强激光长程传输中受激拉曼散射对光束特性的影响

EFFECTS OF STIMULATED RAMAN SCATTERING ON HIGH POWER LASER BEAM PROPAGATION IN LONG-PATH AIR

依照受激拉曼散射(SRS)半经典理论从物质激发感应出的三阶非线性极化率X^(3)出发,在强激光长程传输受激拉曼散射中建立了泵浦(传输)激光相位与斯托克斯(Stokes)光强相耦合的理论模型。它清楚地说明了SRS效应影响泵浦激光光束特性,其中包括时间特性及空间特性(光束质量)的物理机制及影响因素。理论分析与实验结果符合得较好。实验还测得了不同转换效率η_s情况下传输后的泵浦光及Stokes光的时间过程特性。结果表明,在小转换效率的条件下,泵浦光时间特性基本不受SRS效应的影响;而当转换效率η_s>5％时,泵浦光脉宽随η_s增大而被逐渐压缩。

A theoretic model is made to interpret how the phase of the pump laser beam couples with the intensities of Stokes waves produced by the pump laser beam transimitting in a long path air, This model has clariffied the physical mechanism of how the stimulated Raman scattering (S RS) influences the quality of the transimitted pump laser beam. the results of the theoretic research have shown it is the SRS and the nonhomogeneity of the pump laser beam that make the wave plane of the transimitted pump laser beam distorted, resulting in its quality degradation. This conclusion agrees qualitatively with the experimental results in which the far-field divergency angle of the transimitted pump laser beam θp increases with the increment of the Stokes energy conversion ratio ηs. In addition, the experimental results also show that the pulse duration of the transimitted pump laser beam is almost unaffected by SRS when the ηs <5%. but becomes compressed and ' Red shift' takes place in the pulse tail of the transimitted pump beam when ηs>5%.