Experimental analysis of Raman-induced transverse mode instability in a core-pumped Raman fiber amplifier.

The effect of transverse mode instability is a limitation for the power scaling of fiber laser systems, that can originate due to heat caused by stimulated Raman scattering. In this contribution, we experimentally investigate the threshold of transverse mode instability caused by stimulated Raman scattering in a passive fiber. Both, the Stokes seed power and the fiber length of a core-pumped Raman fiber amplifier are varied to systematically study this effect. Mode resolved measurements reveal that the threshold occurs at approximately the same Stokes output power for all tested configurations, independent of the total Raman conversion efficiency. These results increase the understanding of this type of mode instability and show which parameters are important for a further power scaling of high-power Raman fiber amplifiers.

Continuous-wave cascaded second Stokes diamond Raman laser at 1477 nm.

We report a high brightness cascaded Stokes diamond Raman laser with a diffraction limited beam quality pumped by an Yd-doped fiber laser. The Raman laser operated at 1477 nm and reached an output power of 63 W with 214 W pump power in continuous-wave mode. Conversion efficiency over 30% was achieved using a single pump pass concentric cavity that was highly resonant at the first Stokes and had high outcoupling at the second Stokes (45%). Thermal limitations were investigated as well as the temporal behavior of the first and second Stokes intra-cavity power.