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In work that could lead to completely new devices, systems and applications in computing and telecommunications, MIT researchers are bringing the long-sought goal of "optics on a chip"
one step closer to market.
In the January 2007 inaugural issue of the journal Nature Photonics,
the team reports a novel way to integrate photonic circuitry on a
silicon chip. Adding the power and speed of light waves to
traditional electronics could achieve system performance
inconceivable by electronic means alone.
The MIT invention will enable such integrated devices to be
mass-manufactured for the first time. And, depending on the growth of
the telecom industry, the new devices could be in demand within five
years, said co-author Erich P. Ippen, the Elihu Thomson Professor of
Electrical Engineering and Physics.
The new technology will also enable supercomputers on a chip with
unique high-speed capabilities for signal processing, spectroscopy
and remote testing, among other fields.
"This breakthrough allows inter- and intra-chip communications
networks that solve the wiring problems of today's computer chips and
computer architectures," said Franz X. Kaertner, a professor of
electrical engineering and computer science.
In addition to Ippen and Kaertner, other members of the MIT team are
Tymon Barwicz (PhD 2005), Michael Watts (PhD 2005), graduate students
Milos Popovic and Peter Rakich, and Henry I. Smith, professor of
electrical engineering and co-director of MIT's Nanostructures
Molding light waves
Microphotonics technology aims to "mold" the flow of light. By using
two different materials that refract light differently, such as
silicon and its oxides, photons can be trapped within a miniscule
hall of mirrors, giving them unique properties.
The stumbling block has been that microphotonics devices are
sensitive to the polarization of light.
Light waves moving through optical fibers can be arbitrarily
vertically or horizontally polarized, and microphotonic circuits
don't work well with that kind of random input. This has meant that
devices used in photonic subsystems and optical communication
networks, for instance, couldn't connect to the outside world without
often having to be assembled piecemeal and painstakingly by hand.
Like polarizing sunglasses, which use vertical polarizers to block
the horizontally oriented light reflected from flat surfaces such as
roads or water, the MIT method of integrating optics on a chip
involves separating the two orientations of polarized light waves.
Splitting the difference
The MIT researchers' innovative solution involves splitting the light
emanating from an optic fiber into two arms-one with horizontally
polarized beams and one with vertical beams-in an integrated, on-chip
Setting these two at right angles to one another, the researchers
rotated the polarization of one of the arms, also in an integrated
way. The beams from the two arms, now oriented the same way, then
pass through identical sets of polarization-sensitive photonic
structures and out the other side of the chip, where the two split
beams are rejoined.
"These results represent a breakthrough in permitting the processing
and switching of arbitrarily polarized input light signals in tightly
confined and densely integrated photonic circuitry," said Ippen. The
innovation means that optical components can be integrated onto a
single silicon chip and mass-produced, cutting costs and boosting
performance and complexity.
The advantage in integrating optics with silicon technology is that
silicon fabrication technology "is already highly developed and
promises precise and reproducible processing of densely integrated
circuits," Kaertner said. "The prospect of integrating the photonic
circuitry directly on silicon electronic chips is ultimately also an
In addition to offering a breakthrough in polarization, the MIT chip
also contains first-of-their-kind components in materials meeting
"Our results illustrate the importance of academic research in
nanofabrication and academia's role in breaking new pathways for the
industry to follow," Smith said. "Creating these devices was only
possible due to the unique nanofabrication facilities at MIT,
enabling fabrication with extraordinary precision."
This work was supported by Pirelli Labs in Milan, Italy, and made use
of MIT's Nanostructures Laboratory and MIT's Scanning Electron Beam
Lithography Facility, both within the Research Laboratory of
Today MIT is a world-class educational institution. Teaching and research—with relevance to the practical world as a guiding principle—continue to be its primary purpose. MIT is independent, coeducational, and privately endowed. Its five schools and one college encompass numerous academic departments, divisions, and degree-granting programs, as well as interdisciplinary centers, laboratories, and programs whose work cuts across traditional departmental boundaries.
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Elizabeth A. Thomson, MIT News Office
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