KW-class clad-pumped Raman all-fiber laser with brightness enhancement

The importance of average power scaling of fiber lasers (FL) is well known. However, power scaling is strongly limited by factors such as thermal load, and non-linear effects. An alternative path for reaching high powers utilizes the stimulated Raman scattering mechanism, and harnesses its power and brightness enhancement potential to reach high average power, high brightness FL. kW scale Raman FLs have been demonstrated, however they are in core-pumping configurations, meaning that they require an a-priori existing brighter kW laser that acts as their pump modules. There have been only a few publications of Raman FLs where the generated signal has a higher brightness than the pump source at levels of >= 100W, the highest, being at 250W. Here we report a strictly all-fiber clad pumped Raman FL with a CW power output of 800 W with a conversion efficiency of 80%. To the best of our knowledge this is the highest power and highest efficiency Raman FL demonstrated in any configuration allowing brightness enhancement (i.e clad pumped or graded index fiber, excluding step-index core pumped), thus being the first kW-class Raman FL with brightness enhancement. This result was achieved with a specially designed triple-clad fiber (TCF). The core was 25 mu m, 0.065 NA, and the inner cladding was 45 mu m 0.22NA. The choice of the small inner clad allows obtaining sufficient Raman gain without requiring too long a fiber, as well as being compatible with the waist size of the pump source fiber. In addition this diameter complies with the inner-clad/core ratio which prevents generation of a 2nd Stokes laser beam. Two fiber Bragg gratings at 1120 nm written onto the TCF, were employed as the oscillator's reflectors. The cavity was pumped by a lower beam-quality source with an M-2 of similar to 8 at 1070 nm. The Raman signal generated in the core, at the first Stokes wavelength of 1120 nm, showed an improved beam-quality in relation to the pump.