Photonic System Wins $4.3M

Electrical engineers at the University of Southern California (USC) have received $4.3 million to design a photonic communications system that makes data as easy to transmit by light as it is by silicon.

Alan Willner and Robert Hellwarth, professors in the Viterbi School of Engineering's Ming Hsieh Department of Electrical Engineering, received the funds from DARPA to develop continuously tunable optical delays which they hope will change the rules of manipulating photonic data at ultrahigh speeds.

"The technical community is still missing a simple way to tune the time delay of one photonic data stream relative to another, which is a key building block for many types of data-processing functions," said Willner. "We think we've found it."

According to Willner, optical fibers can carry enormous volumes of information coded in photonic form, with far greater bandwidths than metal electrical cables. Still, "photonics usually can't compete with electronics when it comes to processing data," he said, "because silicon transistors are extremely cheap and can perform processing operations that have long been very difficult to do with light. With electronic systems, it is easy to temporarily store information."

"Therefore we are left with a significant mismatch -- data transmission is performed optically but data processing is done electronically. Normally, transmitted photonic data would need to be turned into electronic data for processing and then turned back into photonic data for further transmission," he said.

If such photonic-electronic conversions can be avoided, great savings in expense and energy are possible, Willner said.

For example: numerous low-speed branch data streams can feed into larger trunk lines. While the trunk cable has more than sufficient capacity to carry all the information, before it can do so, the information has to be multiplexed, which means collating the data bits and packets in such a way that different channels don’t interfere with each other within the same time slot.

To do this now, the laser-coded information has to be converted into electronic form, multiplexed, and then reconverted to laser pulses. "This is energy inefficient, cumbersome, and takes away system capacity," Willner said.

But the new optical system he and Hellwarth are building will provide another way, in which data can stay photonic and pass through at extremely high speed, he said.

Their technique is to convert the photonic information from one color to another one, and then pass the data through an element that has a speed of light dependent on the color of the light -- i.e., red photons could travel slower than blue photons. Each photonic data stream is given its own "color," or delay value, and then seamlessly woven together and sent on their common way without ever going through an electronic interface.

Willner has already succeeded in efficiently delaying a single 80-Gb/s data stream and multiplexing two separate 40-Gb/s data streams. His plans call for upping capacity to the hundreds-of-Gb/s range.

In their proposal to DARPA, the engineers said a tunable system could allow "accurate synchronization for bit-level interleaving, time-slot interchange, multiplexing and demultiplexing, time switching, and data packet synchronization."

The system that the USC team proposes comes out of recent independent research by Willner and others in creating systems that use new techniques to slow light. For all these applications, a high degree of delay control is needed in the ability to continuously tune the timing of the high-speed-data flow.

The design goal is a system that can tunably slow light from 0 up to 5 microseconds -- that is, a slowed stream can arrive up to 5 microseconds later than an untreated one; the 5-microsecond delay would require a roughly fiftyfold improvement on their current published value of 100 nanoseconds. To put this in perspective, a delay of 5 microseconds for a 500-Gb/s data stream is the same as delaying a data stream by millions of bits, an amount that could open up many possible applications.

Willner, past president of the Institute of Electrical and Electronic Engineers' (IEEE) Lasers and Electro-Optics Society (LEOS), has published a series of papers in the last two years describing steady improvements in both the amount of slowing achieved and control over the process.

Key to the proposed tunable system is extremely sophisticated laser technology. Hellwarth, a member of both the National Academy of Sciences and the National Academy of Engineering, is internationally recognized for his photonics work, including his invention of giant pulse lasers and his studies of the effects of lasers upon materials.

For more information, visit: http://viterbi.usc.edu