The Institute of Microelectronics (IME), Singapore, has done some research work on silicon photonics research and MPW prototyping. IME will include components (germanium-on-silicon avalanche photodetectors, silicon modulators and wavelength-division-multiplexing (WDM) optical filters, all of which can be monolithically integrated on the silicon-on-insulator (SOI) platform. Thanks to my good friend, Charles Lee, I was able to set up a conversation with Dr. Patrick Guo-Qiang Lo, director of Nanoelectronics & Photonics Programme, IME. Elaborating on IME’s silicon photonics research and MPW prototyping, he said that, traditionally, an optical transceiver is fabricated by assembling discrete photonic and electronic components in order to achieve integrated functionality. Such an approach is not cost-effective and has limited scalability in terms of performance, component size and functionality. Dr. Guo-Qiang Lo said, "The fully monolithic silicon photonic transceiver chip, which involves the convergence of electronics and photonics on a single silicon chip, will be an enabling building block for future low-cost small form-factor photonic transceivers. In anticipation of this, the Institute of Microelectronics (IME) has been actively developing and enhancing its technology platform for monolithic silicon photonics. "A key objective is to pack different building blocks into a small yet highly-integrated real estate. Monolithic silicon photonics is undoubtedly fraught with challenges – how to maintain acceptable loss for fiber-to-waveguide coupling (mode-size mismatch) and propagation, how to detect the signal with high sensitivity and yet with high speed and low noise, how to achieve efficient and high-speed modulation, how to create compact filters with good crosstalk and loss performance, etc." In addition to the design innovations and experience in packing different layers of structures, the IME has also developed a set of process modules (e.g., CMOS-compatible low-temperature technologies) for photonic device construction, and is constantly expanding and refining its existing set of photonic device building blocks. It would be interesting to know the reasons for selecting components like germanium-on-silicon avalanche photodetectors, silicon modulators and wavelength-division-multiplexing optical filters. The IME spokesperson said that some of the latest technology developments at IME for realizing high performance monolithic silicon photonic transceivers are introduced: germanium-on-silicon avalanche photodetectors (APDs), silicon modulators and WDM optical filters, all of which can be monolithically integrated on the SOI platform. "Photodetectors and modulators are essential components in an optical transceiver. Silicon is transparent at the commonly used optical communication wavelength bands of 1.3 and 1.55 um, which makes it ideal for fabricating photonic waveguides and other passive photonic devices or circuits. However, silicon itself cannot be used for photodetection at these wavelengths. Instead, germanium, which has a lower bandgap, is used to form the photodetectors, enabling optical signal detection at these wavelengths. "The avalanche carrier-multiplication mechanism is exploited in the APD, further improving its sensitivity to incoming optical signals. Silicon also does not exhibit significant electro-optic behavior, unlike dedicated materials such as lithium niobate. Nevertheless, modulators can also be monolithically realized on silicon by utilizing the free carrier dispersion mechanism. WDM enables simultaneous multi-channel transmission on a single fiber, making it possible to scale the aggregate data-rate into the terabit regime. WDM filters fabricated on silicon have a small footprint, and can potentially be used to realize highly compact WDM transceivers." Applications benefitting from silicon photonicsNaturally, what would be the applications that can benefit from silicon photonics and how will all of those work? According to Dr. Guo-Qiang Lo, the possible applications of silicon photonics include on-chip interconnects, as well as optical connections between servers, boards and chips for high speed computers. More importantly, monolithic silicon photonics can potentially extend high data-rate communication capabilities even to consumer applications such as fiber-to-the-home (FTTH). FTTH involves deploying high bandwidth fiber optic infrastructure all the way to individual homes. The FTTH subscriber base, as well as its infrastructure, is rapidly growing worldwide. At the end of September 2008, worldwide FTTH subscribers totaled 23 million. A FTTH transceiver can potentially be fully integrated on a single silicon chip, dramatically reducing cost and complexity. As such, silicon photonics will facilitate the widespread deployment of high performance communications hardware at reasonable cost. In the next post featuring IME, I will talk about the convergence of electronics and photonics, and how it will change the technology landscape, as well as its MPW service. I will also provide some research insights from the IME on: a) plasmonic and b) photonic crystal devices. Pradeep July 25, 2009