Page 215 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

Optical Fibers and Optical FIber Amplifiers 209

modulation rate of 2.5 GHz, the pulse width is four times longer and,
at the same time the modulation broadening is four times smaller,
so the dispersion problem becomes practically negligible.

Material dispersion (also called chromatic dispersion) is not the
only source of dispersion. There is also structural dispersion that de-
pends on the geometry of the fiber. The geometry of the fiber and the
index contrast are linked through the fiber V number, as we have
seen earlier. The very interesting feature of the structural dispersion
is that it depends on the wavelength in the opposite way from materi-
al dispersion. That is,
Total dispersion = Material dispersion – Structural dispersion (9.20)

This has the important implication that structural dispersion can be
used to compensate for material dispersion. In real optical fiber sys-
tems, lengths of special fiber, with a structure designed for just this
purpose, are spliced in periodically to correct for material dispersion.
Material dispersion and structural dispersion combine to form the
static dispersion of the fiber. This dispersion is built-in when the fiber
is drawn and does not change with time. The structural dispersion
can be represented as
d n
n
S( ) = A V = A NA (9.21)
2
c
c
where A is a constant of proportionality.
The effect of the structural dispersion in a conventional fiber is to
shift the zero dispersion point from 1280 nm toward longer wave-
lengths. Popular high-performance, single-mode fiber made by Alcatel
©
©
(Teralight ) and Corning (LEAF ) use this trick to shift the zero dis-
persion all the way to 1550 nm, thereby achieving zero dispersion and
minimum loss at the same wavelength.
More complicated designs involving an intermediate cladding layer
are used to flatten the dispersion over a range of wavelengths. This
design is shown schematically in Fig. 9.11. Design rules for achieving
a specific wavelength spectrum of dispersion are given by Jeunhomme
(see Bibliography).
The engineering control over the optical propagation properties of
optical fibers is a key technology in the optical fiber telecommunica-
tion business. The dispersion characteristics that we have discussed
are summarized in Fig. 9.12
In addition to the dispersion that is built into the fiber, there is a
second kind of dispersion, called polarization-mode dispersion. The
calculation of the transmission modes of a fiber assumes that the core
and the cladding have perfect circular symmetry. The lowest-order

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.