Tracing the origins of mode coupling for beam quality optimization in high-power fiber laser delivery systems

Suppressing mode degradation is the key issue for high-power laser delivery; however, diagnosing mode degradation in its entirety, ranging from the contents and origins to locations, has always been a major obstacle. Here, a versatile approach for tracing the origins of mode coupling is demonstrated through addressing the differential intermodal dispersions of fiber modes. Full recognition for modal contents and the origins of mode degradation are experimentally completed in a two-mode fiber laser delivery system, which assists a significant improvement of beam quality M 2 from 1.35 to 1.15 at the highest power of over 300 W. This method yields a quantitative characterization for manipulating the individual mode of dual-mode coupling origins or their combinations. This work points toward a promising strategy for the online tracing of mode coupling in cascade fiber links, thus enabling further pursuit of seeking extreme beam quality in high-power fiber laser systems.