Relationship Between Transverse Mode Instability and Stimulated Raman Scattering in Ytterbium Doped All-Fiber Laser Oscillator

The relationship between transverse mode instability effect and stimulated Raman scattering (SRS) in Ytterbium doped all-fiber laser oscillator is studied. In the single-end pumped 1.5 kW class all-fiber laser oscillator with core diameter of 20 mu m, when the stimulated Raman scattering reaches to a certain value, the transverse mode instability effect appears, in this case, the power in the laser oscillator decreases. The decreased power is stripped out of the laser oscillator by the cladding light stripper. It is found in the experiment that the enhancement of the stimulated Raman scattering spectrum, the decrease of the fiber laser power, and the increase of the temperature of the cladding light stripper have an inherence relationship. By shortening the fiber in the laser to mitigate the stimulated Raman scattering, we can increase the transverse mode instability threshold in the single-end pumped laser oscillator up to 2 kW. Similar results by increasing transverse mode instability threshold employing suppression the stimulated Raman scattering are also validated in the double-side pumped all-fiber laser oscillator with fiber core diameter of 25 mu m, and laser output greater than 5 kW is demonstrated. The experimental results validates that when the nonlinear effect is somewhat strong, the stimulated Raman scattering is the reason for the transverse mode instability. By mitigating the stimulated Raman scattering effect, we can also increase the transverse mode instability threshold.