7 Semiconductor amplifiers (SOA) and Raman amplifiers
7.2 Raman amplifier

Raman amplification uses the Stimulated Raman Scattering (SRS).

A longer wavelength photon induces the inelastic scattering of a shorter wavelength pump photon (mother wavelength) in an optical fibre. Another photon is produced at the wavelength shifted by about 100 nm (valid for standard telecom fibres).

This phenomenon is successfully used for the construction of an optical amplifier. If the Raman pump is located locally, where gain is to be achieved, we call such amplifier:

- LRA – Lumped Raman Amplifier.

However, since the SRS accumulates along the entire fibre, it is more optimal to place Raman pump at the opposite end of a fibre:

- DRA – Distributed Raman Amplifier.

Raman gain in different types of optical fibres: Single Mode Fibre (SMF), Dispersion Shifted Fibre (DSF) and Dispersion Compensating Fibre (DCF) .
Commercially available EDFA, SOA and Raman amplifier.

Raman effect is not always desired. In DWDM systems, SRS can produce crosstalk between transmission channels. Raman crosstalk is a generally undesirable, non-linear optical effect in networks that occurs due to minute imperfections in the fibre. It is an inelastic photon scattering effect (i.e. there is a change in photonic energy). In practical terms this results in crosstalk between channels and a migration of channel power from shorter to longer wavelengths (or from faster to slower frequencies).