Page 263 - Optical Communications Essentials

Performance Impairments

Performance Impairments 253

Now one can make some application tradeoffs. Suppose one selects a compensa-
tion value of 500ps/nm. Then, for example, since for G.655 fiber the total disper-
sion that arises over a 100-km length is about 500ps/nm, in order to use a 10-cm
chirped Bragg grating to compensate for this dispersion, the gratings must be
used over a very narrow bandwidth of less than 1nm. That means the short grat-
ings made prior to 2000 only could be used to compensate for individual wave-
lengths in a WDM system.
In 2001, 3M Corporation developed a manufacturing technique that can make
fiber gratings several meters long. For example, now it is possible to fabricate a
chirped fiber Bragg grating that is about 2m long, has a bandwidth of over
2
30nm, an insertion loss of about 1dB, and a delay slope of 1.1ps/nm . These
types of chirped Bragg gratings are multichannel components that are capable
of compensating for chromatic dispersion across the entire C-band or L-band in
individual fibers.

15.4. Polarization Mode Dispersion (PMD)
Polarization mode dispersion (PMD) results from the fact that light signal energy
at a given wavelength in a single-mode fiber actually occupies two orthogonal
polarization states or modes. Figure 4.10 shows this condition. At the start of
the fiber the two polarization states are aligned. However, fiber material is not
perfectly uniform throughout its length. In particular, the refractive index is
not perfectly uniform across any given cross-sectional area, which is known as
the birefringence of the material. Consequently each polarization mode will
encounter a slightly different refractive index, so that each will travel at a
slightly different velocity, and the polarization orientation will rotate with dis-
tance. The resulting difference in propagation times between the two orthog-
onal polarization modes will result in pulse spreading. This is the basis of
polarization mode dispersion. PMD is not a fixed quantity but fluctuates with
time due to factors such as temperature variations and stress changes on the
fiber. Since these external stresses vary slowly with time, the resulting PMD
also fluctuates slowly. It varies as the square root of distance and thus is speci-
fied as a maximum anticipated value in units of ps/ k m .
A typical PMD value for a fiber is D PMD 0.05ps/ k m , but the cabling
process can increase this value. The PMD value does not fluctuate widely for
cables that are enclosed in underground ducts or in buildings. However it can
increase periodically to over 1 ps/ k m for outside cables that are suspended on
poles, since such cables are subject to wide variations in temperature,
wind-induced stresses, and elongations caused by ice loading.
Pulse spreading ∆t PMD resulting from polarization mode dispersion is given by
∆t PMD D PMD fi b er l en g th (15.4)

As an example, consider a 100-km-long fiber for which D PMD 0.5ps/ k m .
Then the pulse spread over this distance is ∆t PMD 5.0ps. Suppose one wants

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