Raman and Brillouin scattering are inelastic processes in which part of the power is lost from an optical wave and absorbed by the transmission medium. The remaining energy is then re-emitted as a wave of lower frequency. Raman and Brillouin scattering processes can become nonlinear in optical fibres due to the high optical intensity in the core and the long interaction lengths afforded by these waveguides. Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering (SBS) occur when the light launched into the fibre exceeds a threshold power level for each process. Under the conditions of stimulated scattering, optical power is more efficiently converted from the input pump wave to the scattered Stokes wave.

The scattered wave is frequency-shifted from the pump and in the case of SBS propagates in the opposite direction. This means that the amount of optical power leaving the far end of the fibre no longer increases linearly with the input power. The maximum launch power becomes clamped and the excess is simply reflected back out of the fibre. For long distance or highly-branched fibre links, it is important that as much power as possible can be launched into the fibre to compensate for attenuation and power splitting. Limits on the maximum output power due to SBS must therefore be avoided.

In SRS, the Stokes wave can be shifted from the pump wave by typically 10 to 100-nm and continues to propagate forwards along the fibre with the pump wave. If the pump is actually one channel of a multi-wavelength WDM communication system, then its Stokes wave may overlap with other channels at longer wavelengths - leading to crosstalk and Raman amplification. Raman amplification causes shorter wavelength channels to experience power depletion and act as a pumps for the amplification of longer wavelength channels. This can skew the power distribution among the WDM channels - reducing the signal-to-noise ratio of the lowest frequency channels and introducing crosstalk on the high frequency channels. Both of these effects can lower the information-carrying capacity of the optical system.

A report is available giving the results of a literature survey into techniques for measuring SBS and SRS-related parameters in optical fibre. The aim of the study was to identify the most promising approaches to the development of measurement facilities traceable to national standards. The theory behind each effect is summarised and experimental SBS and SRS data used to illustrate techniques for the determination of the SBS threshold and SRS gain coefficient of fibres.

The results of the investigation are available in a report entitled "Measurement Methods for Stimulated Raman and Brillouin Scattering in Optical Fibres".