Fan Chenchen, An Yi, Yao Tianfu, Xiao Hu, Huang Liangjin, Xu Jiangming, Leng Jinyong, Zhou Pu
Opt Lett. 2021 Sep 1;46(17):4220-4223. doi: 10.1364/OL.433750.
Due to the beam cleanup effect, brightness enhancement (BE) can be achieved in a Raman fiber amplifier (RFA) based on multimode (MM) graded-index (GRIN) fiber. In this Letter, a novel, to the best of our knowledge, diagnostic tool of mode decomposition (MD) based on a stochastic parallel gradient descent algorithm is demonstrated to observe the beam cleanup effect in a GRIN-fiber-based RFA for the first time, to our knowledge. During output power boosting up to 405 W at 1130 nm, the output beam quality factor improves from 3.45 to 2.88, with a BE factor of 10.5. The MD results based on the near-field beam profiles from RFA indicate that the modal weight of the fundamental mode increases from 74.5% to 87%, confirming that the fundamental mode dominates with higher Raman gain. Moreover, the beam quality is found to be limited by the existence of a higher-order (Laguerre-Gaussian) mode, which is insensitive to the beam cleanup effect. The correlation coefficient reaches over 0.98 for all MD results. Thus, the accuracy of the MD method is high enough to provide further valuable insight into the physics of spatiotemporal beam dynamics in MM GRIN fiber.
由于光束净化效应，基于多模（MM）渐变折射率（GRIN）光纤的拉曼光纤放大器（RFA）可以实现亮度增强（BE）。在本信函中，据我们所知，首次展示了一种基于随机并行梯度下降算法的新型模式分解（MD）诊断工具，用于观察基于GRIN光纤的RFA中的光束净化效应。在1130nm处输出功率提升至405W的过程中，输出光束质量因子从3.45提高到2.88，亮度增强因子为10.5。基于RFA近场光束轮廓的模式分解结果表明，基模的模式权重从74.5%增加到87%，证实了基模在拉曼增益较高时占主导地位。此外，发现光束质量受高阶（拉盖尔 - 高斯）模式的存在限制，该模式对光束净化效应不敏感。所有模式分解结果的相关系数均超过0.98。因此，模式分解方法的精度足够高，能够为MM GRIN光纤中的时空光束动力学物理提供进一步有价值的见解。
