Specifications

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Cisco ONS 15530 Planning Guide
OL-7708-01
Chapter 4 Optical Loss Budgets
Fiber Plant Testing
ORL
ORL is a measure of the total fraction of light reflected by the system. Splices, reflections created at
optical connectors, and components can adversely affect the behavior of laser transmitters, and they all
must be kept to a minimum of 24 dB or less. You can use either an OTDR or an LTS equipped with an
ORL meter for ORL measurements. However, an ORL meter yields more accurate results.
PMD
PMD has essentially the same effect on the system performance as chromatic dispersion, which causes
errors due to the “smearing” of the optical signal. However, PMD has a different origin from chromatic
dispersion. PMD occurs when different polarization states propagate through the fiber at slightly
different velocities.
PMD is defined as the time-averaged DGD (differential group delay) at the optical signal wavelength.
The causes are fiber core eccentricity, ellipticity, and stresses introduced during the manufacturing
process. PMD is a problem for higher bit rates (10 GE and above) and can become a limiting factor when
designing optical links.
The time-variant nature of dispersion makes it more difficult to compensate for PMD effects than for
chromatic dispersion. “Older” (deployed) fiber may have significant PMD—many times higher than the
0.5 ps/Ð km specification seen on most new fiber. Accurate measurements of PMD are very important
to guarantee operation at 10 Gbps. Portable PMD measuring instruments have recently become an
essential part of a comprehensive suite of tests for new and installed fiber. Because many fibers in a cable
are typically measured for PMD, instruments with fast measurement times are highly desirable.
Chromatic Dispersion
Chromatic dispersion testing is performed to verify that measurements meet your dispersion budget.
Chromatic dispersion is the most common form of dispersion found in single-mode fiber. Temporal in
nature, chromatic dispersion is related only to the wavelength of the optical signal. For a given fiber type
and wavelength, the spectral line width of the transmitter and its bit rate determine the chromatic
dispersion tolerance of a system. Dispersion management is of particular concern for high bit rates
(10 Gbps) using conventional single-mode fiber. Depending on the design of the 10-Gbps transceiver
module, dispersion compensation might be needed to accommodate an upgrade from GE to 10 GE in
order to keep the same targeted distances.
Portable chromatic dispersion measurement instruments are essential for testing the chromatic
dispersion characteristics of installed fiber.
Fiber Requirements for 10-Gbps Transmission
Do not deploy 10-Gbps wavelengths, or even 2.5-Gbps wavelengths, over G.653 fiber. This type of fiber
causes enormous amounts of nonlinear effects.