Dispersion

As they travel down an optical fiber, optical pulses will broaden. This is called dispersion. Since eventually the pulses will overlap and the data will be lost, dispersion determines the data carrying capacity.

Step-index optical fibers generally have several modes of propagation. Modes are represented by different angles with which the light rays hit and reflect from the core-clad interface. These various angles can be caused by the various angles that light enters the fiber or by other effects in the fiber such as scattering or changes in angle due to bending. The time it takes the light in a particular mode to travel from one end to the other is directly proportional to the distance it has traveled in the optical fiber. Low order modes get to the output end quicker than higher order modes. There are three main types of dispersion:

• Chromatic dispersion … is a result of different wavelengths of light traveling at different velocities in the core material. Since typical light sources provide power distributed over a range of wavelengths, rather than a single, discrete "line," the pulses spread out as they travel through the fiber. This is predominant in single-mode fibers.
• Modal dispersion … is due to different fiber modes traveling along different paths through multimode fiber. The result is broadening of the pulse as the higher-order modes reach the output behind the fundamental mode. This is predominant in step-index, multimode fibers.
• Waveguide dispersion … is due to the geometry of the fiber and results in different velocities of the various modes for different wavelengths. It is the least important of the causes of dispersion.

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