Physical Properties of Optical Fiber
All common types of fibers are characterized by two the most important properties: attenuation and dispersion.
There are modal and material dispersion – distortion of a signal, which caused by specialty of light-wave propagation.
Material dispersion appears because of the speed difference between waves with diverse length, which is based on specialty of physical structure of fiber. This effect particularly noticeable while singlemode optical fiber using. Reduction of emission bandwidth of the source and optimal wave-length selection decrease the material dispersion.
Modal dispersion appears in multimode fiber because of different length of the path, that the various mod light rays go through. Reduction of the fiber core diameter and mode numbers, using fiber with gradient profile help to reduce this dispersion.
An attenuation of the signal in the fiber optic cable depends on the material properties and environments. Attenuation characterizes the power loss of transmitted signal over defined distance. It is measured in dB/km, where decibel is a logarithmic relation expression of power, that comes out from source P1, to power, that comes into receiver P2, dB=10*log(P1/P2). The leak of 3 dB means that the half of power is lost. The leak of 10 dB means that only 1/10 of source power gets to the receiver.
Usually, fiber-optic lines can normally work with leak of 30 dB (taking 1/1000 of power).
There are two radically different physical mechanisms, which lead to this effect: fiber absorption loss and dispersion loss. The effects will be describe in the next article.