Optical System Simulates PMD
Daniel S. Burgess
Polarization mode dispersion (PMD) in legacy fiber is one of the greatest obstacles to upgrading telecommunications networks from 2.5 Gb/s to 10 and 40 Gb/s. Although means are emerging to compensate for polarization mode dispersion, it has been difficult to adequately simulate the effects of the phenomenon in order to test these systems. Now, researchers at JDS Uniphase Corp. in San Jose, Calif., have developed an optical system that generates a mixture of first-, second- and higher-order polarization mode dispersion in nearly the same distributions observed in fiber links.
An optical system of alternating linear retarders and birefringent crystals generates a mixture of first-, second- and higher-order polarization mode dispersion in a nearly Maxwellian distribution for testing telecommunications components. A proprietary athermalization technique ensures that the system reproduces the desired differential group delay within 1 ps for typical changes in temperature. Courtesy of JDS Uniphase Corp.
Polarization mode dispersion is the result of the inherent asymmetry in the core of optical fiber, attributable to both the manufacturing process and to the mechanical stresses on the fiber caused, for example, by cabling, by vibration or by changes in temperature. First-order polarization mode dispersion describes how the polarization of an input pulse splits into two principal states of polarization that travel at different speeds through the fiber, resulting in an effect that Russell Chipman, a researcher on the project, likened to an echo. The principal states of polarization in second-order polarization dispersion vary with frequency over the spectral bandwidth of the pulse, also leading to differential group delay and an increased bit error rate.
Chipman and his associate, Ravinderkumar Kinnera, devised a system of alternating linear retarders and birefringent crystals. A series of motors rotate the retarders, changing the coupling of the crystals and thus enabling the system to generate the desired static and dynamic polarization mode dispersion. The emulator for 10 Gb/s produces first-order polarization mode dispersion with a differential group delay of between 0 and 120 ps, second-order between 0 and 3000 ps2, and an appropriate mix of higher-order polarization mode dispersion. The emulator can generate reproducible, well-calibrated high-order settings and can produce time-varying distributions that model months of thermal variation in a few minutes to a few hours.
"The more stages that are used, the closer the emulator's PMD statistics approach the Maxwellian statistics of a fiber link," Chipman said. "Twelve stages is a good compromise between the statistics and the complexity of the emulator."
A serious problem with such emulators has been their sensitivity to temperature fluctuations as small as less than 0.1 °C, the result of the temperature dependence of the retardance in the hundreds of millimeters of birefringent crystal. The researchers report that they have developed a proprietary athermalization technique that ensures the system produces reproducible differential group delay and high-order polarization mode dispersion across typical changes in temperature.
Anyone hoping to add such a system to the lab will have to wait, however. "Due to the level of complexity and customization, the high-order PMD emulator is not a catalog item at this time -- but it is available on a special-order basis," Chipman said.
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