A laser used as an energy pump in an optical amplifier or other laser by transferring or injecting energy from one wavelength to another in a medium, often a doped fiber. Raman amplification uses a high-energy pump wavelength sent in the reverse direction from that of the operating laser from the output end of the optical fiber span, where the incoming signal is weakest. The pump wavelength, which generally is in the 1450 nm (E-band) window, interacts with atoms in the crystalline lattice of the fiber core. The atoms absorb the photons and, when stimulated by the counter-flowing signal, quickly Cladding LED Cladding Core release photons with energy equal to the signal photon plus/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 excites atoms that shift energy to longer-wavelength signals, typically in the 1550 nm window (C-band), in the forward direction, thereby serving to amplify them. See also amplifier, C-band, E-band, frequency, laser, optical fiber, photon, propagate, Raman amplifier, signal, span, and wavelength.