Brightness enhancement with Raman fiber lasers and amplifiers using multi-mode or multi-clad fibers

A large variety of high power, high brightness, rare earth doped fiber lasers have been demonstrated throughout recent years. Several obstacles, such as modal instabilities, heat removal and photo-darkening, have been identified as the limiters and have complicated the further power increase. Raman fiber sources have inherent advantages in regard to these challenges and therefore may offer a high power high brightness alternative. Up to over kW power Raman fiber laser and amplifier sources have recently been demonstrated with multimode graded index fiber (1002 W) as well as with multi-clad clad-pumped fiber (1200 W) in brightness enhancing configurations. In these schemes, the output Stokes-shifted-signal obtained higher brightness than the input pump source, due to the higher Raman gain associated with the lower order modes. A review of the recent results obtained with various configurations is presented, followed by discussion on the challenges and the paths towards further increase in power and brightness towards achieving near diffraction limited multi-kW Raman fiber sources.