【作者】 孙运强；

【导师】 陆启生； 陈洪斌；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2011， 博士

【摘要】 激光在内通道中传输时,通道内的气体会吸收激光能量,引起气体温度的起伏进而导致气体折射率的变化,形成气体热效应。激光在远距离传输过程中,气体热效应会降低激光到达远场时的能量集中度。因此,国内外对激光系统内通道中气体热效应的研究都非常重视。本文在已有关于气体热效应的研究基础上,采用简化的理论分析模型,对内外通道中的气体热效应进行了研究。对于外传输通道,研究了信标自适应校正所引入的聚焦非等晕误差和角度非等晕误差对激光远距离传输的影响。对于内传输通道,在有管道轴向流存在的情况下,理论上建立了内通道气体热效应的缩比定标规律,确定了描述内通道气体热效应的能量定标参数N_E = n_TαPLR²/ρC_pεπR₀²和几何定标参数N_G= vR²/εL。同时,分析了管道半径与中空光束外半径之比对气体热效应的影响。运用现有流体力学计算软件FLUENT提供的用户自定义子程序功能,本文建立起一套较为完整的光场与流场耦合相互作用数值仿真平台。仿真模型中,利用自编程序对通道内的光传输方程进行求解;对离散流场控制方程,则采用现有的计算流体力学软件FLUENT进行求解,通过用户自定义子程序功能完成光场与流场之间的耦合。考虑到试验中气体吸收系数难以准确测量,本文通过比较理论解析计算结果和数值仿真计算结果的方法,验证了该仿真平台的可靠性。依托该数值仿真平台,对理论分析得到的内通道气体热效应缩比定标规律进行了验证。结果表明,已建立的内通道气体热效应缩比定标规律,不仅适用于直圆型管道结构,对于较为复杂的“L”型管道结构,内通道气体热效应缩比定标规律仍然成立。对复杂光传输通道内的气体热效应研究表明,气体速度在管道横截面内的分布是非常不均匀的,在有光场存在的情况下,非均匀的速度分布改变了通道内的气体温度分布,最终导致了管道出口处光场相位的非均匀分布。为了削弱光强非均匀分布对内通道出口光束质量的影响,研究了光束强度旋转情况下的内通道气体热效应问题。仿真结果表明,光束强度旋转不改变气体热效应到达稳态的时间,光程差中沿光轴方向对称的离焦项和球差项也保持不变,但光束强度旋转使光强不均匀分布带来的二阶像散项基本消失。因此,采取光束强度旋转的方式,在一定程度上可以削弱非均匀光强气体热效应引起的光程差的非对称分布。通过在已建立的气体热效应仿真平台中引入自适应共轭校正模型,研究了共轭相位自适应校正对激光远场峰值斯特列尔比的影响。得到了内通道气体热效应自适应共轭校正的阈值特性。在不考虑镜面热变形对光传输影响的情况下,本文还仿真分析了单独加热镜面、单独加热气体以及镜面和气体同时加热时通道内气体的温度分布。仿真结果表明,激光加热气体引起的光程差以活塞项和离焦项为主,激光加热镜面导致气体温升引起的光程差以高频成份为主。在同时考虑镜面和气体加热效应时,激光加热镜面导致气体温升引起的光程差与激光加热气体引起的光程差,二者基本是叠加的。针对在内通道中注入低吸收系数气体的热效应处理方式,开展了内通道气帘以及在通道内注入不同气体成分的数值分析和试验研究。从气帘的数值仿真和试验结果看,用气帘作为通道的密封设备,对激光内传输通道进行抽真空充氮气处理,能够削弱通道内气流不均匀而引起的附加光程差,而气帘本身的气流不均匀引起的光程差也较小。在有轴向气流存在的情况下,分别以室内空气、氮气以及氮氧混合气作为注入气体,进行了相应的内通道气体热效应试验。通过对比分析可以看出,激光在室内空气中的气体热效应非常严重,而在氮气或者氮气与氧气混合气体中传输时,由于气体吸收激光的能量减小,使得内通道中的气体热效应变得很弱,气体热效应引起的光程差远小于分光镜的透射热变形引起的光程差。因此,在激光内传输通道中注入低吸收系数的氮气或者氮气与氧气混合气体的气流,是一种比较有效的内通道气体热效应处理方式。更多还原

【Abstract】 As a laser beam propagates through a pipe in which the air would absorbe some energy of the laser beam, the absorbed laser energy heats the air and form a distributed thermal blooming effects along the heated air path. As it can limit the maximum power that can be efficiently transmitted through the air, thermal blooming is one of the most serious problems encountered in the propagation of laser. Many theoretical and experimental works have been concerned with it.In this paper, based on the available research of the blooming thermal effect, thermal effects of the inside and outside channel are analyzed by the simplified model. For the outside laser path, the expressions of the angular anisoplanatic error and the focus anisoplanatic error are derived for the beacon adaptive correction system. For the laser beam path indoor, using the simplified straight round pipe model with the axial pipe flow, scaling laws for the thermal effects of the beam path indoor is derived theoretically, the description of the energy scaling parameters N_E = n_TαPLR²/ρC_pεπR₀²and geometric scaling parameters N_G= vR²/εL are expressed. The effects of the ratio of the pipe radius to the beam radius on the thermal blooming are also analyzed based on the flattened hollow gaussian beam.Using the user-defined function provided by the fluid dynamics software FLUENT, a more completed simulation platform for the laser interacting with the fluid is established. In the simulation platform, the laser propagation equation is solved by the self-compiled program. For the discrete equations of the flow field, the fluid dynamics software FLUENT is used. The coupling effects between the flow field and the laser are solved by the user-defined function.In view of the uncertainty of the gas absorption coefficient in the experiment, the theoretical analysis is compared with the numerical simulation results for the simple mode of the straight pipe, and it is verified that the results derived from the simulation platform are reliable. Based on the numerical simulation platform, scaling law of gas thermal effect of the inside channels is verified by simplified theoretical analysis.The result shows that the established scaling law not only can be applied for straight circular pipe structure, but also for the more complex“L”shaped pipe structure.By studying gas thermal effect in the complex channels for laser propagation, it is shown that the gas velocity distribution in the pipe cross-section is very nonuniform, and furthermore the nonuniform velocity distribution changes the gas concentration distribution in channels and finally brings about optical phase distribution into the“double fishes eyes type”. In order to weak the affect of the beam distribution to the beam quality, the problem on the gas thermal effect of the rotation beam in the inside channels is simulated and calculated based on the rotation beam model mentioned in the previous literatures,. Simulation results show that the beam rotation does not change the time duration which the gas thermal effect reaches steady state, and symmetrical defocus and spherical aberration along the optical axis remains constant. But the second-order astigmatism caused by the uneven beam intensity distribution is disappeared. Therefore, the way of the beam rotation can weaken the asymmetrical distribution of optical path difference caused by the thermal blooming of asymmetrical beam intensity distribution.By introduction of adaptive conjugate correction model to the established simulation platform of thermal effects, the change of the peak far-field Strehl-ratio with output is studied. The threshold property of conjugate adaptive phase correction for the thermal effects is obtained. Without considering the thermal deformation of the mirror, alone heating effect of the mirror, alone heating effect of the gas and heating effect of simultaneously considering mirror and gas are separately simulated. Simulation results show that the optical path difference caused by the heating of gas is meanly composed by the piston items and focus items, the optical path difference caused by the heating of mirror is meanly composed by the high frequency component. Taking into account of the gas heating effect and the mirror heating effect synchronously, the optical path difference caused by the heating effect of gas and mirror can be linearly superposed.Acording to the thermal effect treatment manner of pipe filled with low absorption coefficient gas, the numerical and experimental studies on air curtain and thermal effects of different gas are carried out. The results indicate that the method of vacuum pipe filled with low absorption coefficient gas with the air curtain as the seals can reduce the optics phase difference caused by the heterogeneous beam intensity distribution. However the optics phase difference induced by the air curtain is small. With the axial flow exists, with the air, nitrogen and nitrogen-oxygen mixture as the injected gas respectively, the corresponding thermal effects experiments in the pipe are carried out. The results indicate that the heating effect of air is serious as the laser transmitts through the pipe, while the thermal effects are very weak as the laser transmitts though the pipe filled with nitrogen or nitrogen and oxygen mixed gas. The optical path difference caused by thermal effect of the gas is much smaller than that caused by the permeating thermal deformation of the splitter. Therefore, the transmission path of the laser system filled with the low absorption coefficient of nitrogen or nitrogen and oxygen mixed gas flow will be a more effective thermal treatment manner.更多还原