SOI is at the heart of silicon photonics. Here’s an overview of past, present and future trends.
The existence of Silicon Photonics owes much to serendipity. During the early years of the development of SOI wafer technology probably nobody anticipated that SOI would be a perfect medium for short distance transmission and modulation of light beams. Only in 1986 Richard Soref pointed out that SOI structure had the right properties for light confinement in near infrared1, and some years later Si waveguides started being designed.
A very large refractive index contrast between the Si and the SiO2 means that the light is very well confined inside the Si waveguide core, which can have sharp bends. This leads to very compact photonic integrated circuits (PICs) with densely spaced micron-scale photonic devices.
By coupling optical fibers with Si waveguides etched in SOI substrates, the light that is going into or coming out of such fibers can be processed.
In recent years the photonics community has developed all the devices needed for such processing, from light modulators and wavelength filters built in SOI to photodetectors made in germanium that was selectively grown on Si, all capable of handling data streams with a bandwidth of at least 10 Gb/s and often much higher.
Just as multiple wavelengths can propagate in a fiber (wavelength division multiplexing or WDM), they can also propagate together in a Si waveguide, and devices to multiplex and demultiplex these data streams can also be built in SOI.
We can expect that the future silicon PICs will be built with higher complexity and reduced cost, and with reduced power/bit of data. Fully integrated silicon photonics technology for transceivers used in short (10 – 100 m) and medium (1 km) range optical interconnects in data centers and supercomputers exists now at a few companies.
The enormous bandwidth requirements and power dissipation constraints in large IT systems will advance Si photonics for inter-board and inter-chip communication, and eventually for intra-chip links.
The MIT Microphotonics Center and about 20 industrial partners released Communication Technology Roadmaps in 2005 and 2009 that address the challenges of high bandwidth communication at the lowest possible power.
At the latest Microphotonics Center Spring Meeting in April 2011, presentations covered a broad range of topics. The audience heard a provocative statement by a speaker from Alcatel-Lucent that integrated photonic circuits are unlikely to succeed since photonics devices do not scale as their size is defined by the wavelength of light. Others agreed that this indeed is the reason to aim for hybrid electronic/photonic integration since different device scales are less conducive to monolithic integration.
Speakers from NTT and AIST in Japan, Kotura, Analog Devices, and even Alcatel-Lucent presented new designs for fully integrated photonic circuits on SOI platforms.
Si PIC development will accelerate when a new photonic foundry, OpSIS (Optoelectronic System Integration in Silicon), for multi-user wafers comes on line. It is being organized by the University of Washington, with the financial backing of US government agencies and industry. BAE will be the first fab qualified to process the photonic chips for OpSiS, while two more facilities may join later.
All these developments lend credence to the emergence of silicon (in an SOI structure) as an important photonic material. Dr. Soref’s observation of 25 years ago is paying off.
1 R.A. Soref, J. P, Lorenzo, “All-Silicon Active and Passive Guided-Wave Components for wavelength=1.3 and 1.6μm”, IEEE J. Quantum Electron. Vol. QE-22, 873 (1986).
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