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  • Dispersion Increases Fiber Capacity
Dec 2000
HOLMDEL, N.J. -- A researcher at Lucent Technologies Inc.'s Bell Labs has taken a working solution to a wireless communications problem and applied it to optical communication over multimode fiber. Although still in the early stages of research, the technique may significantly increase the amount of data that can be transmitted over multimode fiber in short-haul communications systems.
Several years ago, researchers at Bell Labs found that the capacity of a wireless transmission could be increased by using multiple antennas at both the transmitter and the receiver, said Howard R. Stuart, developer of the new technique. "Each antenna transmits independent data on the same carrier frequency. At the receiver, the signals arrive all mixed up; each antenna receives some energy from all of the transmitters. The received signal is decoded using signal processing." Most importantly, the capacity of the system is linearly dependent on the number of transmitting antennas.
Stuart saw an analogy between this concept and the modal dispersion of light in multimode fiber. In his optical system, multiple lasers and detectors are used. The wavelengths of the lasers are irrelevant, and there is no need for wavelength-multiplexing optics. "Each laser carries an independent bit stream," he said. "The system works because each laser couples to the fiber a bit differently. This difference in the coupling provides a means by which the data can be unraveled at the receiver by the signal processor."
One of the drivers behind this research is the need to increase capacity of high-speed Ethernet systems inside buildings. Multimode fiber is used in this application because it aligns easily, making it inexpensive to install. The problem is that the growing demand for bandwidth is pushing the limits of the systems, leaving system providers with the choice of either installing more fiber or making the existing fiber work harder.
"There is an extensive installed base of multimode fiber out there now," he said. "The technique I have developed provides a possible route to upgrading the capacity of this fiber."
Stuart, who described the technique in the July 14 issue of Science, has demonstrated its feasibility but has not tested its ability to expand a system. "What I have done is to couple two lasers to a single multimode fiber, put an independent data stream on each and show that I can separate the two data streams in the receiver using two detectors and a signal processor," he said.
It remains to be seen if the system can compete with wavelength division multiplexing (WDM). "At this point ... we don't really know how cheaply one could build a WDM system, and we don't know how complex the signal processor [for the new technique] will be, so we don't yet know ... which technique would be more cost-effective."

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