Fabrication of cascaded wideband LPFGs by CO2 laser and Raman suppression in a 5 kW one-stage MOPA fiber laser

Long period fiber Bragg grating (LPFG) has been proved to be an effective component for stimulated Raman scattering (SRS) suppression in high power fiber lasers. The rejection bandwidth has significant influence on the Raman suppression effect, usually the wider the better. In this paper, a simple cascaded inscription method for achieving wideband LPFGs, to the best of our knowledge, is proposed and experimentally validated for the first time. The influence of grating periods on the transmission spectrum was simulated, and then a wideband cascaded LPFG was designed and fabricated by CO2 laser. To test its performance, a 5-kW level oscillator one-stage power amplification fiber laser was established and several comparative experiments were done. The validity of the cascaded LPFG has been demonstrated by the better Raman suppression effect. This work provides a simple and effective method for obtaining wideband LPFGs, which have many other applications besides Raman suppression, such as mode conversion in fiber lasers.