Standards bodies struggle with PMD specification
By WILLIAM B. GARDNER
The statistical nature of polarization mode dispersion (PMD) in singlemode fibers contributes significant uncertainty to the measurement process and complicates efforts to create PMD specifications. Most vendors indicate that the PMD in their cables does not exceed 0.5 psec/(km)1/2. A worst-case calculation with such a "specification" yields a maximum regenerator spacing of about 25 km for STM-256 (40-Gbit/sec) systems. This amount would imply an unacceptable power penalty at the 80-km distance that the International Telecommunication Union (ITU) will be considering for STM-256 systems at its Working Party 4/15 meeting in Geneva in April.
The problem here lies not in the cable, but in the worst-case assumption. It has been pointed out (see Lightwave, June 1997, page 74) that using a realistic statistical analysis can raise the PMD limit on distance between regenerators by more than an order of magnitude. The specter of STM-256 looming on the horizon lends urgency to reaching an agreement on a statistical approach for PMD.
The International Electrotechnical Commission (IEC) is balloting several such documents: "Statistical PMD Cable Specification" (IEC 60794-2) in IEC SC86A and "Guideline for the Calculation of PMD in Fiber Optic Systems" (IEC 61282-3) in IEC SC86C. "Method 1" in the latter document characterizes the distribution of cabled PMD with a metric called PMDQ, which is based on both the process mean and variability. PMD measurements of at least 100 cabled fibers are used in determining the value of this metric for a particular manufacturing process. "Method 2" in IEC 61282-3 shows how to predict, for a broad range of distributions, the probability of exceeding a given differential group delay (DGD) in links up to 400 km in length.
At the April meeting of ITU Working Party 4/15, the United States is proposing adding an appendix, "Information on PMD Statistics," to each of the ITU's singlemode-fiber Recommendations. This appendix, authored by Tom Hanson of Corning Inc., summarizes the ideas contained in IEC 61282-3.
Until a statistical specification gains general acceptance, maximum PMD coefficients are sometimes cited in industry documents. For example, the European standards organization EuroTelCab has circulated Draft EN 60794-3-11 for Family Specification "Single mode optical fiber cable for duct/direct buried installation." Section 3.2.7 in this document says that "the cable PMD coefficient shall be <0.5 psec/(km)1/2."
Efforts have also arisen in Europe to impose a maximum PMD coefficient of perhaps 0.2 or 0.3 psec/(km)1/2 on uncabled fiber. The latest proposals in IEC and ITU contain the following note: "An optional maximum PMD coefficient on uncabled fiber may be specified by cablers to support the primary requirement on cable PMDQ of 0.5 psec/(km)1/2, if it has been demonstrated for a particular cable construction." Objections to such uncabled fiber requirements are twofold: (1) no procedure has been sanctioned for mapping PMD between fibers and cables, and (2) the measured fiber PMD can depend on how the fiber is wound on its spool.
William B. Gardner represents Lucent Technologies, Norcross, GA, on sev eral fiber standards committees. He can be contacted at tel: (770) 798-2674; fax: (770) 798-4654; e-mail: firstname.lastname@example.org.