Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB1104400) and the National Natural Science Foundation of China (Grant No. 61735007).
Received Date:24 December 2018
Accepted Date:22 February 2019
Available Online:01 June 2019
Published Online:05 June 2019
Abstract:The phenomenon mode instability is the most limiting factor for further scaling the output power and beam quality in high power fiber lasers. Thus, it is meaningful and necessary to study the influencing factor of mode instability and finally find the approaches to mitigating its influence. Theoretical calculations reveal that the fiber V-parameter has a negative effect on fiber amplifier mode instability threshold. Nevertheless, the influence of fiber core numerical aperture (NA) on fiber oscillator mode instability threshold has rarely been investigated compared with that on the fiber amplifier. In this paper, we build a high-power all-fiber laser oscillator pumped by 976nm laser diodes and measure its laser efficiency and mode instability threshold of 20/400 step-index ytterbium doped fiber with different fiber core NA. Experimental result reveals that at the same 976 nm pump power, the fiber with relatively low core NA (~0.059) has a higher mode instability threshold power than that with relatively high core NA (~0.064), and that even a higher core NA (~0.064) fiber has a higher laser efficiency than lower core NA (~0.059) fiber. The fact shows that the fiber core NA has a significant influence on mode instability threshold, and a relatively high core NA results in a lower mode instability threshold. Also, numerical simulations explain the reason why the fiber core NA has a negative effect on mode instability threshold in fiber oscillator. First of all, the higher fiber core NA will support more propagating modes in fiber, and the lower fiber core NA will result in higher order mode (HOM) content leaking into fiber cladding and the overlap of HOM content and gain area is reduced, thus the gain of HOM is relatively reduced. Also, the bending loss of HOM is very sensitive to fiber core NA variation, and the increase of fiber core NA will reduce the bending loss of HOM dramatically. In conclusion, the fiber core NA has a significant negative effect on fiber oscillator mode instability threshold, and numerical simulationscan explain the physical origin of the negative effect of fiber core NA on laser oscillator mode instability threshold. Thus, for the mode instability mitigation in high power laser oscillator, optimizing the NA of active fiber conduces to the increase of mode instability threshold, which is helpful and necessary for further scaling the output power and beam quality. Keywords:mode instability/ fiber laser/ ytterbium-doped fiber
3.实验结果与分析当使用光纤1搭建的光纤振荡器测试时, 其输出功率-抽运功率曲线如图2所示. 在注入的抽运功率在1400 W时, 经过包层光滤除器后的输出功率为1140 W, 对应的光光效率为81.4%; 当继续增加注入抽运功率到1500 W时, 输出功率从1140 W下降到1120 W, 此时对应的光光效率仅为74.7%, 光光效率出现了大幅度的降低, 并伴随有一定程度的功率起伏, 在光光效率下降的同时, 光电探测器测得的散射光信号的标准差也出现了明显的增加, 说明模式不稳定现象已经出现. 为进一步分析光纤1中的MI, 对光电探测器采集到的时域信号进行了处理, 如图3所示. 图 2 光纤1的输出功率-抽运功率曲线 (a) 输出功率/光光效率-抽运抽运功率关系; (b) 输出功率/归一化标准差-抽运功率关系 Figure2. Output power and pump power curve of fiber 1: (a) Correlation between output power/optical-optical efficiency and pump power; (b) output power/normalized standard deviation and pump power.
图 3 光纤1的输出功率时域信号 (a) 1400 W抽运源; (b) 1500 W抽运源 Figure3. Output time domain of different power of fiber 1: (a) 1400 W pump power; (b) 1500 W pump power.
由图3可知, 当注入的抽运功率为1400 W时(此时输出功率为1140 W), 对应的光光效率为81.4%, 时域输出信号基本保持稳定; 而当注入功率增至1500 W时(此时输出功率为1120 W), 对应的光光效率仅为74.7%, 时域输出信号较之前发生明显的波动, 对应在图2中表示为明显的标准差值的增加, 证明此时模式不稳定已发生. 同时, 输出功率在注入抽运功率从1400 W增加到1500 W时不升反降, 主要是由于发生模式不稳定时, 基模向高阶模耦合时因为弯曲泄露到包层之中被包层光滤除器滤掉, 在实验中观察到包层光滤除器在模式不稳定发生时温度显著升高也说明了这一点. 进一步对光纤1输出的时域信号做Fourier变换后, 得到的频域图如图4所示. 由图4可以清晰地看出, 在注入抽运功率为1400 W时, 频率信号除直流分量之外没有其他明显的频率成分; 而在增加注入抽运功率到1500 W时, 出现了一些直流分量之外的其他频率成分, 与1400 W注入功率时的频域信号大不相同. 根据以上分析, 我们认为光纤1在注入功率为1500 W时确实发生了模式不稳定. 图 4 光纤1的输出功率频域信号 (a) 1400 W抽运源; (b) 1500 W抽运源 Figure4. Output frequency domain of different power of fiber 1: (a) 1400 W pump power; (b) 1500 W pump power.
为进一步研究光纤NA对激光振荡器中的模式不稳定的影响, 换用光纤2重新测量了其激光性能, 其输出功率-抽运功率如图5所示. 在注入功率为1500 W时, 输出功率为1140 W, 其对应的光光效率为76.0%; 继续增加注入功率到1600 W时, 输出功率为1210 W, 此时对应的光光效率为75.6%, 光光效率较光纤1不发生模式不稳定时较低. 由光电探测器测量得到的标准差归一化值均在一个较小的范围之内波动, 如图5所示, 表征输出信号功率的波动较小, 无明显的剧烈波动, 而且在包层光滤除器上也未观察到明显的温度增加, 说明对于光纤2而言, 保持1600 W的注入功率未观察到明显的模式不稳定现象. 此时功率的进一步提升仅仅受限于抽运功率. 图 5 光纤2的输出功率-抽运功率曲线 (a) 输出功率/光光效率-抽运功率关系; (b) 输出功率/归一化标准差-抽运功率关系 Figure5. Output power and pump power curve of fiber 2: (a) Correlation between output power/optical-optical efficiency and pump power; (b) output power/normalized standard deviation and pump power.
为进一步确认光纤2在1600 W注入功率的条件下未发生模式不稳定, 对光纤2的输出时域信号进行了处理, 结果如图6所示. 当注入抽运功率为1500 W时, 输出的时域信号基本保持稳定, 未见明显的波动, 仅有因噪声导致的小幅度变化, 当注入抽运功率达到1600 W时, 输出的时域信号与注入抽运功率为1500 W时相比无明显变化, 也基本保持稳定且无明显波动, 而且频域信号在注入抽运功率为1500 W和1600 W时几乎保持一致(见图7), 无明显高频分量出现, 因此结合以上判据, 确认光纤2在整个输出阶段均未发生模式不稳定. 图 6 光纤2的输出功率时域信号 (a) 1500 W抽运源; (b) 1600 W抽运源 Figure6. Output time domain of different power of fiber 2: (a) 1500 W pump power; (b) 1600 W pump power.
图 7 光纤2的输出功率频域信号 (a) 1500 W抽运源; (b) 1600 W抽运源 Figure7. Output frequency domain of different power of fiber 2: (a) 1500 W pump power; (b) 1600 W pump power.
以上实验结果表明, 尽管光纤1在未发生模式不稳定之前, 其光光效率(81.4%)较光纤2的光光效率(75.6%)高, 从转换效率而言光纤1的热负载较光纤2更低, 但光纤2却表现出更高的模式不稳定阈值, 主要是因为光纤1的NA较光纤2的NA更大. 一方面, NA较大的光纤支持传输的模式越多; 另一方面, 较大的纤芯NA通常意味着较重的掺杂, 因此一般会有较大的吸收. 如图8所示, NA的减小对基模在光纤中的分布相对影响较小, 而对LP11模的影响较大(图8中的$\tau$表达了对应的模式在纤芯中功率的比例), 主要会使LP11模更多地延伸进入光纤包层中, 从而减小了LP11模的纤芯部分与光纤掺杂(增益)区的重叠, 因此LP11模的增益会随着NA的减小而降低, 模式不稳定阈值相应地上升. 图 8 具有不同NA光纤中的LP01和LP11模式分布 (a) LP01; (b) LP11 Figure8. LP01 and LP11 mode profile in fiber at different NA: (a) LP01; (b) LP11.
此外, LP11模的弯曲损耗对于光纤NA的变化极其敏感, 图9给出了在不同的光纤NA下光纤的弯曲损耗随着光纤的弯曲直径变化的关系. 由图9可知, 当NA减小时, 高阶模式的弯曲损耗会极大地增加, 这会导致更多的高阶模式由于光纤的弯曲泄露进入包层之中, 减少了高阶模式和光纤掺杂(增益)区的重叠, 从而导致LP11模的增益会随着NA的减小而降低, 模式不稳定阈值相应地增加. 图 9 具有不同NA光纤的LP11弯曲损耗随弯曲直径变化曲线 Figure9. Bending loss of LP11 versus bending radius at different fiber core NA.