Scientists led by Chalmers University of Technology professor Peter Andrekson have achieved a 40-Gb/s soliton pulse transmission over a 400-km distance on commercial dispersion-shifted fiber optic cable. The researchers describe their success as the highest-capacity field transmission experiment ever reported at a single-wavelength bit rate beyond 20 Gb/s. Soliton pulses are very short pulses of light that show the peculiar characteristic of maintaining their pulse width in the presence of chromatic dispersion as they propagate in optical fibers. Solitons, and any encoded information they carry, have a very strong resistance to interference such as polarization-mode dispersion. "Polarization dispersion makes an initially polarized pulse fade off in time, which causes a problem for high-speed communications that is difficult, if not impossible, to compensate for," said Andrekson. "Solitons have a real tendency to hold together in the presence of polarization dispersion. That was a key finding of the experiment that was not part of its objective." He added that this is the first time that this robust nature of soliton pulses has been observed experimentally in a real transmission system. Researchers anticipate that the aggregate bit rate could be increased through wavelength division multiplexing techniques. Andrekson pointed out that soliton-based bit rates higher than 40 Gb/s have been achieved, but only under controlled lab conditions. "For instance, lab experiments often use in-line control such as sliding guiding filters, which improve signal-to-noise ratio and reduce timing jitter, but may have problems in a real commercial system," he said. The 400-km system over which Andrekson's group performed the experiment incorporated no in-line soliton control and used erbium-doped amplifiers at intervals of 57 km. The transmission penalty was as small as 0.5 dB with an overall polarization-dependent sensitivity of ±0.3 dB caused by the relatively high polarization-mode dispersion of 0.3 ps/km in the dispersion-shifted fiber. The solitons were 10 ps wide, and the chromatic dispersion was 0.2 ps/km nm. The experiment was performed in collaboration with the Swedish network operator Telia.