针对高功率强抽运条件下,热光效应及其引起的模式不稳定效应对高功率大模场光纤功率及亮度提升的限制,理论仿真了有源光纤吸收系数变化对光纤热沉积、热致折射率以及光纤数值孔径的影响,分析表明高吸收系数光纤带来更高热负荷密度,在热光效应调制下加剧光纤数值孔径的增大程度,从而降低光纤激光的模式不稳定阈值。在理论研究基础上设计并制备了两款不同抽运吸收系数光纤样品并展开了高功率模式不稳定实验研究。实验结果证明,吸收系数为1.71 dB/m的光纤样品的模式不稳定阈值激光功率约为800 W,吸收系数为1.20 dB/m的光纤样品在输出激光功率达到1700 W时仍未观测到任何模式的不稳定现象,实验结果验证了降低光纤的抽运吸收系数可提升热致模式不稳定阈值的理论分析。该研究结果为设计研制大模场有源光纤,并获得更高功率光纤激光输出提供了一种新颖有效的技术途径。

To study the limits of improvements of the high-power large-field fiber power and brightness caused by the thermo-optic effect and thermal-induced mode instability under strong pumping conditions, we analyze the influence of the variation of absorption coefficient on the thermal deposition, thermal-induced refractive index, and numerical aperture of a fiber. Results show that a fiber with high absorption coefficient causes a high heat load density, and its numerical aperture increases under the thermal-optic effect modulation, thereby reducing the mode instability threshold. Based on the theoretical research, fibers with two different pump absorption coefficients are designed and fabricated,and high-power mode instability experiments are performed. Experimental results show that the mode instability threshold power is 800 W for the high pump absorption fiber with an absorption coefficient of 1.71 dB/m. However, for the fiber with a low pump absorption coefficient of 1.20 dB/m, the mode instability is not observed when the output power reaches 1700 W. Thus, the mode instability threshold can be considerably improved by decreasing the pump absorption coefficient of the active fiber. These results are significant to the development of high-power active optical fibers and provide a novel and effective technical approach for obtaining high-power fiber laser output.