Filtering of stimulated Raman scattering in a monolithic fiber laser oscillator using chirped and tilted fiber Bragg gratings

We have demonstrated here, for the first time to the best of our knowledge, the filtering of stimulated Raman scattering (SRS) in a monolithic fiber laser oscillator, using chirped and tilted fiber Bragg gratings (CTFBGs). We designed and inscribed CTFBGs in large-mode-area fibers according to the operating wavelength of the fiber laser oscillator. With a CTFBG inserted before the output coupling FBG in the oscillator, a maximum SRS suppression ratio more than 10 dB was achieved in spectrum with 976 nm and 915 nm laser diode pumps respectively at a given power level. Obvious decreasing of the Stokes power and increasing of the equivalent Raman threshold was observed. By reducing the insertion loss and improving the transmission spectrum of the CTFBG, an improvement in laser efficiency could be achieved. This work provides a novel idea for SRS filtering in a high-power all-fiber oscillator system, which is very useful for the power enhancing of a fiber oscillator. (paper)