High-speed modal analysis of dynamic modal coupling in fiber laser oscillator

Up till now, the spatial and temporal dynamics of transverse mode instability (TMI) in fiber laser oscillator have increasingly attracted a worldwide attention. Here, we develop a high-speed modal decomposition (MD) system to analyze the modal coupling for fiber laser oscillator above the TMI threshold. A set of angular-multiplexing transmission functions (TFs) are designed for simultaneous MD and monitoring the far-field beam profile. The TMI threshold of the deployed fiber laser oscillator is 181 W at a co-pumping power (CPP) of 279 W. As the CPP increases from 318 W to 397 W, the power fluctuations of the output laser become more drastic. The changes of the far-field beam profile and the centroid of far-field spot (COFFS) indicate an increased velocity of energy transfer between modes. The high-speed MD verifies above process and analyzes the modal components, indicating that the single cycle of modal coupling decreases from 11 ms to 4 ms. Otherwise, the strong mode coupling occurs between modes with relatively large weights. The high-speed MD provides a powerful tool to research the TMI effect.

Automated summary

In this study, a high-speed modal decomposition (MD) system is developed to analyze modal coupling in fiber laser oscillator above the transverse mode instability (TMI) threshold. A set of angular-multiplexing transmission functions (TFs) are designed for simultaneous MD and monitoring of the far-field beam profile. Experimental results show that as the co-pumping power increases from 318 W to 397 W, the power fluctuations of the output laser become more drastic, accompanied by changes in the far-field beam profile and the centroid of far-field spot (COFFS), indicating increased velocity of energy transfer between modes. The high-speed MD verifies this process and analyzes the modal components, revealing a decrease in the single cycle of modal coupling from 11 ms to 4 ms, as well as strong mode coupling between modes with relatively large weights.