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自模式不稳定现象公开报道之后,研究人员从多个方面对模式不稳定现象展开分析,大量实验研究表明,高功率光纤激光器中模式不稳定阈值具有明显的“阈值性”[19]。模式不稳定阈值附近的时域特性具有“周期性”,特征频率耦合在kHz量级,输出信号光经包层光滤除器之后功率滞涨等特征[20]。利用模式不稳定这些特征属性,可以准确的判定模式不稳定阈值。实验中,由于采用了小弯曲结构盘绕增益光纤和包层光滤除器过滤高阶模,在达到模式不稳定阈值之后不会出现光束质量下降现象。
实验中,首先采用工业高效率激光器泵浦源976 nm LD作为该振荡器的泵浦源进行实验,得到实验结果如图4所示。图4(a)为不同泵浦功率情况下输出功率及对应的光光转换效率。可以看到,在低功率时输出功率随泵浦功率成线性增长趋势,但是当泵浦功率达到412 W之后,输出功率出现下降,对应的光光转化效率由最高时的67%下降到48%。利用光谱仪测量得到的光谱数据如图4(b)所示,光谱中没有明显的拉曼成分,故功率降低与光光转化效率下降与受激拉曼散射效应无关。利用光电探测器测量输出功率为279 W时的时域信号以及傅里叶变换后的频域信号如图4(c)所示。在泵浦功率为412 W,输出功率为279 W时,时域上没有观测到明显的波动,频域也没有出现模式不稳定效应特征峰。图4(d)为泵浦功率483 W,输出功率235 W时的时频域信号,结果显示,在泵浦功率为483 W时,时域上出现周期性的起伏,频域上可以观察到明显的模式不稳定效应特征峰。在泵浦功率为483 W时测得光斑形态如图4(e)所示,对应的光束质量因子为M2x=1.4、M2y=1.4。根据实验中测得的功率和光光转化效率下降现象和时频域信号,可以判定976 nm LD作为泵浦源时,该振荡器的模式不稳定阈值约为279 W。
Figure 4. Output characteristic of fiber oscillator when pumped with 976 nm LD. (a) Output power and optical-to-optical(O-O) efficiency under different pump power; (b) Optical spectrum at the output power of 279 W; (c)-(d) Time domain signals and their Fourier spectra at the output power of 279 W and 235 W, respectively; (e) Beam quality factor and beam profile when the pump power is 483 W
随后采用工业应用中稳定性较好的激光器泵浦源915 nm LD对光纤激光器进行泵浦,得到实验结果如图5所示。图5(a)为不同泵浦功率情况下输出功率及对应的光光转换效率曲线。可以看到,与976 nm LD泵浦时类似,在低功率时输出功率随泵浦功率成线性增长趋势,在泵浦功率为912 W时,最高输出功率达到502 W。当泵浦功率继续增加到952 W时,输出功率下降为468 W,对应的光光转化效率从最高时的61%下降到49%。利用光谱仪测量得到的光谱数据如图5(b)所示,由于功率较低光谱中没有明显的拉曼成分,故功率降低和光光转化效率下降与受激拉曼散射效应无关。利用光电探测器测量输出功率为399 W时的时域信号以及傅里叶变换后的频域信号如图5(c)所示。在泵浦功率为749 W,输出功率为399 W时,时域上没有观测到明显的波动,频域也没有出现模式不稳定特征峰。图5(d)为泵浦功率912 W,输出功率502 W时的时频域信号,可以看到,时域上出现周期性的起伏,频域上可以观察到明显的模式不稳定效应特征峰。在泵浦功率为952 W时测得激光器远场光斑形态如图5(e)的插图所示,对应的光束质量因子为M2x=1.5、M2y=1.5。根据实验中测得的功率和光光转化效率下降现象以及光谱和时频域信号,使用915 nm LD泵浦时,该激光器的模式不稳定阈值约为502 W。
Figure 5. Output characteristic of fiber oscillator when pumped with 915 nm LD. (a) Output power and O-O efficiency under different pump power; (b) Optical spectrum at the output power of 502 W; (c)-(d) Time domain signals and their Fourier spectra at the output power of 502 W and 468 W, respectively; (e) Beam quality factor and beam profile when the pump power is 952 W
最后,采用工业应用不多,但有望提高模式不稳定阈值的940 nm LD作为该振荡器的泵浦源进行实验,得到实验结果如图6所示。图6(a)为不同泵浦功率情况下输出功率及对应的光光转换效率。与976 nm LD和915 nm LD泵浦时类似,在低功率时输出功率随泵浦功率成线性增长趋势,但是当泵浦功率达到1144 W之后,输出功率出现下降,对应的光光转化效率由最高时的63%下降到59%。利用光谱仪测量得到的光谱数据如图6(b)所示,光谱中没有明显的拉曼成分,故功率降低与光光转化效率下降与受激拉曼散射效应无关。利用光电探测器测量泵浦功率1144 W,输出功率为697 W时的时域信号以及傅里叶变换后的频域信号如图6(c)所示。在输出功率为697 W时,时域上没有观测到明显的波动,频域也没有出现模式不稳定效应特征峰。图6(d)为泵浦功率1149 W,输出功率693 W时的时频域信号,结果显示,在输出功率为693 W时,时域上出现周期性的起伏,频域上可以观察到明显的模式不稳定效应特征峰。在泵浦功率为1149 W时测得光斑形态如图6(e)所示,对应的光束质量因子为M2x=1.4、M2y=1.4。根据实验中测得的功率和光光转化效率下降现象和时频域信号,可以判定,940 nm LD作为泵浦源时,该振荡器的模式不稳定阈值约为697 W。
Figure 6. Output characteristic of fiber oscillator when pumped with 940 nm LD. (a) Output power and O-O efficiency under different pump power; (b) Optical spectrum at the output power of 697 W; (c)-(d) Time domain signals and their Fourier spectra at the output power of 697 W and 693 W, respectively; (e) Beam quality factor and beam profile when the pump power is 1149 W
如表1所示,分别利用976、915、940 nm LD泵浦纤芯/包层直径为30/400 μm的掺镱光纤振荡器时,激光器的模式不稳定阈值分别为279、502、697 W,光光转化效率分别为67.7%、61%和63%,且均能保持近单模光束质量。对比仿真结果可以看到,泵浦吸收系数越高,相同泵浦功率水平下增益光纤温度越高,激光器实际模式不稳定阈值越低,理论与实验结果相吻合。
Pump wavelength/nm Pump absorption coefficient/dB.m−1 Active fiber maximum temperature/℃ TMI threshold/W O-O conversion efficiency 976 2.73 110.89 279 67.7% 915 0.88 70.93 502 61% 940 0.68 53.26 697 63% Table 1. TMI threshold for different pump wavelengths
Transverse mode instability effect of fiber lasers with different pump wavelengths
doi: 10.3788/IRLA20210256
- Received Date: 2022-01-20
- Rev Recd Date: 2022-02-25
- Accepted Date: 2022-03-07
- Publish Date: 2022-05-06
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Key words:
- fiber laser /
- transverse mode instability /
- thermal effect /
- pump absorption coefficient
Abstract: It is generally believed that the transverse mode instability (TMI) of fiber laser mainly comes from the thermal effect caused by the quantum defect of the pump source and the pump absorption of gain fiber. Theoretically, based on the analysis of the heat source in optical fiber, it was found that the thermal effect of induced mode instability mainly comes from pump absorption, followed by the quantum defect. And the simulation software SeeFiberLaser developed by the research group was used to verify the conclusion. The simulation results showed that the lower the pump absorption coefficient, the lower the maximum temperature and temperature gradient in the fiber, the more beneficial it was to restrain the formation of thermally induced refractive index grating and increase the TMI threshold. Experimentally, a co-pumped ytterbium-doped fiber laser oscillator with a core/inner cladding diameter of 30/400 μm was built and comparatively studied on TMI threshold characteristics of lasers pumped by laser diodes (LDs) with central wavelengths of 976 nm, 915 nm and 940 nm. The results showed that when 976 nm LD, 915 nm LD and 940 nm LD were used as pump sources, the laser TMI threshold was 279 W, 502 W and 697 W respectively, and the laser optical-to-optical (O-O) conversion efficiency was 67.7%, 61% and 63%, respectively. It was be found that the influence of the pump absorption coefficient on the TMI threshold was greater than that of the quantum defect on the TMI threshold, and the TMI threshold could be effectively increased by changing the pump wavelength and reducing the pump absorption coefficient. Optimizing pump wavelength, taking into account both quantum efficiency and pump absorption coefficient, was one of the important technical routes for fiber lasers to achieve high beam quality and high mode instability threshold.