France sees future in solitons
By STEPHEN HARDY
French researchers are investigating the use of solitons--short light pulses potentially capable of propagating the entire length of an optical fiber without distortion--for future use in communications systems. Laboratory research indicates that the low chromatic dispersion of solitons could benefit both long-haul and comparatively short-run applications.
For example, scientists at the Burgundy Laboratory of Computing, Electronics, Signals, and Images in Dijon, France, have created a mechanical model that simulates soliton propagation (see photo). The simulator replicates the optical environment through a line of small mechanical cells, each consisting of a pendulum. A wave propagates through the simulator, reflects off the back wall, and returns--completely unaltered by interference, say researchers quoted by the French Technology Press Office.
Meanwhile, France Telecom`s National Center for Telecommunications Studies (cnet) also is looking into soliton applications. Part of this research is being conducted with Pirelli Cavi spa in Milan, Italy, the prime contractor for the European esther project. The cnet soliton facility in Lannion, Brittany, has established a 500-mi link in its laboratory to test soliton use. The link features an amplification interval of 60 mi and attenuation of 21 to 22 dB. For the sake of simplicity, no online soliton control devices reside in the network, which will soon expand to 600 mi, according to researchers.
An integrating source on an indium phosphide substrate, two electroabsorption modulators, and an amplifier generate soliton pulses for the network. Powered by a sinusoidal voltage, this combination emits a train of 10-psec pulses at a frequency of 20 GHz. The pulses are coded at 20 Gbits/sec for the addition of data using an electro-optical modulator. The lab uses polarization time-division multiplexing to simulate a 40-Gbit/sec train with 25 psec between pulses.
The laboratory network uses erbium-doped fiber amplifiers and singlemode fiber with stepped dispersion. The fiber retains a small amount of the light`s chromatic dispersion and thus further reduces distortion, say researchers. However, amplification noise remains a problem, resulting in time jitter. Both sliding frequency filtering and inline modulation have been used to combat such jitter. The inline modulation technique, which uses fixed optical filters and an optical modulator to produce the equivalent of an optical regeneration, has resulted in error-free transmission at ranges to 600,000 mi, according to the press office. q
PULL QUOTE
"Low chromatic dispersion of solitons could benefit both long-haul and comparatively short-run applications."