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Principal Sponsors:
 | Dense Wavelength Division Multiplexing (DWDM) Testing |
8. Characterizing Fiber for DWDM Applications
Theory predicts—and field experience confirms—that the characteristics of the fiber itself can have significant impact on the performance of DWDM networks and that the particular characteristics that are most important are not necessarily those of greatest concern in conventional single-wavelength links.
Chromatic Dispersion
Chromatic dispersion, the variation of the index of refraction of the fiber with wavelength, can be a critical determinant of system performance in DWDM systems, especially those that use a judiciously selected amount of dispersion to minimize certain undesirable nonlinear effects in the fiber itself. Its value is determined during fiber manufacture, however, and few situations have arisen in which it is necessary to verify this value in the field.
As DWDM systems are operated ever closer to their limits, however, a need is likely to emerge to verify that this parameter is adequately controlled at every point in the optical path. The eventual development of field instrumentation to measure chromatic dispersion is likely, especially if the management of chromatic dispersion on installed fiber turns out to be more complex than expected.
Polarization Mode Dispersion
Polarization mode dispersion (PMD), in which various polarization states of the optical signal propagate at different velocities, is especially difficult to deal with. Its effects prevent many present-day optical systems from using high-bandwidth transmission equipment meeting 10–Gbps, OC–192 specifications. As current state-of-the-art DWDM technology offers eight such OC–192 channels, where the fiber can support the rate, PMD can be a serious limitation to system performance and to prospects for upgrading that performance.
PMD affects the transmission quality by spreading signal pulses and, therefore, raising the bit error rate (BER) of the system. It arises in the first place because of asymmetries in the fiber itself, so the primary remedy must be applied at the manufacturing level. But the damage does not necessarily end there. During installation, the fiber can be crushed, kinked, or otherwise overstressed. Environmental and climatic changes can also affect its circular geometry and thus worsen its PMD characteristics. Post-installation testing may be needed to ensure that a network does not overly suffer from PMD and that the installed facilities can be upgraded to support tomorrow's higher bit rates (see Figure 12).
Second-order PMD, the variation of polarization mode dispersion with wavelength, is considered to have a negligible effect on network performance. However, it acts as a completely random contribution to the network's chromatic dispersion, possibly negating deliberate steps taken in network design to provide the exact amount of this dispersion to reduce nonlinear disturbances in signal propagation. Although this parameter bears watching, its long-term importance cannot yet be predicted.
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