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Chromatic Dispersion
Chromatic, or intramodal, dispersion occurs in both single mode and multimode optical
fibers. Chromatic dispersion occurs because different colors of light travel through the
fiber at different speeds. Since the different colors of light have different velocities,
some colors arrive at the fiber end before others. This delay difference is called the
differential group delay τ(λ) per unit length. This differential group
delay leads to pulse broadening.
Chromatic dispersion is measured using EIA/TIA-455-168 in the time domain. Chromatic
dispersion is also measured in the frequency domain using EIA/TIA-455-169 and
EIA/TIA-455-175. These methods measure the composite optical fiber material and waveguide
dispersion. To understand the contribution that material and waveguide dispersive
mechanisms have on multimode and single mode fiber dispersion, refer to chapter 2. In this
chapter we limit the discussion on chromatic dispersion to the time domain method
described in EIA/TIA-455-168.
The chromatic dispersion of multimode graded-index and single mode fiber is obtained by
measuring fiber group delays in the time domain. These measurements are made using
multiwavelength sources or multiple sources of different wavelengths. A multiwavelength
source could be a wavelength-selectable laser.
The pulse delay for both a long test sample fiber and a short reference fiber are
measured over a range of wavelengths. The pulse delay for the reference fiber as a
function of wavelength is &tgr;in(λ). The pulse delay for the
test fiber as a function of wavelength is &tgr; out(λ). The
group delay &tgr;(λ). per unit length at each wavelength is
where Ls is the test sample fiber length in
kilometers (km) and Lref is the reference sample length in km.
The fiber chromatic dispersion is defined as the derivative, or slope, of the fiber
group delay curve with respect to wavelength. Generally, the group delay as a function of
wavelength is fit to a simple mathematical function and the derivative calculated. The
range of wavelengths over which meaningful data is obtained depends on the wavelength
range of optical source(s) used. The zero-dispersion wavelength (λ0)
and the zero-dispersion slope (S0) are determined from the chromatic dispersion
curve.
Q.12 Delay differences between the source wavelengths occur as the optical signal
propagates along the fiber. What is this called?
Q.13 What determines the range of wavelengths over which meaningful data is obtained for
calculating the chromatic dispersion?
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