Abstract:Based on the thermal blooming effect theory for high-energy laser propagating in atmosphere, the vector model concept of thermal distortion parameter $ {{\boldsymbol{N}}_{\text{D}}} $ is put forward. Based on the vector model concept of thermal distortion parameter and the laser system simulation software EasyLaser, the scaling law between the centroid offset of laser beam farfield and the vector thermal distortion parameter is simulated and analyzed. The simulation results indicate that the centroid offset quantity is in direct proportion to the modulus of vector thermal distortion parameter $ {{\boldsymbol{N}}_{\text{D}}} $, and the centroid offset direction is opposite to the direction of vector thermal distortion parameter $ {{\boldsymbol{N}}_{\text{D}}} $. Based on the scaling law, by real-time measuring the atmosphreic parameters on laser beam propagation path, the beam deviation of laser system can be conveniently estimated in practical application. Keywords:thermal blooming effect/ beam deviation/ thermal distortion parameter/ vector model
激光系统实际应用场景下, 传输路径上自然风速风向是随机变化的. 取激光发射口径$D = 0.9 \;{\rm{m}}$, 平台光束或中心遮拦比$ \varepsilon = 0.4 $的环形光束, 光束传输路径自然风速和风向、大气吸收及消光特性如图2所示, 通过对不同距离处自然风向$ {V_{\text{d}}} $加载随机扰动, 并调节发射功率P, 获得不同的热晕效应, 评估远场光斑质心偏移与热畸变参数$ {{\boldsymbol{N}}_{\text{D}}} $的规律关系. 仿真分析时自然风向$ {V_{\text{d}}} $随机扰动的RMS值约5°—30°. 图5和图6是平台光束远场光斑质心偏移随矢量热畸变参数$ {{\boldsymbol{N}}_{\text{D}}} $的变化规律. 图5仿真计算时取自然风向廓线A, 图6仿真计算时取自然风向廓线B. 图中黑线是理论公式(9)的计算结果, 离散点表示不同发射功率和自然风向$ {V_{\text{d}}} $随机扰动时的模拟结果. 由图5和图6可看出: 针对光束传输路径自然风速风向随机变化的非均匀分布, 采用热畸变参数矢量模型, 远场光斑质心偏移与矢量热畸变参数$ {{\boldsymbol{N}}_{\text{D}}} $模的大小呈近线性增长, 质心偏移方向与矢量热畸变参数$ {{\boldsymbol{N}}_{\text{D}}} $的方向相反. 质心偏移模拟值与理论值的偏差的RMS值约小于0.2 μrad. 图 5 自然风向廓线A条件下平台光束光束偏折与矢量热畸变参数$ {{\boldsymbol{N}}_{\text{D}}} $的变化规律 (a) X方向质心偏移; (b) Y方向质心偏移; (c) 质心总偏移 Figure5. Relation between centriod offset of flat circular beam and thermal blooming parameter $ {{\boldsymbol{N}}_{\text{D}}} $ while wind direction outline A is used: (a) Centriod offset in X axis; (b) centriod offset in Y axis; (c) all centriod offset.
图 6 自然风向廓线B条件下平台光束光束偏折与矢量热畸变参数$ {{\boldsymbol{N}}_{\text{D}}} $的变化规律 (a) X方向质心偏移; (b) Y方向质心偏移; (c) 质心总偏移 Figure6. Relation between centriod offset of flat circular beam and thermal blooming parameter $ {{\boldsymbol{N}}_{\text{D}}} $ while wind direction outline B is used: (a) Centriod offset in X axis; (b) centriod offset in Y axis; (c) all centriod offset.
图7和图8是中心遮拦比$ \varepsilon = 0.4 $的环形光束远场光斑质心偏移随矢量热畸变参数$ {{\boldsymbol{N}}_{\text{D}}} $的变化规律. 图7仿真计算时取自然风向廓线A, 图8仿真计算时取自然风向廓线B. 图中黑线是基于(9)式的计算结果, 离散点表示不同发射功率和自然风向$ {V_{\text{d}}} $随机扰动时的仿真计算结果. 图7和图8所得结论与图5和图6相同. 质心偏移模拟值与理论值的偏差的RMS值约小于0.3 μrad. 图 7 自然风向廓线A条件下环形光束光束偏折与矢量热畸变参数$ {{\boldsymbol{N}}_{\text{D}}} $的变化规律 (a) X方向质心偏移; (b) Y方向质心偏移; (c) 质心总偏移 Figure7. Relation between centriod offset of hollow circle beam and thermal blooming parameter $ {{\boldsymbol{N}}_{\text{D}}} $ while wind direction outline A is used: (a) Centriod offset in X axis; (b) centriod offset in Y axis; (c) all centriod offset.
图 8 自然风向廓线B条件下环形光束光束偏折与矢量热畸变参数$ {{\boldsymbol{N}}_{\text{D}}} $的变化规律 (a) X方向质心偏移; (b) Y方向质心偏移; (c) 质心总偏移 Figure8. Relation between centriod offset of hollow circle beam and thermal blooming parameter $ {{\boldsymbol{N}}_{\text{D}}} $ while wind direction outline B is used: (a) Centriod offset in X axis; (b) centriod offset in Y axis; (c) all centriod offset.