受激拉曼散射对高功率激光传输特性影响研究

Influence of Stimulated Raman Scattering on Propagation Properties of High-Power Laser

研究了高功率光谱组束系统内光路的气体热效应对激光远场光束质量的影响。通过同等功率密度传输的子光束与合成光束的远场光斑分布对比研究,明确了子光束中的受激拉曼散射是造成组束光路气体热效应的主要因素。当拉曼光功率密度仅为180 W/cm^(2)时,远场光斑即出现了中心强度明显下降、能量分散等劣化状态。研究了激光传输路径长度对气体热效应的影响,当光程从100 mm增加至450 mm时,远场光斑的峰值强度逐渐下降,光斑逐渐变散。通过向密封的组束装置中充入氮气,基本可以消除气体热效应对远场光束质量的影响,可作为一种有效的气体热效应抑制手段。研究成果为光谱组束激光器的光束质量优化提供了有效支撑。

Objective Spectral beam combining(SBC)is an effective method to achieve a high-power,high beam quality fiber laser.In the SBC system,multi-channel incident lasers are arranged spatially and are combined into a single laser beam via an optical element.The transmitted laser power density in the SBC system is very high due to the high power and small beam diameter.In this case,the thermal blooming effect becomes a nonnegligible factor that influences the far-field beam quality.In addition,a narrow linewidth is required in SBC to eliminate the dispersion effect.As a result,nonlinear effects are easy to stimulate,e.g.,stimulated Brillouin scattering(SBS)and stimulated Raman scattering(SRS),which generates a new wavelength laser.The new wavelength laser may cause enhanced atmospheric absorption and degrade the far-field beam quality.Atmospheric thermal blooming of high-power laser propagation has been studied extensively in the outer path(optical path in atmosphere)and inner path(optical path in the launching system).However,relevant studies of the inside optical path of a high-power laser have not been sufficiently thorough.Therefore,in this paper,the influence of thermal blooming on far-field beam quality in a high-power spectral beam combining system is studied.SRS in the incident narrow linewidth fiber amplifier is verified to be the dominant factor that induces thermal blooming in the beam combining system.In addition,N2 injection into the combining system essentially eliminates the influence of thermal blooming on far-field beam quality,which can be considered an efficient suppression method.Methods A three-channel spectral beam combining system is constructed with central wavelengths of 1064,1072,and 1084 nm,respectively.Each incident laser can deliver 2 kW power.Taking the SRS effect into consideration,we employ 1400 W and 1800 W laser power in this experiment.First,the far-field beam patterns of each incident laser at 1400 W and 1800 W are tested.Then,the far-field beam patterns of the three-channel combined beam at different power are measured.By analyzing the measured results,the causing factor of thermal blooming in SBS system is confirmed.Then,the relationship between the optical path length and thermal blooming effect is investigated by adjusting the beam splitter position of a 1084 nm laser.Finally,suppression of the thermal blooming effect by N2 injection is verified.Results and Discussions When the power of each incident laser increases from 1400 W to 1800 W,apparent degradation is observed in the far-field beam pattern[Fig.5(a)--(c)].The peak intensity degrades severely,and the beam distribution disperses badly,which reduces the laser’s focusing property.For the combined beam,when the power reaches 2000 W to 3800 W,no obvious beam quality degradation is observed[Fig.5(d)].The results demonstrate that the 1μm signal laser does not cause obvious thermal blooming effect in the SBC system.The thermal blooming effect of the incident laser at 1800 W can be attributed to the SRS light.When the incident laser power is 1800 W,the SRS is measured as 100 W.In addition,the SRS wavelength covers from 1.1μm to 1.3μm for the 1μm signal laser,which is in the strong absorption band of the H_(2)O molecule in atmosphere.Thus,air in the beam propagation path is heated to cause the thermal blooming effect.We found that optical path length has direct influence on the thermal blooming effect.When the optical path length increases from 100 mm to 450 mm,the thermal blooming effect becomes increasingly significant(Fig.6).The far-field beam patterns of the 1084 nm laser at 1400 W and 1800 W after N2 injection are measured(Fig.7).The peak intensity increases significantly from 1400 W to 1800 W,and no beam quality degradation is observed,which means that the thermal blooming effect did not occur.This result demonstrates that N2 injection can be an effective method to eliminate the thermal blooming effect in the SBC system.Conclusions In this paper,we investigate the thermal blooming effect in an SBC system.By comparing the far-field beam patterns of a sub-beam and combined beam with identical power density,stimulated Raman scattering in the incident narrow linewidth fiber amplifier is verified as the dominant factor that induces thermal blooming in the SBC system.When the power density of Raman light reaches 180 W/cm^(2),the peak intensity of the far-field beam is reduced significantly,and the energy spreads.The influence of optical path length on thermal blooming is investigated.The focusing property of the far-field beam degrades gradually and finally spreads as the length increases from 100 mm to 450 mm.By injecting N2 into the combining system to reduce H_(2)O content,the thermal blooming effect is effectively suppressed.The results presented in this paper are expected to facilitate optimization of the beam quality of high-power spectral beam combining.