4 Aug 2011 Pasadena - Stretching for thousands of miles beneath oceans, optical fibers now connect every continent except for Antarctica. With less data loss and higher bandwidth, optical-fiber technology allows information to zip around the world, bringing pictures, video, and other data from every corner of the globe to your computer in a split second. But although optical fibers are increasingly replacing copper wires, carrying information via photons instead of electrons, today's computer technology still relies on electronic chips.
Now, researchers led by engineers at the California Institute of Technology (Caltech) are paving the way for the next generation of computer-chip technology: photonic chips. With integrated circuits that use light instead of electricity, photonic chips will allow for faster computers and less data loss when connected to the global fiber-optic network.
"We want to take everything on an electronic chip and reproduce it on a photonic chip", stated Liang Feng, a postdoctoral scholar in electrical engineering and the lead author on a paper published in the August 5 issue of the journalScience. Liang Feng is part of Caltech's nanofabrication group, led by Axel Scherer, Bernard A. Neches Professor of Electrical Engineering, Applied Physics, and Physics, and co-director of the Kavli Nanoscience Institute at Caltech.
In that paper, the researchers describe a new technique to isolate light signals on a silicon chip, solving a longstanding problem in engineering photonic chips.
An isolated light signal can only travel in one direction. If light weren't 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 analogue of a diode, a device called an optical isolator. "This is something scientists have been pursuing for 20 years", Liang Feng stated.
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 stated. In order to isolate light, its properties need to somehow change when going in the opposite direction. An optical isolator can then block light that has these changed properties, which allows light signals to travel only in one direction between devices on a chip.
"We want to build something where you can see me, but I can't see you", Liang Feng explained. "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, Liang Feng and his colleagues designed a new type of optical waveguide, a 0.8-micron-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.
A light wave'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", Liang Feng stated. "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's the basis for the integrated circuit, is a linear material. If computers were to use optical isolators made out of non-linear 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.
Although this work is just a proof-of-principle experiment, the researchers are already building an optical isolator that can be integrated onto a silicon chip. An optical isolator is essential for building the integrated, nanoscale photonic devices and components that will enable future integrated information systems on a chip. Current, state-of-the-art photonic chips operate at 10 gigabits per second (Gbps) - hundreds of times the data-transfer rates of today's personal computers - with the next generation expected to soon hit 40 Gbps. But without built-in optical isolators, those chips are much simpler than their electronic counterparts and are not yet ready for the market. Optical isolators like those based on the researchers' designs will therefore be crucial for commercially viable photonic chips.
In addition to Liang Feng and Axel Scherer, the other authors on theSciencepaper, "Non-reciprocal light propagation in a silicon photonic circuit", are Jingqing Huang, a Caltech graduate student; Maurice Ayache of UC San Diego and Yeshaiahu Fainman, Cymer Professor in Advanced Optical Technologies at UC San Diego; and Ye-Long Xu, Ming-Hui Lu, and Yan-Feng Chen of the Nanjing National Laboratory of Microstructures in China. This research was done as part of the Center for Integrated Access Networks (CIAN), one of the National Science Foundation's Engineering Research Centers. Yeshaiahu Fainman is also the deputy director of CIAN. Funding was provided by the National Science Foundation, and the Defense Advanced Research Projects Agency.
"This discovery will help to realize a long-term goal of combining electronics with photonics to enable scalable, energy-efficient and cost-effective technology that will have a tremendous impact on such information systems as supercomputers, the Internet, and data centres", stated Yeshaiahu (Shaya) Fainman, professor and chair of the UC San Diego Department of Electrical and Computer Engineering. "Computer technology will be able to handle a lot more data, faster and at lower cost, which will benefit large-scale business and government users as well as gadget-loving consumers."
Maurice Ayache is a Ph.D. candidate in electrical engineering at UCSD. His work on near-field imaging enabled the team to prove that they had in fact realized nonreciprocal propagating light beams as intended. Maurice Ayache said he used a near-field scanning optical microscope (NSOM) to capture the light confined inside the waveguide device. The NSOM is part of a heterodyne interferometer which enables it to measure variations of the light wave in space. The experiment showed clearly that the light behaves differently moving in one direction from the other. Maurice Ayache compared the NSOM to an "optical stethoscope" enabling the team to see light trapped inside the waveguide device.
"It's like water in an insulated water pipe", Maurice Ayache stated. "You can't see or hear the water inside, but if you held a stethoscope to the pipe you could hear the current moving and know what's happening. The integration of NSOM and the interferometer was key to being able to prove that what we thought was going on inside the waveguide was actually going on."
Maurice Ayache said Yeshaiahu Fainman's team at UCSD is one of a few groups in the world that have the capability and capacity to combine near-field imaging and heterodyne interferometry to achieve this kind of measurement and analysis of light signals.