Nonlinear effects

Nonlinear effects

Usually lightwaves or photons transmitted through a fiber have little interaction with each other, and are not changed by their passage through the fiber (except for absorption and scattering). There are exceptions, however, arising from the interactions between lightwaves and the material transmitting them, which can affect optical signals. These processes are normally referred to as nonlinear effects or phenomena because their strength typically depends on the square (or some higher power) of the optical intensity. Hence nonlinear effects are weak at low powers but they can become much stronger at high optical intensities. This situation can result either when the power is increased, or when it is concentrated in a small area such as the core of a single-mode optical fiber.

Although the nonlinear effects in optical fibers are small, they accumulate as light passes through many kilometers of single-mode fiber. The small core diameters, together with the long transmission distances that may be obtained with these fibers, have enabled the occurrence of nonlinear phenomena at power levels of a few milliwatts which are well within the capability of semiconductor lasers. Furthermore, the optical power levels become much larger when wavelength division multiplexing packs many signal channels into one single-mode fiber such that the overall power level is the summation of the individual channel optical powers.

There are two broad categories of nonlinear effects that can be separated based on their characteristics: namely, scattering and Kerr effects.