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.

/pdf/silicon-optical-modulators-578o7o6kvr.pdf

Silicon optical modulators

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.

/pdf/slow-light-in-photonic-crystals-59fjoaj79v.pdf

Slow light in photonic crystals

Achieving optical gain and/or lasing in silicon has been one of the most challenging goals in silicon-based photonics because bulk silicon is an indirect bandgap semiconductor and therefore has a very low light emission efficiency. Recently, stimulated Raman scattering has been used to demonstrate light amplification and lasing in silicon. However, because of the nonlinear optical loss associated with two-photon absorption (TPA)-induced free carrier absorption (FCA), until now lasing has been limited to pulsed operation. Here we demonstrate a continuous-wave silicon Raman laser. Specifically, we show that TPA-induced FCA in silicon can be significantly reduced by introducing a reverse-biased p-i-n diode embedded in a silicon waveguide. The laser cavity is formed by coating the facets of the silicon waveguide with multilayer dielectric films. We have demonstrated stable single mode laser output with side-mode suppression of over 55 dB and linewidth of less than 80 MHz. The lasing threshold depends on the p-i-n reverse bias voltage and the laser wavelength can be tuned by adjusting the wavelength of the pump laser. The demonstration of a continuous-wave silicon laser represents a significant milestone for silicon-based optoelectronic devices.

/pdf/a-continuous-wave-raman-silicon-laser-1lmeebkhha.pdf

A continuous-wave Raman silicon laser

An electrically pumped light source on silicon is a key element needed for photonic integrated circuits on silicon. Here we report an electrically pumped AlGaInAs-silicon evanescent laser architecture where the laser cavity is defined solely by the silicon waveguide and needs no critical alignment to the III-V active material during fabrication via wafer bonding. This laser runs continuous-wave (c.w.) with a threshold of 65 mA, a maximum output power of 1.8 mW with a differential quantum efficiency of 12.7 % and a maximum operating temperature of 40 degrees C. This approach allows for 100's of lasers to be fabricated in one bonding step, making it suitable for high volume, low-cost, integration. By varying the silicon waveguide dimensions and the composition of the III-V layer, this architecture can be extended to fabricate other active devices on silicon such as optical amplifiers, modulators and photo-detectors.

Electrically pumped hybrid AlGaInAs-silicon evanescent laser

About the Authors.Foreword.Acknowledgements.1. Fundamentals.2. The Basics of Guided Waves.3. Characteristics of Optical Fibres for Communications.4. Silicon-on-Insulator (SOI) Photonics.5. Fabrication of Silicon Waveguide Devices.6. A Selection of Photonic Devices.7. Polarisation-dependent Losses: Issues for Consideration.8. Prospects for Silicon Light-emitting Devices.Index.

Andrew P. Knights

Papers

Silicon Photonics: An Introduction

Silicon‐On‐Insulator (SOI) Photonics

A Selection of Photonic Devices

Prospects for Silicon Light‐Emitting Devices