A team from Corning Inc. (Corning, NY) has developed a duobinary format WDM system that can make use of half the theoretically available information-carrying potential of an optical fiber with a signal length much longer than has previously been realized. Transmission over 170 km with 20-GHz-spaced channels has been achieved, and researchers say they could get even higher spectral efficiency with better lasers. The Q measurement for the system was more than 17 dB and would be higher in a full demonstrator, say the researchers.
Duobinary format is a means of encoding signals that is more efficient than nonreturn to zero (NRZ) in terms of bandwidth and allows errors to be detected without the need for additional data bits. It is a three-level code (+1, 0, -1) where the value of each bit is determined not just by its own value, but the value of the bit that went immediately before it. It is efficient because only one extra bit is required per arbitrary string (pseudo-random bit sequence or PSBR). As a result, the larger the strings that can be transmitted, the more efficient the format becomes. Its error-correcting ability comes from the fact that the bits are correlated (not independent), so most errors will cause an inconsistency in the received code that can be detected easily.
In the Corning setup, the output of 16 lasers centered around 1554 nm are brought together and modulated by a Mach-Zehnder (interferometric) modulator (see figure).1 The output, in turn, is controlled by two erbium-doped fiber amplifiers that are receiving the data and an inverted copy of the data, respectively. The result is the transmission of the three-level signal through the system on all wavelength channels. The transmission takes place through two different fibers: 80 km of single-mode fiber—which has a larger core and higher chromatic dispersion, thus reducing nonlinearities—and 90 km of large effective area fiber (LEAF). The latter has a smaller effective core, thus increasing the gain of a Raman amplifier placed at the end of the optical link.
The fact that the spectral efficiency and signal length used in this system were so high, the researchers say, demonstrates that the duobinary format is applicable to commercial systems. Some crossphase modulation was evident in the experiment but was partly induced by the fact that all channels were carrying the same signal, which would not be true in a real network.
- K. S. Cheng et al., Elect. Lett. 36 (21), 1791 (Oct. 12, 2000).
Sunny Bains is a scientist and journalist based in London, England.