As demand for streaming video over the Web, voice over Internet protocol (VoIP) calling services and other forms of Internet-based multimedia communication skyrockets, content creators and consumers are counting on fiber-optic networks to handle these increasing loads quickly and efficiently. One way to ensure this happens is to enhance the ability of such networks, which transmit data over glass or plastic threads, to capture and retain data even for very brief intervals.
Toward that end, a team of researchers from Duke University and University of Rochester's Institute of Optics recently reported in Science that it successfully transferred encoded information from a laser beam to sound waves and back to light waves, a breakthrough that could speed development of faster optical communication networks. Swapping data between optics and acoustics allows it to be stored in pockets of acoustic vibration created when laser beams interact along a short strand of optical fiber.
The research is significant, because it addresses how memory can be created for optical pulses. "The primary thrust is investigating slow light via stimulated Brillouin scattering, where we slow down a pulse as it propagates through an optical fiber," says study co-author Daniel Gauthier, chairman of Duke's department of physics. Brillouin scattering occurs when light traveling through a medium, such as glass, changes its path as it encounters varying densities.
The main goal of the research is to pave the way for better fiber-optic communication systems, which today consist of fiber placed underground and linked by routers. The typical way to send data over an optical network is to break it up into chunks called packets. When a packet comes into a router, its address information is read. The problem with routers is that they each contain a single switch that can only process one packet at a time. As a result, some packets are dropped unless others coming in are buffered (saved) or can wait until it is their turn to be routed. "If you drop the packet, you reduce the throughput of the entire network," Gauthier says. "If you buffer, then the packets are processed one after the other."
As greater demands are placed on telecommunication infrastructures, "it's important to start to investigate parallel technologies," he adds.
Gauthier and his colleagues discovered that when two laser beams of slightly different frequencies are pointed at one another along a piece of glass fiber, they create acoustic vibrations called phonons. When co-author Zhaoming Zhu, Gauthier's postdoctoral research associate, encoded information onto one of these beams, the data could be imprinted on these newly created phonons and retained for 12 billionths of a second, long enough to be transferred back to light again by shining a third laser through the fiber.
"When thinking about how to store light in optical fibers," Zhu says, "we realized that we can convert optical information to acoustic vibration, something that hasn't been done before."
The researchers are seeking ways to create longer storage times and reduce the peak power of the laser beam needed for retaining and reading out the information, a process that will take years before a commercial version of the technology is available.
"There is still a great need for developing new strategies for optimizing the flow of information over the Internet," says Robert Boyd, a professor of optics and physics at the Institute of Optics and a research co-author. "If two data packets arrive at a switch at the same time, you need to store one until the other packet clears the switch, maybe 100 nanoseconds later. Our technique is aimed at … building buffers for high-speed telecommunications."
During the first phase of the project—which is part of the Defense Advanced Research Projects Agency's (DARPA) Defense Sciences Office slow-light program—Zhu says he learned that pulses could be stored and read out at a later time. The second phase was the actual experiment in which data pulses were stored (as acoustic waves in an optical fiber) and retrieved after a certain period of time.
"We really want to demonstrate that methods for storing optical information are much broader than people thought," Gauthier says. "In the current telecommunication systems, you turn the optical signal into an electronic signal and store it in RAM. The optical data pulses are then regenerated by using the electrical signals to turn on and off an auxiliary laser source. But this process generates heat. The faster this is done, the more heat is generated."
For this to work in the real world, the scientists say the communication fibers must be made of a material that provides an acoustic time frame long enough to allow the information to move from optical to sound, then return to optical. One option, Gauthier says, is to work with a new type of glass made from a chalcogenide, which has good semiconductor properties and contains one or more elements from the periodic table's chalcogenide group, also known as the "oxygen family," which includes oxygen, sulfur, selenium and tellurium.
Another option that researchers are exploring is to run the laser beams through a hollow optical fiber filled with gas (such as xenon), which would allow them to use a less powerful laser to induce longer lasting sound waves in the gas. This could potentially create a sound wave 50 times longer and allow the lasers used to be 100 times less powerful—and less energy intensive—thereby delivering more data more quickly at a lower cost.