22-09-2012, 12:24 PM
Chromatic Dispersion
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Abstract:
All forms of dispersion degrade a lightwave signal, reducing the datacarrying
capacity through pulse-broadening. Chromatic dispersion results from a
variation in propagation delay with wavelength, and is affected by fiber materials
and dimensions.
INTRODUCTION
Chromatic dispersion is a broadening of the input signal as it travels down the length of
the fiber. The concept to consider when talking about chromatic dispersion (CD) should
be optical phase. It is important to mention optical phase before any explanations of CD
or group delay because of their mathematical relationship. Group delay is defined as the
first derivative of optical phase with respect to optical frequency. Chromatic dispersion
is the second derivative of optical phase with respect to optical frequency.
Both of these phenomena occur because all optical signals have a finite spectral width,
and different spectral components will propagate at different speeds along the length of
the fiber. One cause of this velocity difference is that the index of refraction of the fiber
core is different for different wavelengths. This is called material dispersion and it is the
dominant source of chromatic dispersion in single-mode fibers. Another cause of
dispersion is that the cross-sectional distribution of light within the fiber also changes for
different wavelengths. Shorter wavelengths are more completely confined to the fiber
core, while a larger portion of the optical power at longer wavelengths propagates in the
cladding. Since the index of the core is greater than the index of the cladding, this
difference in spatial distribution causes a change in propagation velocity. This
phenomenon, illustrated in Figure 2, is known as waveguide dispersion. Waveguide
dispersion is relatively small compared to material dispersion.