Raman amplifier

A type of amplifier used in long haul, single-mode (SMF) fiber optic transmission systems (FOTS). Raman amplification usually occurs throughout the length of the transmission fiber itself in a process known as distributed amplification, rather than in a discrete amplification, or lumped amplification configuration such as that employed by an erbium-doped fiber amplifier (EDFA). Raman amplification occurs as a high-energy pump wavelength is sent in the reverse direction from the output end of the fiber span, where the incoming signal is weakest. The pump wavelength, which generally is in the 1450 nm range (E-Band), interacts with and excites atoms in the crystalline lattice of the fiber core.The atoms absorb the photons, and quickly release photons with energy equal to the original photon, plus or minus atomic vibration. In other words, a frequency/wavelength shift occurs as the pump wavelength propagates along the fiber in the reverse direction.The energy lost in the pump wavelength shifts to longer-wavelength (within about 100 nm) signals, generally in the 1550 nm window (C-Band), in the forward direction, thereby serving to amplify them. Raman amplifiers offer the advantage of amplifying signals in the broad range extending from 1300 nm to 1700 nm. Further, they perform better than EDFAs in terms of signalto-noise ratio (SNR). Raman amplifiers often are used as preamplifiers to enhance the performance of EDFAs in dense wavelength division multiplexing (DWDM) systems.

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