'Optics on a Chip' Melds Photonic Circuitry, Silicon

Using microphotonics, researchers have split the light from an optic fiber in an integrated, on-chip way, controlled the random polarization of the two beams and then reconnected them. The breakthrough in "optics on a chip" technology could lead to completely new devices, systems and applications in computing and telecommunications.

MIT graduate student Peter Rakich, who worked on developing optics that can fit atop a microchip. (Photo: Donna Coveney, MIT)

The Massachusetts Institute of Technology invention is 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 researchers said, and will allow such integrated devices to be mass-manufactured for the first time.

Depending on the growth of the telecom industry, the new devices could be in demand within five years, said team member 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 applications.

"This breakthrough allows inter- and intrachip communications networks that solve the wiring problems of today's computer chips and computer architectures," said team member Franz X. Kaertner, a professor of electrical engineering and computer science. Other members of the MIT team are Tymon Barwicz and Michael Watts, graduate students Milos Popovic and Peter Rakich, and Henry I. Smith, professor of electrical engineering and co-director of MIT's Nanostructures Laboratory.

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 tiny hall of mirrors, giving them unique properties. The stumbling block has been that microphotonics devices are sensitive to the polarization of light.

Top-view electron micrograph of MIT's "optics on a chip." Such photonic circuits could bring the speed of light waves to traditional electronics. (Image: Tymon Barwicz, MIT)

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.

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 fashion. 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.

Illustration of MIT's solution to polarization sensitivity, which until now prohibited most real-world applications of "optics on a chip." (Image: Tymon Barwicz, MIT)

The advantage in integrating optics with silicon 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 important driver."

The research was supported by Pirelli Labs in Milan, Italy, and made use of MIT's Nanostructures Laboratory and Scanning Electron Beam Lithography Facility, both within the Research Laboratory of Electronics. A report on the research, co-authored by Ippen, appears in the Jan. 2007 issue of the journal Nature Photonics.

For more information, visit: http://web.mit.edu/newsoffice/