Volume 3, Issue 4-5 - Special issue: Silicon Photonic Integrated Devices / Guest Editors: Chris Doerr, Haisheng Rong and Michal Lipson

Rapid progress has been made in recent years repurposing CMOS fabrication tools to build complex photonic circuits. As the field of silicon photonics becomes more mature, foundry processes will be an essential piece of the ecosystem for eliminating process risk and allowing the community to focus on adding value through clever design. Multi-project wafer runs are a useful tool to promote further development by providing inexpensive, low-risk prototyping opportunities to academic and commercial researchers. Compared to dedicated silicon manufacturing runs, multi-project-wafer runs offer cost reductions of 100× or more. Through OpSIS, we have begun to offer validated device libraries that allow designers to focus on building systems rather than modifying device geometries. The EDA tools that will enable rapid design of such complex systems are under intense development. Progress is also being made in developing practical optical and electronic packaging solutions for the photonic chips, in ways that eliminate or sharply reduce development costs for the user community. This paper will provide a review of the recent developments in silicon photonic foundry offerings with a focus on OpSIS, a multi-project-wafer foundry service offering a silicon photonics platform, including a variety of passive components as well as high-speed modulators and photodetectors, through the Institute of Microelectronics in Singapore.

Silicon photonic devices and integrated circuits have undergone rapid and significant progresses during the last decade, transitioning from research topics in universities to product development in corporations. Silicon photonics is anticipated to be a disruptive optical technology for data communications, with applications such as intra-chip interconnects, short-reach communications in datacenters and supercomputers, and long-haul optical transmissions. Bell Labs, as the research organization of Alcatel-Lucent, a network system vendor, has an optimal position to identify the full potential of silicon photonics both in the applications and in its technical merits. Additionally it has demonstrated novel and improved high-performance optical devices, and implemented multi-function photonic integrated circuits to fulfill various communication applications. In this paper, we review our silicon photonic programs and main achievements during recent years. For devices, we review high-performance single-drive push-pull silicon Mach-Zehnder modulators, hybrid silicon/III-V lasers and silicon nitride-assisted polarization rotators. For photonic circuits, we review silicon/silicon nitride integration platforms to implement wavelength-division multiplexing receivers and transmitters. In addition, we show silicon photonic circuits are well suited for dual-polarization optical coherent transmitters and receivers, geared for advanced modulation formats. We also discuss various applications in the field of communication which may benefit from implementation in silicon photonics.

The majority of the most successful optical modulators in silicon demonstrated in recent years operate via the plasma dispersion effect and are more specifically based upon free carrier depletion in a silicon rib waveguide. In this work we overview the different types of free carrier depletion type optical modulators in silicon. A summary of some recent example devices for each configuration is then presented together with the performance that they have achieved. Finally an insight into some current research trends involving silicon based optical modulators is provided including integration, operation in the mid-infrared wavelength range and application in short and long haul data transmission links.

Group IV photonics hold great potential for nonlinear applications in the near- and mid-infrared (IR) wavelength ranges, exhibiting strong nonlinearities in bulk materials, high index contrast, CMOS compatibility, and cost-effectiveness. In this paper, we review our recent numerical work on various types of silicon and germanium waveguides for octave-spanning ultrafast nonlinear applications. We discuss the material properties of silicon, silicon nitride, silicon nano-crystals, silica, germanium, and chalcogenide glasses including arsenic sulfide and arsenic selenide to use them for waveguide core, cladding and slot layer. The waveguides are analyzed and improved for four spectrum ranges from visible, near-IR to mid-IR, with material dispersion given by Sellmeier equations and wavelength-dependent nonlinear Kerr index taken into account. Broadband dispersion engineering is emphasized as a critical approach to achieving on-chip octave-spanning nonlinear functions. These include octave-wide supercontinuum generation, ultrashort pulse compression to sub-cycle level, and mode-locked Kerr frequency comb generation based on few-cycle cavity solitons, which are potentially useful for next-generation optical communications, signal processing, imaging and sensing applications.

Silicon microring resonators have been hailed for their potential use in next-generation optical interconnects. However, the functionality of silicon microring based devices suffer from susceptibility to thermal fluctuations that is often overlooked in their demonstrated results, but must be resolved for their future implementation in microelectronic applications. We survey the emerging efforts that have been put forth to resolve these thermal susceptibilities and provide a comprehensive discussion of their advantages and disadvantages.

An effective solution to enhance the capacity of an optical-interconnect link is utilizing advanced multiplexing technologies, like wavelength-division-multiplexing (WDM), polarization-division multiplexing (PDM), spatial-division multiplexing (SDM), bi-directional multiplexing, etc. On-chip (de)multiplexers are necessary as key components for realizing these multiplexing systems and they are desired to have small footprints due to the limited physical space for on-chip optical interconnects. As silicon photonics has provided a very attractive platform to build ultrasmall photonic integrated devices with CMOS-compatible processes, in this paper we focus on the discussion of silicon-based (de)multiplexers, including WDM filters, PDM devices, and SDM devices. The demand of devices to realize a hybrid multiplexing technology (combining WDM, PDM and SDM) as well as a bidirectional multiplexing technologies are also discussed to achieve Peta-bit optical interconnects.

Silicon photonics offers the possibility of a reduction in size weight and power for many optical systems, and could open up the ability to build optical systems with complexities that would otherwise be impossible to achieve. Silicon photonics is an emerging technology that has already been inserted into commercial communication products. This technology has also been applied to analog signal processing applications. MIT Lincoln Laboratory in collaboration with groups at MIT has developed a toolkit of silicon photonic devices with a focus on the needs of analog systems. This toolkit includes low-loss waveguides, a high-speed modulator, ring resonator based filter bank, and all-silicon photodiodes. The components are integrated together for a hybrid photonic and electronic analog-to-digital converter. The development and performance of these devices will be discussed. Additionally, the linear performance of these devices, which is important for analog systems, is also investigated.

The mid-infrared (IR) wavelength region (2–20 µm) is of great interest for a number of applications, including trace gas sensing, thermal imaging, and free-space communications. Recently, there has been significant progress in developing a mid-IR photonics platform in Si, which is highly transparent in the mid-IR, due to the ease of fabrication and CMOS compatibility provided by the Si platform. Here, we discuss our group’s recent contributions to the field of silicon-based mid-IR photonics, including photonic crystal cavities in a Si membrane platform and grating-coupled high-quality factor ring resonators in a silicon-on-sapphire (SOS) platform. Since experimental characterization of microphotonic devices is especially challenging at the mid-IR, we also review our mid-IR characterization techniques in some detail. Additionally, pre- and post-processing techniques for improving device performance, such as resist reflow, Piranha clean/HF dip cycling, and annealing are discussed.