ABSTRACT
Raman fibre lasers (RFLs) are based on a non-linear process of stimulated Raman scattering in optical fibres. RFLs are applied for a laser light generation at new wavelengths. At most cases, RFL converts a single-mode pump radiation into a single-mode output radiation at increased wavelength. However, it was recently demonstrated that an almost single-mode radiation can be generated by the RFL based on a multi-mode fibre. Moreover, this multi-mode fibre was pumped by a multi-mode radiation. Thus, RFLs can increase the brightness of radiation: the RFL output radiation is much brighter than the pump. In this respect, RFLs are similar to usual double clad rare-earth lasers. The value of the frequency shift of the RFL radiation with respect to the pump (the Raman shift) is mainly determined by three factors. Those are properties of the core material, properties of a pump radiation and the scheme of RFL. In particular, it is possible to increase the resulting value of the Raman shift with the use of a cascaded scheme of the RFL.
