Abstract
This chapter describes the general principles of fiber Raman amplifiers and refer to their applications in the updated optical communications systems or wavelength-division multiplexing (WDM) transmission systems. The process of Raman scattering can be described by using nonlinear polarization. An electron of the molecule absorbs an incident pump photon, which is excited to the virtual intermediate states of the molecule. The electron is then quickly de-excited down to the molecule in the first energy state of the optical phonon. If a photon with Stokes-shifted frequency enters when the electron is de-excited, then the stimulated emission can occur. Stimulated Raman scattering is a process in which a Stokes-shifted photon receives gain when it enters into the molecules together with a pump photon. It is found that in the counter-pumping scheme, the pump and Stokes waves pass by each other so rapidly over distance that the relative orientation of the two polarization states will rapidly change and thus averages out to be completely random. This randomization process is strictly through the concatenation of local birefringent segments distributed over fiber. The efficient combining of two pump lasers into a single-mode fiber is also elaborated.
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