Theoretical Study of the Threshold Power of Mode Instability in High-Power Fiber Amplifiers

Rumao, 陶汝茂 Tao; Xiaolin, 王小林 Wang; Hu, 肖虎 Xiao; Pu, 周朴 Zhou; Zejin, 刘泽金 Liu

doi:10.3788/aos201434.0114002

Cited by 3 publications

(2 citation statements)

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“…Since the Ytterbium (Yb)-doped fiber (YDF) of the fiber laser oscillator to be test has a core diameter of 30 μm (NA = 0.064), the supported eigenmodes at an operating wavelength λ of 1,080 nm are LP 01 , LP 11e , LP 11o , LP 21e , LP 21o , LP 02 , LP 31e , LP 31o , LP 12e , and LP 12o . The mode filtering can be realized via certain bending and coiling of fiber [24][25][26]. After repeated experiments, we find ten eigenmodes can be limited to six by coiling the YDF in a ∞ shape with a minimum bend diameter of 85 mm.…”

Section: The MD Based On Ocfmentioning

confidence: 90%

High-speed modal analysis of dynamic modal coupling in fiber laser oscillator

et al. 2023

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Up till now, the spatial and temporal dynamics of transverse mode instability (TMI) in fiber laser oscillator have increasingly attracted a worldwide attention. Here, we develop a high-speed modal decomposition (MD) system to analyze the modal coupling for fiber laser oscillator above the TMI threshold. A set of angular-multiplexing transmission functions (TFs) are designed for simultaneous MD and monitoring the far-field beam profile. The TMI threshold of the deployed fiber laser oscillator is 181 W at a co-pumping power (CPP) of 279 W. As the CPP increases from 318 W to 397 W, the power fluctuations of the output laser become more drastic. The changes of the far-field beam profile and the centroid of far-field spot (COFFS) indicate an increased velocity of energy transfer between modes. The high-speed MD verifies above process and analyzes the modal components, indicating that the single cycle of modal coupling decreases from 11 ms to 4 ms. Otherwise, the strong mode coupling occurs between modes with relatively large weights. The high-speed MD provides a powerful tool to research the TMI effect.

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Section: The MD Based On Ocfmentioning

confidence: 90%

High-speed modal analysis of dynamic modal coupling in fiber laser oscillator

et al. 2023

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“…不稳定性产生的根源是热效应，而量子亏损是光纤激光中最主要的热源，因此，同带抽运可以提高模式不稳定现象出现的功率阈值 [39] ，理论计算表明可以提高至少 2 倍 [40] [13] 。另一方面，模式不稳定现象与光纤中的热负荷有关，可以通过增加光纤长度、降低掺杂浓度、减小抽运吸收 [41] 等减小光纤中的热负荷，提高模式不稳定现象出现的阈值。但是，对于单频和窄线宽光纤激光，增加光纤长度会降…”

Section: -unclassified

Progress of Study on Mode Instability in High Power Fiber Amplifiers

Rumao¹,

Pu²,

Hu³

et al. 2014

激光与光电子学进展

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Mode instability (MI) is an abrupt mode change when the average output power increases above a certain threshold power, which results in degradation of beam quality and currently limits the power scaling of diffraction-limited high power fiber laser. The investigation progress on MI in high power fiber amplifiers is introduced in detail. The characteristics of MI are described and the ways to mitigateMI and increase the threshold power are summarized. The development trends of study on MI in high power fiber amplifier are briefly discussed.

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Experimental study on influence of fiber numerical aperture on mode instability threshold of ytterbium fiber oscillator

Chen¹,

Liao²,

Li³

2019

Acta Phys. Sin.

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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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Theoretical Study of the Threshold Power of Mode Instability in High-Power Fiber Amplifiers

Cited by 3 publications

References 0 publications

High-speed modal analysis of dynamic modal coupling in fiber laser oscillator

High-speed modal analysis of dynamic modal coupling in fiber laser oscillator

Progress of Study on Mode Instability in High Power Fiber Amplifiers

Experimental study on influence of fiber numerical aperture on mode instability threshold of ytterbium fiber oscillator

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