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Light Isolated on a Photonic Chip

PASADENA & SAN DIEGO, Calif., Aug. 5, 2011 — Information systems increasingly are relying on fiber optic networks carrying data via photons instead of electrons, but computer technology still relies heavily on electronic chips, which are slower and more prone to data loss than photonic chips. Now, researchers at the California Institute of Technology (Caltech) and the University of California, San Diego, have developed an integrated photonic chip that could replace electronic chips as the backbone of computer and information technology.

"We want to take everything on an electronic chip and reproduce it on a photonic chip," said Liang Feng, a postdoctoral scholar in electrical engineering from Caltech and the lead author on a paper published in the journal Science.

An isolated light signal can travel in only one direction. If the light is not isolated, signals sent and received between different components on a photonic circuit could interfere with one another, causing the chip to become unstable. In an electrical circuit, a device called a diode isolates electrical signals by allowing current to travel in one direction but not the other. The goal, then, is to create the photonic analog of a diode — a device called an optical isolator. "This is something scientists have been pursuing for 20 years," Feng said.


Caltech engineers have developed a new way to isolate light on a photonic chip, allowing light to travel in only one direction. This finding can lead to the next generation of computer chip technology: photonic chips that allow for faster computers and less data loss. (Image: Caltech/Liang Feng)

Normally, a light beam has exactly the same properties when it moves forward as when it's reflected backward. "If you can see me, then I can see you," he said. For light to be isolated, its properties somehow need to change when going in the opposite direction. An optical isolator can block light that has these changed properties, which allows light signals to travel in only one direction between devices on a chip.

"We want to build something where you can see me, but I can't see you," Feng said. "That means there's no signal from your side to me. The device on my side is isolated; it won't be affected by my surroundings, so the functionality of my device will be stable."

To isolate light, the team designed a new type of optical waveguide, a 0.8-µm-wide silicon device that channels light. The waveguide allows light to go in one direction but changes the mode of the light when it travels in the opposite direction.


This image was captured using UCSD Jacobs School of Engineering's unique capacity for near-field imaging. Heterodyne interferometry shows that the waveguide device built by the Caltech-UCSD team prevents backscattered light from interfering in the operations of a photonic silicon chip. (Image: Caltech-UC San Diego research team)

A lightwave's mode corresponds to the pattern of the electromagnetic field lines that make up the wave. In the researchers' new waveguide, the light travels in a symmetric mode in one direction but changes to an asymmetric mode in the other. Because different light modes can't interact with one another, the two beams of light thus pass through each other.

Previously, there were two main ways to achieve this kind of optical isolation. The first way — developed almost a century ago — is to use a magnetic field. The magnetic field changes the polarization of light — the orientation of the light's electric-field lines — when it travels in the opposite direction, so that the light going one way can't interfere with the light going the other way.

"The problem is, you can't put a large magnetic field next to a computer," Feng said. "It's not healthy."

The second conventional method requires so-called nonlinear optical materials, which change light's frequency rather than its polarization. This technique was developed about 50 years ago but is problematic because silicon, the material that forms the basis of the integrated circuit, is a linear material. If computers were to use optical isolators made out of nonlinear materials, silicon would have to be replaced, which would require revamping all of computer technology. But with their new silicon waveguides, the researchers have become the first to isolate light with a linear material.

Lab versions of photonic chips being developed across the industry already are supporting data transfer rates of 10 Gb/s, and in just five years, photonic chips could achieve data transfer rates of more than 40 Gb/s — an order of magnitude higher than the speed of today's networks. The shift toward optical networks will make information sharing faster, more energy-efficient and less costly.

For more information, visit: www.caltech.edu or www.ucsd.edu  


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