Transverse mode coupling in monolithic few-mode fiber laser oscillators.

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 10.07 kW 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.