高能激光短距离传输光束质量测量仿真与实验研究

Simulation and experimental study of beam quality measurement for short distance transmission of high power laser

准确测量光束质量对于高能激光系统性能的评估至关重要,相比传统的远距离传输测量后再修正的方法,短距离传输后的光束质量更接近激光系统出口处的真实情况。建立激光短距离传输模型,采用多层相位屏法,分析空气湍流等因素对激光远场光斑形态和光束质量的影响。结果表明,在20 m的较短传输路径上,风速、空气相对湿度、功率等影响热晕效应的因素对光束质量影响较小,常规条件范围内,光束质量因子β值劣化不超过15%,而空气湍流效应对高能激光光束质量的影响较明显,随着空气湍流强度C_(n)^(2)由1×10^(-18)m^(-2/3)增大到1×10^(-12)m^(-2/3),β值从1.0848增大至8.9933;湍流强度对不同口径激光光束的光束质量影响也不尽相同,口径越大,受湍流强度影响越大。文中搭建实物光路,使用700 mm口径平行光管进行验证,在C_(n)^(2)<1×10^(-14)m^(-2/3)的情况下,测得2.63,与模拟结果相符,实验中还发现,可以通过搅散空气的方法来减小湍流强度。实验结果表明:将空气湍流强度控制在1×10^(-14)m^(-2/3)以下,光束质量因子不小于3的高能激光系统光束质量测量偏差小于5%,为大口径激光系统出口光束质量测量提供一种解决方案。

Objective Accurate measurement of beam quality is crucial to the evaluation of laser performance,and compared with the traditional method of measuring and then correcting after long-distance transmission,the beam quality after short-distance transmission is closer to the real situation at the export of the laser.When high-energy laser propagates in the air,as the power increases,the interaction with the air will also become more obvious,mainly reflected in atmospheric turbulence and thermal blooming effects,which will affect the distortion and expansion of the spot shape,leading to a decrease in beam quality.However,current simulations and experiments on high-power large-caliber laser transmission often involve long-distance transmission to obtain target power.After accounting for the effects of atmospheric turbulence and thermal blooming on the transmission path,they then deduce beam quality information for further evaluation.Due to the complexity and randomness of atmospheric turbulence and thermal blooming,as well as the inability to obtain real-time relevant parameters of each point on the transmission optical path,the deduction amount is much greater than the true value of the beam quality under poor atmospheric conditions,resulting in significant errors.Therefore,measuring beam quality directly at the system export,given the short transmission distance,is less affected by nonlinear effects such as atmospheric turbulence and thermal blooming,and thus produces more accurate results.Methods Firstly,the theories regarding the atmospheric turbulence effect and thermal blooming effect in short-distance transmission were elaborated.Then,we simplified the optical path diagram for measuring beam quality(Fig.1),and established a model simulation based on this optical path diagram,by changing different environmental factors and conducting simulation experiments.Using a parallel light tube with a diameter of 700 mm,at a distance of 20 m,a Charge Coupled Device(CCD)was used to receive the focused spot and calculate its beam quality.While measuring the beam quality,a temperature pulsation meter was used to collect the indoor turbulence intensity during the experimental process,and methods for reducing turbulence intensity were analyzed.Results and Discussions In the case of 20 m transmission,the beam quality factor was severely affected by turbulence intensity,and the spot shape underwent significant distortion and expansion as turbulence increases(Fig.2).Further analysis was conducted on the variance of the factors under different intensities of turbulence.The variance did not exceed 0.08%under weak turbulence,approximately 12.63%under moderate turbulence,and over 100%under strong turbulence(Fig.3).For lateral winds of different speeds,the difference in factors was within 10%.For different relative humidity of the air,the difference in factors did not exceed 15%.For different apertures,factors increased with the increase of aperture size.In this case,the difference in factors between different apertures was about 15%(Fig.4).When strong turbulence intensity is reached,the difference in factors between different apertures could reach up to 80%(Fig.5).For different powers,the error in factors calculation was within 2%.The beam transmission with different initial factors was also simulated,and the beam quality measurement deviation of the laser system was less than 5%for the laser system with the air turbulence control intensity controlled below 1×10^(-14)m^(-2/3) and the beam quality β factor not less than 3(Fig.6).Different transmission distance also had an impact on the factors of laser beam quality.When the factor measurements are stable in the moderate turbulent intervals for a transmission distance of 2 m,whereas it increases approximately linearly for 20 m(Fig.7).The actual measured beam quality of the 700 mm caliber collimator(Fig.8)was consistent with the simulation results(Fig.9).Collecting indoor turbulence intensity variation maps,both static and stirred air could achieve lower turbulence intensity values(Fig.10),which means that in actual measurement processes,atmospheric turbulence intensity can be reduced by stirring air to obtain more accurate values.Conclusions This article introduces the research progress of high-power laser beam quality measurement.We separately discussed the effects of atmospheric turbulence and thermal blooming on laser transmission over short distances.After short distance transmission,the atmospheric turbulence effect has a greater impact on beam quality,and as C_(n)^(2) increases,the β factors increases from 1.0848 to 8.9933.The influence of factors such as thermal blooming effect and humidity is minimal.In case of moderate and weak turbulence,the difference in β factors for different lateral wind speeds,relative air humidity,and power is within 3%.But for different apertures,when reaching C_(n)^(2)≤10^(-14)m^(-2/3),the difference in factors for different apertures is about 15%.When strong turbulence intensity is reached,the difference in factors for different apertures can reach up to 80%.And in actual measurements,it has been verified that stirring air can also reduce atmospheric turbulence to a certain value.Therefore,it is necessary to minimize the disturbance of the air within the distance from the outlet to the beam splitter,making stirring air more practical.The factors deviation is within 8% when C_(n)^(2)≤10^(-14)m^(-2/3)at 20 m transmission distance.The beam quality degradation after transmission is different for lasers with different initial factors,and the more excellent the initial beam quality is,the more serious the degradation is.The experimental results of this paper show that the beam quality measurement deviation of the high-power,large-caliber laser system with beam quality factor not less than 3 is less than 5%by controlling the air turbulence control intensity below C_(n)^(2)<10^(-14)m^(-2/3),which provides a solution for the beam quality measurement of large-caliber laser export.