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Fiber dispersion can be classified into two categories based on the types of fiber. If the core of a fiber is thick enough, this fiber is a multimode fiber, which allows multimode signals to transmit within it. In contrast, if the core of a fiber is thin enough, this fiber allows a single mode to be transmitted within it, which is called single mode fiber.
Intermodal fiber dispersion
In a multimode fiber there are multiple signals with different modes transmitted. Each optical pulse is made up of the components of these multimode signals. The signal of each mode has a different propagation speed in the fiber to reach a receiver. Some modes arrive earlier, while other modes arrive later. The different arrival times thus generates a temporal extension of the signal pulse. This extension is called intermodal fiber dispersion.
Chromatic dispersion
In contrast to the multimode signals in a multimode fiber, there is only a single mode in a single mode fiber. Thus, there is no intermodal dispersion. For a signal model signal, there are dispersions caused by fiber material (silica) and fiber cross-plane shape. It is found that silica can cause different propagation speeds for signals at different frequency. Because each signal pulse is made of multiple frequency components, when it transmits over a single-mode fiber, different frequency components will arrive at a receiver at different times. This therefore generates a temporal extension of the signal pulse, which is called material dispersion. Similarly, due to the non-consistency of fiber manufacture, the shape of fiber cross-plane cannot always be homogenous, which also causes different signal frequency components to arrive at a receiver at different times and a temporal extension of the signal pulse. Such an extension is called waveguide dispersion, as a fiber is a waveguide. There is a more general name to cover the above two types of dispersion, called chromatic dispersion.
It should be noted that in multimode fibers, there is also the chromatic dispersion. However, because the intermodal dispersion is more dominant, the chromatic dispersion is often ignored. Polarization mode dispersion (PMD)
PMD is another type of dispersion in a single mode fiber. A single-mode optical signal transmitted in a fiber is essentially made up of two signal components, which are perpendicular to each other in electrical fields, i.e., Ex and Ey. Due to the non-circularity of fiber core, these two components travel at different speeds in the fiber, thereby causing the spread of optical signal pulse. This phenomenon is called polarization mode dispersion.
Compared to the chromatic dispersion, PMD is generally negligible when the system transmission rate is low, e.g., less than 10Gb/s. However, if some dispersion compensation is used to count for the chromatic dispersion, and the system transmission rate goes beyond 10Gb/s, e.g., 40Gb/s, then the effect of the polarization mode dispersion will dominate and should be considered for some PMD dispersion.
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