Definition: dispersion

In optical fibers, the broadening of the waveforms over long distances by the time they reach the receiving end, which makes them difficult to interpret. There are three major causes. One is the multiple transmission paths (modes) possible in large-core multimode fibers where each path results in a different travel distance.

A second cause has to do with the varying of the refractive index due to changes in frequency (or correspondingly, changes in wavelength). The speed of light in a fiber is based on the frequency of light and the refractive index of the fiber. Thus, different frequencies travel at different speeds. The problem is that there are always multiple frequencies. Analog signals are naturally many frequencies, but digital pulses are also more than one frequency, because it is difficult to create a perfect single frequency.

The third cause of dispersion is the random fluctuations of light polarization inside the fiber. Following are the common types of dispersion.

Modal Dispersion (or Intermodal Dispersion)
Occurs in multimode fibers, because light travels in multiple modes (reflective paths), and each path results in a different travel distance. Modal dispersion is a major problem with multimode fibers.

Chromatic Dispersion
The sum of material dispersion and waveguide dispersion. "Material dispersion" is caused by the variation in refractive index of the glass in the fiber. "Waveguide dispersion" is due to changes in the distribution of light between the core and the cladding of a singlemode fiber.

Polarization Mode Dispersion (PMD)
Light travels in two polarization states in singlemode fibers. Over long distances, conditions such as stress and slight irregularities in the fiber core cause random fluctuations in how the two polarizations travel through the fiber. As a result, they gradually spread over the square root of the distance. See refractive index, dispersion compensator, step index fiber, graded-index fiber, dispersion-shifted fiber and fiber optics glossary.