NZD fiber undergoes international scrutiny
WILLIAM B. GARDNER
Nonlinear optical effects have led to the introduction of non-zero dispersion (NZD) fiber (see Lightwave, December 1994, page 1, and November 1995, page 40). As optical power levels and unregenerated distances are increased in wavelength-division multiplexed (WDM) networks, optical lightwave degradation effects such as four-wave mixing can impair system operation.
For example, four-wave mixing can cause the various wavelengths present in WDM networks to interact with each other. The very low chromatic dispersion present in conventional dispersion-shifted fiber ensures that the wavelength phase relationships remain constant along the fiber. However, this characteristic results in the steady growth of nonlinear component effects.
On the other hand, NZD fiber introduces a small but significant amount of dispersion (typically a few ps/nm-km). This dispersion causes the wavelength phase relationships to constantly change, suppressing the growth of nonlinear component effects.
This year, the Telecommunications Industry Association (TIA), International Electrotechnical Commission (IEC) and International Telecommunication Union (ITU) have all made important progress on the standardization of NZD fiber. Sectional and Blank Detail Specifications for NZD fiber have been successfully balloted by TIA Subcommittee FO-6.6 on optical fibers and materials. These documents are now undergoing Technical Standards Subcommittee review--the last stage of the TIA approval process.
Committee Document 86A/370/CD was circulated in the IEC last September for comments, a prelude to balloting there. The IEC has designated the NZD fiber type as "Category B4."
At its June 1996 meeting in Geneva, ITU Study Group 15 "determined" the new Recommendation G.655 on NZD fiber. This recommendation was submitted to the World Telecommunication Standardization Conference last October for final approval. The increasing demand for NZD fiber in the marketplace has prompted the accelerated procedure.
Whereas G.655 is strictly a fiber and cable recommendation, NZD fiber is also working its way into the application codes of ITU system Recommendations G.scs (optical interfaces for single-channel SDH [synchronous digital hierarchy] systems with optical amplifiers, and STM-64 systems) and G.mcs (optical interfaces for multichannel systems with optical amplifiers). These application codes offer long-distance terrestrial system design parameters for different fiber types, wavelengths and bit rates.
These standard NZD documents recognize that either positive or negative chromatic dispersion can be used to suppress four-wave mixing. Although the sign of the dispersion must not change over the wavelength range of intended use within a given fiber, fibers of opposite dispersion sign can be spliced together. Where enough residual dispersion exists to limit system performance, a number of compensation schemes that balance positive and negative dispersion are available to system designers.
The Italian delegation to the ITU expressed concern about the unintentional variation in dispersion along the length of NZD fibers. If the dispersion assumes a low enough value over long distances, four-wave mixing could become excessive. Dispersion uniformity is not easily measured in optical fibers, but new noninvasive mapping techniques were reported at the National Institute of Standards and Technology Symposium on Optical Fiber Measurements, held recently in Boulder, CO.
The solution proposed by the Italian delegation was to raise the minimum allowable dispersion from 0.1 to 0.5 ps/nm-km. However, ITU Working Party 4/15 decided that the necessity for this change has not been sufficiently demonstrated. q
William B. Gardner bio and mug (see Sept. 96, page 36 for the last time it appeared)
"Positive or negative chromatic dispersion can be used to suppress four-wave mixing."
William B. Gardner represents Lucent Technologies, Norcross, GA, on several fiber standards committees. He received a B.S. from the University of Alabama and a Ph.D. from Johns Hopkins University, both in physics.