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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.
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Keywords:
- mode instability /
- fiber laser /
- ytterbium-doped fiber
[1] Gapontsev V, Fomin V, Ferin A, Abramov M 2001 Proceedings Advanced Solid-State Photonics Seattle, Washington, United States, January 28, 2001 pAWA1
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图 1 光纤振荡器结构图(抽运源为976 nm波长的激光器)
Figure 1. Experimental setup of fiber oscillator pumped by 976 nm LDs.
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
Figure 2. 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
Figure 3. Output time domain of different power of fiber 1: (a) 1400 W pump power; (b) 1500 W pump power.
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
Figure 4. Output frequency domain of different power of fiber 1: (a) 1400 W pump power; (b) 1500 W pump power.
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
Figure 5. 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.
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
Figure 6. 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
Figure 7. Output frequency domain of different power of fiber 2: (a) 1500 W pump power; (b) 1600 W pump power.
图 8 具有不同NA光纤中的LP01和LP11模式分布 (a) LP01; (b) LP11
Figure 8. LP01 and LP11 mode profile in fiber at different NA: (a) LP01; (b) LP11.
图 9 具有不同NA光纤的LP11弯曲损耗随弯曲直径变化曲线
Figure 9. Bending loss of LP11 versus bending radius at different fiber core NA.
表 1 实验中使用掺镱光纤的参数
Table 1. Yb-doped fiber parameter applied in the experiment.
参数 光纤1 光纤2 光纤长度/m 20 20 NA(纤芯/包层) 0.064/0.460 0.059/0.460 主要掺杂元素 Yb/Al Yb/Al 弯曲直径/cm 15 15 吸收系数@976 nm/(dB·m–1) 1.2 1.2 掺镱浓度/wt.% 0.65 0.65 
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[1] Gapontsev V, Fomin V, Ferin A, Abramov M 2001 Proceedings Advanced Solid-State Photonics Seattle, Washington, United States, January 28, 2001 pAWA1
[2] Richardson D, Nilsson J, Clarkson W 2010 JOSA B 27 B63
Google Scholar
[3] Eidam T, Hanf S, Seise E, Andersen T V, Gabler T, Wirth C, Schreiber T, Limpert J, Tünnermann A 2010 Opt. Lett. 35 94
Google Scholar
[4] Eidam T, Wirth C, Jauregui C, Stutzki F, Jansen F, Otto H J, Schmidt O, Schreiber T, Limpert J, Tünnermann A 2011 Opt. Express 19 13218
Google Scholar
[5] Jauregui C, Eidam T, Limpert J, Tünnermann A 2011 Opt. Express 19 3258
Google Scholar
[6] Smith A V, Smith J J 2011 Opt. Express 19 10180
Google Scholar
[7] Ward B, Robin C, Dajani I 2012 Opt. Express 20 11407
Google Scholar
[8] Jauregui C, Eidam T, Otto H J, Stutzki F, Jansen F, Limpert J, Tünnermann A 2012 Opt. Express 20 440
Google Scholar
[9] Jansen F, Stutzki F, Otto H J, Eidam T, Liem A, Jauregui C, Limpert J, Tünnermann A 2012 Opt. Express 20 3997
Google Scholar
[10] Haarlammert N, Vries O D, Liem A, Kliner A, Peschel T, Schreiber T, Eberhardt R, Tünnermann A 2012 Opt. Express 20 13274
Google Scholar
[11] Otto H J, Stutzki F, Jansen F, Eidam T, Jauregui C, Limpert J, Tünnermann A 2012 Opt. Express 20 15710
Google Scholar
[12] Otto H, Stutzki F, Eidam T, Limpert J, Tünnermann A 2013 Proc. SPIE 8601, Fiber Lasers X: Technology, Systems, and Applications San Francisco, California, United State, February 2−7, 2013 p86010A
[13] Hansen K R, Alkeskjold T T, Broeng J, Lægsgaard J 2011 Opt. Express 19 23965
Google Scholar
[14] Hansen K R, Alkeskjold T T, Broeng J, Lægsgaard J 2012 Opt. Letters 37 2382
Google Scholar
[15] Hansen K R, Alkeskjold T T, Broeng J, Lægsgaard J 2013 Opt. Express 21 1944
Google Scholar
[16] Smith A V, Smith J J 2012 Opt. Express 20 24545
Google Scholar
[17] Smith A V, Smith J J 2013 Opt. Express 21 2606
Google Scholar
[18] Smith A V, Smith J J 2013 Opt. Express 21 15168
Google Scholar
[19] Dong L 2013 Opt. Express 21 2642
Google Scholar
[20] 陶汝茂, 周朴, 肖虎, 王小林, 司磊, 刘泽金 2014 激光与光电子学进展 51 020001
Google Scholar
Tao R M, Zhou P, Xiao H, Wang X L, Si L, Liu Z J 2014 Laser & Optoelectroincs Progress 51 020001
Google Scholar
[21] 陶汝茂, 王小林, 肖虎, 周朴, 刘泽金 2014 光学学报 34 0114002
Google Scholar
Tao R M, Wang X L, Xiao H, Zhou P, Liu Z J 2014 Acta Optica Sinica 34 0114002
Google Scholar
[22] Tao R, Ma P, Wang X, et al. 2014 Fiber-Based Technologies and Applications Wuhan, China, 18–21 June, 2014
[23] Tao R, Ma P, Wang X, Zhou P 2015 IEEE J. Quant. Electr. 51 1
[24] Tao R, Ma P, Wang X, Zhou P, Liu Z 2015 Laser Phys. 26 065103
Google Scholar
[25] Tao R, Ma P, Wang X, Zhou P, Liu Z 2015 Laser Phys. Lett. 12 085101
Google Scholar
[26] Yang B, Zhang H, Shi C, Wang X, Zhou P, Xu X, Chen J, Liu Z, Lu Q 2016 Opt. Express 24 27828
Google Scholar
[27] Tao R, Su R, Ma P, Wang X, Zhou P 2017 Laser Phys. Lett. 14 025101
Google Scholar
[28] Tao R, Wang X, Zhou P 2018 IEEE J. Sel. Top. Quant. 24 1
Google Scholar
[29] Tao R, Xiao H, Zhang H, Leng J, Wang X, Zhou P, Xu X 2018 Opt. Express 26 25098
Google Scholar
[30] Otto H, Modsching N, Jauregui C, Limpert J, Tünnermann A 2015 Opt. Express 23 15265
Google Scholar
[31] Ward B 2016 Opt. Express 24 3488
Google Scholar
[32] Lægsgaard J 2016 Opt. Express 24 13429
Google Scholar
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