Page 67 - Optical Communications Essentials

Optical Fibers

Optical Fibers 57

Figure 4.9. Chromatic dispersion as a function of wavelength in various
spectral bands for several different fiber types.

Initial
polarization state

Varying birefringence
along the fiber

∆

Figure 4.10. Variation in the polarization states of an optical pulse as it
passes through a fiber that has varying birefringence along its length.

with time due to factors such as temperature variations and stress changes on
the fiber. It varies as the square root of distance and thus is specified as a mean
value in units of ps/ km . A typical value is D PMD 0.05 ps/ km .

Dispersion Calculation If t mod , t CD , and t PMD are the modal, chromatic, and polar-
ization mode dispersion times, respectively, then the total dispersion t T can be calcu-
lated by the relationship
t ( t mod ) 2
( t ) 2
( t PMD ) 2
CD
T
Note that t mod 0 for single-mode fibers. As a rule of thumb, the information-carrying
capacity over a certain length of fiber then is determined by specifying that the pulse
spreading not be more than 10 percent of the pulse width at a designated data rate.

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