Modal Dynamics in Kilowatt Cladding-Pumped Random Distributed Feedback Raman Fiber Laser With Brightness Enhancement

Random distributed-feedback Raman fiber lasers (RRFLs) are promising contenders in the applications of remote sensing, biotechnology, imaging, thanks to the unique properties on low coherence and wavelength agility. Despite recent advances on RRFL, the lack of brightness enhancement (BE) releasing high requirement on pump brightness remains a significant technological challenge for further power scaling. To this extent, we propose in this paper the first-ever cladding-pumped (CP) RRFL with BE to our best knowledge. The triple-clad purely passive fiber with designed cladding-to-core area ratio is utilized to suppress the generation of higher-order Stokes light. Pumped by two combined Yb-doped fiber lasers at 1080 nm, signal laser with maximum output power of 1 kW at 1132 nm is achieved with an optical-to-optical conversion efficiency of 92% (ratio between signal and absorbed pump power, which is 74% when vs. launched power). Moreover, the modal dynamics indicates the strong mode coupling induced by the complex interaction among the distributed gain, feedback, and other nonlinear effects, leading to the modal instability in the CP RRFL. Even so, this demonstration paves the way for powerful RRFLs beyond kilowatt with BE, which is of interest for diversified potential applications as diverse as industry and security.