dispersion is caused by the wavelength dependence of group velocity in an anoptical
fiber. As a result different spectral components of the optical source propagates
with different delays and results in pulse envelope broadening.
Figure 3.7 Optical
pulse broadening due to the intramodal dispersion.
Figure 3.8 Optical frequency signal chirp due
to intramodal dispersion.
In case of
single mode fibers, it is possible to distinguish two dispersion components:
material and waveguide dispersion. Material dispersion is a wavelength
dependence of the fiber material refractive index. Waveguide dispersion depends on
fiber geometry and refractive index profile. Last component is referred to as
chromatic dispersion may be defined as:
The total pulse
broadening is given by:
where Dl is the spectral width of optical
broadening due to material dispersion occurs when the phase velocity varies
nonlinearly with the wavelength i. e. second differential of refractive index with
respect to wavelength is not equal zero. The pulse broadening caused by
material dispersion may be obtained from the group delay per a unit length:
delay is given by:
broadening may be obtained as the first term of the Taylor series expansion:
dispersion is caused by the wavelength dependence of the group velocity due to
specific fiber geometry. Single mode fibers exhibit a waveguide dispersion when d2b/dl≠0.
fiber dispersion is a combination of the material and waveguide dispersion. For
silica glass, the material dispersion is dominating and the zero dispersion value occurs
approximately at 1280 nm.
Figure 3.9 Dispersion characteristic of a
silica singlemode fiber.
dispersion wavelength can be shifted to a longer wavelength region by using
dopants or modifying the fiber geometry. Decreasing the core diameter or/and
fractional refractive index results in wavegide dispersion term increase.
characteristics of three principal single mode fiber types are illustrated at
fibure below. Dispersion shifted and dispersion flattened characteristics can
be obtained using specific refractive index profiles.
Figure 3.10 Dispersion
characteristics of dispersion non-shifted, dispersion shifted and dispersion
flattened single mode fibers.
Figure 3.11 Measured
refractive profiles for multimode graded index fiber, single mode dispersion
non-shifted, dispersion shifted and dispersion flattened fibers.
Figure 3.12 Comparison of dispersion
characteristics for different modern fibers.