数值孔径对掺镱光纤振荡器模式不稳定阈值影响的实验研究
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摘要
模式不稳定效应已经成为高功率光纤激光器中限制输出功率和光束质量进一步提升的最大障碍.用不同数值孔径的20/400阶跃折射率分布掺镱光纤搭建了光纤振荡器,并测量了它们的光光效率和模式不稳定阈值.实验结果表明,在同等注入抽运功率和抽运波长为976 nm的前提下,具有较低数值孔径的光纤尽管光光效率低于高数值孔径光纤的,但却表现出更高的模式不稳定阈值.这一结果表明数值孔径对光纤振荡器中的模式不稳定阈值有着显著的影响.因此,对于光纤振荡器的模式不稳定抑制而言,适当地优化降低数值孔径是一个抑制模式不稳定效应,进一步提升光纤振荡器模式不稳定阈值的方法,对于进一步提升光纤振荡器的输出功率和光束质量,有着明显的现实意义.
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 976 nm 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.
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 976 nm 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.
引文
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[29]Tao R,Xiao H,Zhang H,Leng J,Wang X,Zhou P,Xu X2018 Opt.Express 26 25098
[30]Otto H,Modsching N,Jauregui C,Limpert J,Tünnermann A2015 Opt.Express 23 15265
[31]Ward B 2016 Opt.Express 24 3488
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[2]Richardson D,Nilsson J,Clarkson W 2010 JOSA B 27 B63
[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
[4]Eidam T,Wirth C,Jauregui C,Stutzki F,Jansen F,Otto HJ,Schmidt O,Schreiber T,Limpert J,Tünnermann A 2011Opt.Express 19 13218
[5]Jauregui C,Eidam T,Limpert J,Tünnermann A 2011 Opt.Express 19 3258
[6]Smith A V,Smith J J 2011 Opt.Express 19 10180
[7]Ward B,Robin C,Dajani I 2012 Opt.Express 20 11407
[8]Jauregui C,Eidam T,Otto H J,Stutzki F,Jansen F,Limpert J,Tünnermann A 2012 Opt.Express 20 440
[9]Jansen F,Stutzki F,Otto H J,Eidam T,Liem A,Jauregui C,Limpert J,Tünnermann A 2012 Opt.Express 20 3997
[10]Haarlammert N,Vries O D,Liem A,Kliner A,Peschel T,Schreiber T,Eberhardt R,Tünnermann A 2012 Opt.Express20 13274
[11]Otto H J,Stutzki F,Jansen F,Eidam T,Jauregui C,Limpert J,Tünnermann A 2012 Opt.Express 20 15710
[12]Otto H,Stutzki F,Eidam T,Limpert J,Tünnermann A 2013Proc.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,L?gsgaard J 2011Opt.Express 19 23965
[14]Hansen K R,Alkeskjold T T,Broeng J,L?gsgaard J 2012Opt.Letters 37 2382
[15]Hansen K R,Alkeskjold T T,Broeng J,L?gsgaard J 2013Opt.Express 21 1944
[16]Smith A V,Smith J J 2012 Opt.Express 20 24545
[17]Smith A V,Smith J J 2013 Opt.Express 21 2606
[18]Smith A V,Smith J J 2013 Opt.Express 21 15168
[19]Dong L 2013 Opt.Express 21 2642
[20]Tao R M,Zhou P,Xiao H,Wang X L,Si L,Liu Z J 2014Laser&Optoelectroincs Progress 51 020001(in Chinese)[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]
[21]Tao R M,Wang X L,Xiao H,Zhou P,Liu Z J 2014 Acta Optica Sinica 34 0114002(in Chinese)[陶汝茂,王小林,肖虎,周朴,刘泽金2014光学学报34 0114002]
[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.26065103
[25]Tao R,Ma P,Wang X,Zhou P,Liu Z 2015 Laser Phys.Lett.12 085101
[26]Yang B,Zhang H,Shi C,Wang X,Zhou P,Xu X,Chen J,Liu Z,Lu Q 2016 Opt.Express 24 27828
[27]Tao R,Su R,Ma P,Wang X,Zhou P 2017 Laser Phys.Lett.14 025101
[28]Tao R,Wang X,Zhou P 2018 IEEE J.Sel.Top.Quant.24 1
[29]Tao R,Xiao H,Zhang H,Leng J,Wang X,Zhou P,Xu X2018 Opt.Express 26 25098
[30]Otto H,Modsching N,Jauregui C,Limpert J,Tünnermann A2015 Opt.Express 23 15265
[31]Ward B 2016 Opt.Express 24 3488
[32]L?gsgaard J 2016 Opt.Express 24 13429