have made them a good candidate for slow-light generation. This article gives the background theory of slow light, as well as an overview of recent experimental demonstrations based on photonic-band engineering.

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Slow light in photonic crystals

On-chip optical buffers based on waveguide delay lines might have significant implications for the development of optical interconnects in computer systems. Silicon-on-insulator (SOI) submicrometre photonic wire waveguides are used, because they can provide strong light confinement at the diffraction limit, allowing dramatic scaling of device size. Here we report on-chip optical delay lines based on such waveguides that consist of up to 100 microring resonators cascaded in either coupled-resonator or all-pass filter (APF) configurations. On-chip group delays exceeding 500 ps are demonstrated in a device with a footprint below 0.09 mm2. The trade-offs between resonantly enhanced group delay, device size, insertion loss and operational bandwidth are analysed for various delay-line designs. A large fractional group delay exceeding 10 bits is achieved for bit rates as high as 20 Gbps. Measurements of system-level metrics as bit error rates for different bit rates demonstrate error-free operation up to 5 Gbps.

Ultracompact optical buffers on a silicon chip

Driven by the increasing demand on handing microwave signals with compact device, low power consumption, high efficiency and high reliability, it is highly desired to generate, distribute, and process microwave signals using photonic integrated circuits. Silicon photonics offers a promising platform facilitating ultracompact microwave photonic signal processing assisted by silicon nanophotonic devices. In this paper, we propose, theoretically analyze and experimentally demonstrate a simple scheme to realize ultracompact rejection ratio tunable notch microwave photonic filter (MPF) based on a silicon photonic crystal (PhC) nanocavity with fixed extinction ratio. Using a conventional modulation scheme with only a single phase modulator (PM), the rejection ratio of the presented MPF can be tuned from about 10 dB to beyond 60 dB. Moreover, the central frequency tunable operation in the high rejection ratio region is also demonstrated in the experiment.

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Photonic crystal nanocavity assisted rejection ratio tunable notch microwave photonic filter

High-index-contrast, wavelength-scale structures are key to ultracompact integration of photonic integrated circuits. The fabrication of these nanophotonic structures in silicon-on-insulator using complementary metal-oxide-semiconductor processing techniques, including deep ultraviolet lithography, was studied. It is concluded that this technology is capable of commercially manufacturing nanophotonic integrated circuits. The possibilities of photonic wires and photonic-crystal waveguides for photonic integration are compared. It is shown that, with similar fabrication techniques, photonic wires perform at least an order of magnitude better than photonic-crystal waveguides with respect to propagation losses. Measurements indicate propagation losses as low as 0.24 dB/mm for photonic wires but 7.5 dB/mm for photonic-crystal waveguides.

Nanophotonic waveguides in silicon-on-insulator fabricated with CMOS technology

The silicon-on-insulator (SOI) platform allows to make ultra-compact photonic integrated circuits by means of standard processes used for silicon CMOS. Basic properties of SOI waveguides and the coupling to fiber are briefly discussed. Afterwards, various WDM-components (filters and demultiplexers) based on high-contrast nanophotonic waveguides in SOI are reported.

https://biblio.ugent.be/publication/327416/file/562147.pdf

Silicon-on-insulator platform for integrated wavelength-selective components (invited paper)

https://biblio.ugent.be/publication/327044/file/562004.pdf

Microring resonators in silicon-on-insulator

High index contrast, wavelength-scale structures are the key to ultra-compact integration of photonic integrated circuits. We discuss the general properties of these nanophotonic components and the rationale for using them. We studied the fabrication of these structures in the Silicon-on-insulator material system using CMOS processing methods and find this technology to be suitable for commercial implementation of nanophotonic IC’s. We review photonic wires as one of the paths towards achieving denser integration and discuss the measurement results of waveguides and ring resonators. To tackle the coupling to fiber and the polarization dependency problem, we studied and fabricated vertical fiber couplers.

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Wiaux

Papers

Nanophotonic waveguides in silicon-on-insulator fabricated with CMOS technology

Silicon-on-insulator platform for integrated wavelength-selective components (invited paper)

Microring resonators in silicon-on-insulator

Low loss nanophotonic waveguides and ring resonators in silicon-on-insulator

Low-loss, single-mode photonic wires and ring resonators in silicon-on-insulator