Page 331 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
19

Test and Measurement

The installation and powering up of an optical fiber communication system
requires measurement techniques for verifying the link has been configured prop-
erly and that its constituent components are functioning correctly. Of particular
importance are accurate and precise measurements of the optical fiber, since this
component cannot be replaced readily once it has been installed. In addition, vari-
ous test methods are needed for continually monitoring the link condition to ver-
ify that the performance requirements are being met during operation.
During the link design phase an engineer can find the operational parameters
of many components on vendor data sheets. These include fixed parameters for
fibers (e.g., core and cladding diameters, refractive index profile, mode-field
diameter, cutoff wavelength); passive splitters, connectors, and couplers; and
electrooptic components such as sources, photodetectors, and optical amplifiers.
Once these parameters are known, there is no need to measure them again.
However, the attenuation and dispersion of a fiber can change during fiber
cabling and cable installation. In single-mode fibers, chromatic and polarization
mode dispersions are important factors that can limit the transmission distance
or data rate. Chromatic dispersion effects are of particular importance in high-
speed WDM links, and polarization mode dispersion ultimately can limit the
highest achievable data rate in single-mode links. Measurement procedures for
these parameters thus are of interest to the user, as are methods for locating
breaks and faults in optical fiber cables.
When a link is being installed and tested, the operational parameters of inter-
est include the bit error rate, timing jitter, and signal-to-noise ratio as indicated
by the eye pattern. During actual operation, measurements are needed for
maintenance and monitoring functions to determine factors such as fault loca-
tions in fibers and the status of remotely located optical amplifiers and other
active devices.
This chapter discusses measurements and performance tests of interest to
installers and operators of fiber optic links and networks. Of particular interest

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