3.3 Intramodal (chromatic) dispersion


Chromatic 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 profile dispersion.


The fiber chromatic dispersion may be defined as:



The total pulse broadening is given by:



where Dl is the spectral width of optical source.


Pulse 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:



The pulse delay is given by:



The pulse broadening may be obtained as the first term of the Taylor series expansion:



Waveguide 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.


The total 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.


The zero 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.


Dispersion 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.