Light amplification is the process of intensifying the amplitude of an electromagnetic light wave. This process is generally classified into three major categories: laser, parametric and scattering.
Laser amplification is associated with the linear polarization which occurs due to first-order atomic susceptibility in an optical medium. The other two processes are related to the effects of non-linear polarization caused by second and third-order susceptibilities.
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Amplification is carried out using amplifiers which receive electromagnetic input signal and generate an output signal with higher optical power. Most of the optical amplifiers are laser amplifiers that carry out amplification based on stimulated emission. The gain medium in these amplifiers will have certain amount of excited molecules, ions or atoms, which can be stimulated by the light signal for the emission of more light. The gain media can be either optically or electrically pumped semiconductors or insulators doped with some laser-active ions.
Doped insulators used for laser amplification purposes include certain types of waveguides or glasses or laser crystals in bulk form. The laser-active ions are either transition-metal ions or rare earth ions. Non-linearity based optical devices, such as optical parametric amplifiers, are operated based on a medium with χ(2) nonlinearity. Parametric fiber devices work based on the χ(3) nonlinearity of a fiber. Brillouin amplifiers and Raman amplifiers are other devices that exploit a nonlinear response of the medium.
One of the key differences between nonlinearity-based amplifiers and laser amplifiers is that some amount of energy can be stored in the latter, while the gain obtained from the former remains only as long as the pump light is present.