基于目标反射回光对高功率光纤激光器影响的理论模型和数值研究

Theoretical model and numerical study of effect of target reflected light on high-power fiber laser

在高功率光纤激光系统中,常会出现激光照射到目标处产生的回光被重新耦合到激光器内部并得到放大,从而损伤激光系统的现象.对于高功率光谱合成光纤激光系统等缺乏有效回光防护的高功率激光系统,该情况尤为严重.为了解决上述问题,有必要综合整个系统链路中的多种物理效应,评估和分析反射回光对系统运转状态的影响,在设计光纤激光器时优化光路布局和系统结构,以尽量避免不必要的损失.本文基于大气传输理论、光纤速率方程和介质热传导方程模型,分析了反向回光对高功率光纤激光器的影响.研究发现,在大气条件一定的情况下,回光功率与传输距离、光轴偏移角度、光束发散角和光束中心位置偏移量等因素有关,并且会影响光纤激光器输出功率、光束质量因子、热效应和受激拉曼散射光谱信噪比.研究结果对于优化高能光纤激光系统的外光路布局和激光器内部器件系统参数设计具有一定的指导意义.

In a high-power fiber laser system,the reflected light generated when the laser hits the target may be recoupled to the laser and amplified by the laser,thereby damaging the laser system.This situation is particularly serious for a high-power laser system,such as spectral beam combining a high power fiber laser,which lacks effective light-return protection.In order to solve the above problems,it is necessary to integrate various physical effects in the whole system link,evaluate and analyze the influence of reflected light on the operating state of the system,so as to optimize the optical path layout and system structure in the beginning of the design of fiber laser to avoid unnecessary losses.Based on the atmospheric transmission theory,fiber rate equation and medium heat conduction equation model,the effect of reflected light on high-power fiber laser is analyzed.In this paper is conducted the numerical simulation of coupling efficiency of reflected light of high-power fiber laser.It is found that under certain atmospheric conditions,the reflected power is related to the transmission distance,the offset angle of optical axis,divergence angle,and the offset of center position of the beam,and will affect the output power,beam quality factor,thermal effect and the signal-to-noise ratio of the stimulated Raman scattering spectrum of the fiber laser.The coupling efficiency of reflected light has a maximum value at a certain transmission distance.For example,the reflected light power up to 140 W is obtained when the transmission distance is 1500 m,which will largely affect the laser system.The reflected power is affected by the offset angle of optical axis far less than by the transmission distance when transmission increases from 500 m to 2000 m.For example,a change of less than 0.1 W can be obtained when offset angle increases from 0.11°to 0.44°.It is also shown that as the divergence angle changes from 0 to 15'',the coupling power decays nearly exponentially with the divergence angle.The coupling efficiency is close to 1%near 12'',which is almost negligible.The output beam quality of the laser system is also affected by the beam quality of reflected light.Deterioration of the beam quality of reflected light will lead to the deterioration of the laser output beam quality.As the reflected light power enters the fiber laser system and increases,the forward output power will decrease and the backward signal power will increase,and the Raman power will increase rapidly near the fiber output end.When the reflected light is present,the thermal effects caused by selecting the gain fiber with different pump absorption coefficients are very important.The stimulated Raman scattering(SRS)rejection ratio decreases with the increase of pump absorption coefficient.For example,the SRS rejection ratio decreases by 5 dB when pump absorption coefficient increases from 1.5 dB/m to 4.5 dB/m,resulting in a decrease in the signal-to-noise ratio of the laser,which will reduce the reliability of the fiber laser system.In the design and test of spectral beam combining systems for high-power fiber lasers,the attention should be paid to the reflected light in the entire process,which includes the outer optical path and the internal optical path of the laser.The comprehensive constraints of multiple key indicators are analyzed,and the probability of system damage or reliability degradation due to reflected light is evaluated.The research results of this paper are of certain guiding significance in selecting suitable outer optical path layout and system parameters and also in optimizing the design of high energy fiber laser system.