We present investigations on the seed source dependence of stimulated Raman scattering (SRS) created in a high power fiber amplifier. It is shown that fiber oscillators are much worse in terms of SRS than other seed sources. The longitudinal mode composition was found to be of less importance. We reinforce the experimental observations by a numerical investigation, which shows that temporal power variations on the ps-scale and their propagation along the fiber are crucial for the SRS creation in high-power fiber systems, extending the well-known but simplified SRS threshold description.
The average output power of fiber lasers have been scaled deep into the kW regime within the recent years. However a further scaling is limited due to nonlinear effects like stimulated Raman scattering (SRS). Using the special characteristics of femtosecond laser pulse written transmission fiber gratings, it is possible to realize a notch filter that mitigates efficiently this negative effect by coupling the Raman wavelength from the core into the cladding of the fiber. To the best of our knowledge, we realized for the first time highly efficient gratings in large mode area (LMA) fibers with cladding diameters up to 400 μm. The resonances show strong attenuation at design wavelength and simultaneously low out of band losses. A high power fiber amplifier with an implemented passive fiber grating is shown and its performance is carefully investigated.