Objective Raman fiber lasers are distinguished by their exceptional capabilities in generating high-power lasers at specific wavelengths, attracting considerable interest and research efforts. In recent years, the utilization of graded-index multimode fibers in Raman lasers has not only increased the injectable pump power but also enhanced laser brightness through beam cleaning characteristics. Currently, significant breakthroughs have been made in Raman fiber lasers based on large-core graded-index multimode fibers, with researchers successfully achieving kilowatt-level near-single-mode output. Methods An all-fiber Raman laser system was built based on the oscillator structure. Extensive design optimization of passive fiber components, including fiber refractive index, core diameter, mode field area, as well as combiners and fiber gratings, was conducted to effectively suppress higher-order Raman effects and prevent degradation of beam quality. The system utilized multiple 1080 nm lasers as the pump source, which were combined by a custom-designed fiber combiner. The Raman gain medium was a piece of graded-index fiber with a length of 18 m and a core diameter of 150 μm. The resonator cavity was constructed using a pair of specially designed fiber gratings with mode-selecting characteristics, featuring a central reflection wavelength of 1130 nm. Results and Discussions The power evolution of the laser system, illustrated in Fig.1(a), demonstrated that with pump power injection of 2392 W, the signal laser output reached 1780W with residual pump power of 340 W, corresponding to a Raman conversion efficiency of 74.4%. Figures 1(b) and (c) indicated that as the power increased, a discernible broadening of the output spectrum and degradation in beam quality were observed, attributed to heightened nonlinear effects. Specifically, as the signal power increased from 200 W to 1780 W, the 3 dB linewidth of the output spectrum widened from 0.7nm to 1.9 nm, and the output beam quality factor M2 degraded from 2.5 to 3.5 (average values derived from multiple measurements). Moreover, at the maximum power level, the intensity of 2nd order Raman laser is 40 dB lower than that of signal laser. Conclusions An all-fiber Raman oscillator with output of 1.8 kW was successfully achieved, setting a new public record for the highest reported power to date. Through the optimization of fiber and passive components, it is expected that higher-power Raman fiber lasers can be realized in the future. © 2024 Chinese Society of Astronautics. All rights reserved.
