Atec EXFO-FTB-5500 Series - Manual (Page 2)

PMD Affects System Performance
Dispersion phenomenon
Polarization mode dispersion (PMD) affects system performance by spreading the pulse over the length of a fiber. Pulse spreading can be compared
to power loss in the system, because it directly affects the loss safety margin.
PMD is caused by asymmetry in the fiber’s geometry and/or by
localized stress throughout the waveguide. Birefringence in the
optical fiber, which causes light to be propagated along two polar-
ization axes that are orthogonal to each other, produces slow
and fast axes. The difference in propagation speed results in tem-
poral spreading or dispersion as shown in the figure to the right.
With a digital signal, PMD increases the BER, which, limits sys-
tem bandwidth. With an analog signal, PMD creates distortion,
limiting the number of channels.
Network planners quickly realized the danger of this situation: an entire
bandwidth upgrade could be jeopardized by PMD. Consequently, plenty
of testing was carried out when the industry recognized the importance
of characterizing PMD in existing networks.
The introduction of the interferometric method subsequently changed
the reality of PMD measurement. The interferometric method, which was
introduced by the GAP Optique team from the University of Geneva,
quickly demonstrated its usefulness as a fast and secure way of meas-
uring PMD, regardless of environmental conditions.
The interferometric method allows field measurement of PMD; this is not
possible with conventional methods such as JME or wavelength scan-
ning. The interferometric method is faster and almost immune to vibra-
tions and adverse field conditions.
With the marketing of OC-192 (STM-64) and the advent of OC-768
(STM-256), fiber and cable manufacturers eventually acknowledged the
importance of PMD and began guaranteeing specifications.
In light of these developments, however, companies who specialize in
cable installation, maintenance, and troubleshooting still need to be con-
cerned about PMD for a number of reasons:
• PMD is statistical in nature.
• Large variation in PMD values can be observed during the life
cycle of a fiber.
• Additional stress can be applied to a fiber during transportation
and cabling.
• Effects of temperature and vibration are still regarded as
potential problems by operators.
• Cable can be crushed, stressed, or physically damaged
during installation.
• Cable handling causes variations in PMD.
The TIA standard
The inteferometric method is the fastest PMD measurement method. It
was approved by the TIA in April 1999 (EIA/TIA-455-124). The test
method documentation demonstrates the validity of the method and its
limitation in accuracy for PMD values smaller than 0.12 ps using a stan-
dard LED source at 1550 nm. With this method, no end-to-end commu-
nication is required. This allows for less equipment and human interven-
tion as well as faster, more efficient data collection.
A major concern for network planners