Plans are under way for PMD calibration

Plans are under way for PMD calibration

By Willilam b. gardner

The statistical nature of polarization-mode dispersion (PMD) makes the accurate measurement of PMD difficult (see Lightwave, September 1996, p. 36). The measured value is affected by random coupling between polarization states in the fiber. Because this coupling fluctuates in time due to fiber flexing, temperature changes, etc., successive measurements of PMD on the same fiber can differ by 20%--or more, if the PMD value is low. National and international standards bodies have approved four different test methods for PMD: Jones Matrix Eigenanalysis (JME), Poincare Sphere (PS), Interferometer (INT), and Fixed Analyzer (FA--also known as Wavelength Scanning). International standards bodies deal with multiple test methods by selecting one as the "Reference Test Method" (RTM). This method is used to resolve any disagreements that may arise from the use of multiple test methods. Because JME and PS are the only methods that measure the differential group delay between the principal states of polarization, the United States has supported them as the best candidates for the RTM.

At this month`s meeting of the International Telecommunication Union`s Study Group 15 in Geneva, the United States will propose consolidations of JME and PS into a single test method. Both JME and PS involve measuring the Stokes Parameters in the test fiber, and JME and PS then become two options for extracting the PMD from the Stokes parameters.

At the June 1998 meeting of the Telecommunications Industry Association`s (TIA`s) Working Group FO-6.6.1 on Round Robin Testing, Paul Williams of the National Institute of Standards and Technology (NIST) in Boulder, CO, described NIST`s plans for PMD calibration support. Williams has pioneered the use of a stack of birefringent quartz waveplates as an artifact for PMD calibration. The plates are cemented together with random orientation of their axes, thus creating mode coupling that helps to simulate the PMD behavior of long fibers. Pigtailed stacks of this type have been used successfully in TIA/NIST round robins and will soon be sold as artifact SRM 2518 in NIST`s Standard Reference Material (SRM) program.

Williams has demonstrated that the waveplate stack`s PMD can be measured to an uncertainty of about 1% using the JME/PS method. This repeatability is due to the fixed waveplate orientation of the stack and the insensitivity of the JME/PS technique to slight birefringences in the fiber leads. Measurements using the INT and FA techniques are much more susceptible to lead birefringence, which significantly increases measurement uncertainty. Averaging over random polarization launches reduces this uncertainty but also complicates the measurement.

For those wishing to calibrate an INT or FA test set, NIST will negotiate to measure customer-supplied non-mode-coupled artifacts on a case-by-case basis. The artifact will be returned with a certificate known as a "Report of Special Test" stating the calibration conditions and the value of PMD measured by NIST.

The appropriate artifact for a non-fiber device (i.gif., non-mode coupled) would be a single quartz plate. If there is sufficient interest, NIST may offer a packaged (pigtailed) quartz plate SRM, certified for PMD measurements over a broad wavelength range and applicable to all of the PMD measurement techniques. q

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: [email protected].