IEC addresses dispersion slope compensating fiber

At 10-Gbit/sec transmission rates and beyond, most optical transmission systems require dispersion compensation to offset the cumulative chromatic dispersion of the transmission fiber. This compensation may consist of dispersion-compensating modules, which contain several kilometers of dispersion-compensating fiber (DCF) packaged in a small-diameter coil. DCF modules are usually located at repeater stations.

Intuitively, the configuration of up to 17 km of DCF packaged in a module having up to 9 dB of attenuation is inefficient. The several kilometers of DCF are not used to transport the signal over an equivalent distance, and the module's attenuation must be overcome by optical amplification, thereby increasing cost and adding noise. A more elegant solution is to use a fiber in a cable that is periodically spliced to the transmission fiber and compensates dispersion while simultaneously providing transmission. Fibers are already commercially available for accomplishing that.

In October, the International Electrotechnical Commission SC86A/WG1, chaired by Allen Cherin of OFS, considered creating a new Category B5 for a dispersion slope compensating fiber to be used with B1 dispersion-unshifted singlemode fibers. Since the dispersion of B1 fibers is positive, the dispersion of B5 fibers must be negative. Additionally, since the dispersion slope (a measure of how a fiber's dispersion varies with wavelength) of B1 fibers is positive, the dispersion slope of B5 fibers needs to be negative, if it is to compensate for dispersion over a broad wavelength range.

To this end, the standards proposal defines a new term, relative dispersion slope (RDS), which is the ratio of dispersion slope at 1550 nm to dispersion coefficient at 1550 nm. Ideally, the RDS of the B5 fiber should be identical to the RDS of the B1 fiber with which it is being used.

The proposal will be refined and discussed again at the next IEC meeting in April.

James J. Refi represents OFS (Norcross, GA) in the Telecommunications Industry Association, where he chairs FO-4.6.1, and the International Electrotechnical Commission, where he is secretary of SC86A/WG1. He can be reached at

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