【作者】 刘飞；

【导师】 季小玲；

【作者基本信息】 四川师范大学 ， 光学， 2011， 硕士

【摘要】 激光束在大气湍流中的传输理论及实验研究,对于激光通讯、激光测距、激光雷达以及激光武器等领域的应用有着重要的意义。近年来,激光列阵合成技术由于在高功率系统、惯性约束聚变和高能武器等方面的应用也越来越受到人们的关注。而实际激光束存在部分相干情况。因此,研究部分相干列阵光束在大气湍流中的传输特性是十分有意义的。本文研究了湍流对部分和完全相干双曲余弦高斯(ChG)列阵光束扩展和光束传输因子的影响。主要内容总结如下:1.研究了ChG列阵激光束通过大气湍流传输的角扩展及方向性。利用积分变换技巧推导出了ChG列阵光束通过大气湍流传输的二阶矩束宽和角扩展的解析公式,给出了ChG列阵光束与一束高斯光束具有相同角扩展的条件。研究表明:相干合成的ChG列阵光束的角扩展比非相干合成的小,但是,非相干合成的ChG列阵光束的角扩展受湍流影响比相干合成的小。此外,相干合成的ChG列阵光束的角扩展随离心参数,束腰宽度和相对子光束间距的变化均出现振荡,但湍流中的振荡减弱。非相干合成的ChG列阵光束的角扩展与相对子光束间距和光束数目无关。2.采用湍流距离(湍流距离是指大气湍流导致的光束横截面积扩展达10%时光束的传输距离)定量地研究了湍流对部分相干ChG列阵光束扩展的影响。基于广义惠更斯—菲涅耳原理,采用Rytov相位结构函数二次近似和积分变换技巧,推导出了部分相干ChG列阵光束通过大气湍流传输时湍流距离的表达式。详细研究了部分相干ChG列阵光束的湍流距离随着大气湍流强度、光束参数(即子光束数,光束相干参数,离心参数,相对子光束间距)以及光束叠加方式(即交叉谱密度函数叠加和光强叠加)的变化情况。研究表明:部分相干ChG列阵光束的光束扩展会随着大气湍流强度的增大而增大,但是当选择合适的光束参数以及光束叠加方式时,可以减小湍流对部分相干ChG列阵光束扩展的影响。3.采用光束传输因子(M~2因子)作为光束质量的评价参数,研究了ChG列阵光束通过大气湍流传输的M~2因子。利用Rytov相位结构函数二次近似和积分变换技巧推导出了ChG列阵光束在大气湍流中传输的M~2因子的解析公式,并采用相对M~2因子研究了湍流对M~2因子的影响。研究表明:在大气湍流中M~2因子不再是一个传输不变量,湍流使得M~2因子增大。非相干合成情况下,M~2因子随着传输距离、离心参数、相对子光束间距和子光束数目的增大而增大。相干合成情况下,M~2因子随离心参数和相对子光束间距的增大呈现振荡上升。相干合成情况下的M~2因子比非相干合成的要小。然而,非相干合成情况下的M~2因子受湍流影响比相干合成的要小。特别地,相干合成情况下,选取适当的相对子光束间距可以减小湍流对M~2因子的影响。此外,随着子光束数目的增大,相干合成的M~2因子受湍流影响增大,而非相干合成的M~2因子受湍流影响减小。更多还原

【Abstract】 The theoretical and experimental study on the laser beams propagating through atmospheric turbulence is of great significance for the applications of laser communications, laser ranging, laser radar, laser weapons, and so on. In recent years, the laser array combining technique has been paid more and more attention for its applications in high-power system, inertial confinement fusion, high-energy weapons, and so on. In practice, partially coherent beams are often encountered. Therefore, it is very meaningful to study the propagation properties of partially coherent array beams in atmospheric turbulence. The influence of turbulence on the spreading and beam propagation factor of the partially coherent and fully coherent cosh-Gaussian(ChG) array beams has been studied in this thesis. The main works are summarized as follows:1. The angular spread and directionality of ChG array beams propagating through atmospheric turbulence are studied. The closed-form expressions for the mean-squared beam width and the angular spread of ChG array beams propagating through atmospheric turbulence are derived by has the same directionality as one single Gaussian beam is given. It is shown that the angular spread of ChG array beams for the coherent combination is smaller than that for the incoherent combination. However, the angular spread of ChG array beams for the incoherent combination is less sensitive to turbulence than that for the coherent combination. In addition, The angular spread of ChG array beams for the coherent combination exists oscillatory behavior with the changes of the decentered parameter, the waist width and the relative separation distance of beams. However, the oscillatory behavior becomes weaker in turbulence. The angular spread of ChG array beams for the incoherent combination is independent of the relative separation distance of beams and the beam number.2. The influence of turbulence on the beam spreading of partially coherent ChG array beams is studied quantitatively by the turbulence distance which represents the distance at which the spreading due to the turbulence accounts for 10% of the cross-sectional area of the beam. Based on the extended Huygens-Fresnel principle, the expression for the turbulence distance of partially coherent ChG array beams propagating through atmospheric turbulence is derived by using the quadratic approximation of Rytov’s phase structure fuction and integral transform technique. The changes of the turbulence distance versus the spatial power spectrum of the refractive index fluctuations, the beam parameters (i.e., the beam number, the beam coherence parameter, the decentered parameter, the relative separation distance of beams) and the type of the beam superposition (i.e., the superposition of the cross-spectral density function and the superposition of the intensity) are studied in detail. It is showed the turbulence distance of the partially coherent ChG array beams will increase with the the spatial power spectrum of the refractive index fluctuations, but the effect of turbulence on the spreading of partially coherent ChG array beams can be reduced by choosing the suitable beam parameters and the suitable type of the beam superposition.3. The beam propagation factor(M~2-factor)is taken as the characteristic parameter of beam quality, and the M~2-factor of ChG array beams propagating through atmospheric turbulence is studied. The analytical formula for the beam propagation factor (M~2-factor) of ChG array beams propagating through atmospheric turbulence is derived by using the quadratic approximation of Rytov’s phase structure fuction and integral transform technique, and the influence of turbulence on the M~2-factor is studied by using the relative M~2-factor. It is shown that the M~2-factor is not a propagation invariant in turbulence, and the turbulence results in an increase of the M~2-factor. For the incoherent combination, the M~2-factor of ChG array beams increases with increasing the propagation distance, the beam parameter, the relative beam separation distance and the beam number. For the coherent combination, the M~2-factor of ChG array beams increases with the oscillatory behavior as the beam parameter or the relative beam separation distance increases. For the coherent combination the M~2-factor is always smaller than that for the incoherent combination. However, for the incoherent combination the M~2-factor is always less sensitive to turbulence than that for the coherent combination. In particular, the influence of turbulence on the M~2-factor can be reduced by a suitable choice of the relative beam separation distance. In addition, with increasing the beam number, the M~2-factor is more sensitive to turbulence for the coherent combination, while for the incoherent combination the M~2-factor is less sensitive to turbulence.更多还原