Raman amplification is a method of amplifying optical signals based on the Raman effect. This effect was predicted as early as 1923 by Adolf Smekal and experimentally demonstrated in 1928 by the Indian physicist Chandrasekhara Venkata Raman (1888 - 1970). Raman received the Nobel Prize in Physics for this in 1930.
In the Raman amplifier, the pump beam from a pump laser is coupled into the optical fiber via wavelength division multiplexing(WDM). The pump beam has a high intensity, but its wavelength is below that of the light signal to be amplified. The coupled photons from the pump laser propagate through the optical fiber and, when they collide with fiber molecules, release their energy to the longer wavelength photons, causing amplification. The states are not stable and decay to lower energy levels after a short time. In the process, they release energy in the form of photons that scatter in different directions. This scattering is called Raman scattering. The gain produced by the energy transfer is between 10 dB and 15 dB.
In practice, Raman amplification is used on the receiving side, the light power is coupled in opposite to the data signal, and thus the useful signal is amplified over the entire fiber path. Thus, the amplifier power is greatest where the useful signal power is lowest. The amplification thus distributed over the entire distance ensures large link distances. The amplification is controlled by the light power of the pump laser.
Powerful pump las ers in the watt range are required to realize Raman amplification. The maximum gain is about 100 nm above the pump wavelength of the pump laser. However, it is particularly advantageous that Raman amplification can be used with already installed fibers, so that existing network structures can be used. There is no need to use specially doped optical fibers, as is the case with EDFA amplifiers. In addition, the optical signal-to- noise ratio( OSNR) is significantly better due to the amplification being distributed over the entire length of the optical fiber.