And That’s Why It’s Not Called Sili-Can’t…
by By Dom Siriani
So why photonics in silicon? After all, compound semiconductors have historically been phenomenal (and, I might add, they continue to be), especially for active devices (the indirect bandgap of silicon makes this a real challenge). Well, as pointed out by a number of the speakers today, the key is that photonics need electronics, which means we’d really like to make our electro-optical devices compatible with conventional CMOS technology in order to improve size, cost, and efficiency.
The topic was kicked off nicely with a plenary talk by John Bowers of UCSB. Prof. Bowers described the size and efficiency bottleneck for big data transmission, which can primarily be traced to switching and routing. By moving to all-optical switching and integrating photonics with electronics, one can significantly reduce size and power. So the question then is what one can do with silicon photonics.
As it turns out, more and more can be done with silicon photonics as time goes by. This was highlighted in the plenary as well as the following sessions on silicon photonics. Although silicon is not a great emitter, modulators, detectors, waveguides, multiplexers, polarization rotators, and so on can all be made in silicon. Here are a few examples of what can be done with silicon:
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By using a reverse biased p-n junction, a depletion mode phase modulator, which operates on the carrier plasma effect on refractive index, can be made
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Adiabatic waveguide transitions can be used to make polarization rotators, an essential component for polarization multiplexed communications
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Mode multiplexers/demultiplexers can be created using asymmetric Y-branch couplers or by using effective index tuning of codirectional evanescent couplers
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Detectors can be made at a variety of wavelengths, including the near-to-mid-IR by using defect doping (e.g., ionic silicon implanted in silicon)
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Clever tricks can be played with photonic crystals to create a unidirectional device, i.e., an isolator
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Tightly confined modes can be propagated in silicon waveguides, or alternatively low-loss guides can be formed in the CMOS-compatible materials of silicon dioxide or nitride
Illustration of a depletion mode modulator based on the carrier plasma effect on refractive index.
With time, these devices continue to have improved performance. Simultaneously, the need for greater data rates pushes the requirements for more size, cost, and power efficient solutions. In time, it looks as though integration of electronics and photonics will certainly be essential, and the use of silicon photonics holds a lot of promise to address this need.
Disclaimer: Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government and MIT Lincoln Laboratory.
Posted: 8 Oct 2013 by By Dom Siriani