中国国防科技大学杨保来团队近日研究了单片少模光纤激光振荡器的横模耦合。该研究于2025年5月12日发表在《光：科学与应用》杂志上。

非线性热光耦合引起的横模不稳定性（TMI）对光纤激光器的功率缩放提出了主要挑战。在光纤振荡器中，由于激光振荡过程中的模式竞争，密封谐振腔的TMI可能会变得特别复杂。虽然传统的TMI理论主要涉及双模耦合，但研究组利用包括求解稳态热光耦合方程在内的整体方法来探索少模光纤振荡器中的TMI现象。模拟表明，弯曲损耗与TMI阈值之间存在非单调相关性，这与双模相互作用理论提出的单调相关性相反。当一个高阶模式经历净增益时，TMI阈值会发生波动，导致非耦合频率区域内新模式的放大，从而影响增益饱和。 

通过设计低反射光栅（LR）的线宽，可以实现非耦合频域中的模态功率管理。过宽的LR线宽加剧了共享频率区域内的模式耦合，从而加剧了TMI。为了验证理论模拟，研究组精心制作了LRs，并优化了纤维卷绕以提高TMI阈值。通过仔细优化LR线宽和弯曲半径，他们实现了创纪录的10.07的激光输出 kW，使用单片光纤振荡器，没有可观察到TMI的证据。该工作表明，独立频域中的模态功率再分配为减轻高功率光纤激光器中的TMI提供了一种新方法。此外，它还为与光纤通信相关的模式解耦策略提供了新的见解。

附：英文原文

Title: Transverse mode coupling in monolithic few-mode fiber laser oscillators

Author: Rao, Binyu, Chen, Jinbao, Wang, Zefeng, Li, Hao, Yang, Baolai, Zhao, Rong, Ye, Xinyu, Tang, Hengyu, Wang, Meng, Li, Zhixian, Chen, Zilun, Cao, Jianqiu, Xiao, Hu, Liu, Wei, Ma, Pengfei, Yao, Tianfu

Issue&Volume: 2025-05-12

Abstract: Transverse mode instability (TMI), induced by nonlinear thermal-optical coupling, poses a primary challenge for the power scaling of fiber lasers. In the fiber oscillator, a sealed resonant cavity, TMI could become particularly complex due to the mode competition during the laser oscillation. While traditional theories of TMI predominantly address two-mode coupling, this paper explores the TMI phenomena in few-mode fiber oscillators utilizing a holistic approach that includes solving steady-state thermal-optic coupling equations. The simulation shows that there is a non-monotonic correlation between bending loss and the TMI threshold, which is contrary to the monotonic associations suggested by two-mode interaction theory. When one high-order mode experiences net gain, fluctuations of the TMI threshold would occur, leading to the amplification of a new mode within the uncoupled frequency region, thus affecting the gain saturation. By designing the linewidth of a low-reflection grating (LR), the modal power management in the uncoupled frequency domain can be achieved. An excessively broad LR linewidth exacerbates mode coupling within the shared frequency region, thus exacerbating TMI. To validate the theoretical simulation, we carefully fabricated LRs and optimized the fiber coiling to elevate the TMI threshold. Through careful optimization of LR linewidth and bending radii, we achieved a record-breaking laser output of 10.07kW using a monolithic fiber oscillator, with no observable evidence of TMI. Our work demonstrates that modal power redistribution in independent frequency domains offers a novel approach to mitigating TMI in high-power fiber lasers. Additionally, it provides new insights into mode decoupling strategies pertinent to fiber communications.

DOI: 10.1038/s41377-025-01862-6

Source: https://www.nature.com/articles/s41377-025-01862-6