涡旋光束在水下湍流中传输的闪烁因子的实验研究

为了研究涡旋光束和高斯光束在水下湍流中的闪烁特性,搭建了一套含有水下湍流的实验系统,利用循环泵控制水槽内湍流的强弱,使用闪烁仪测量光束的闪烁因子。利用这套实验系统,详细研究了涡旋光束和高斯光束在水下传输时的闪烁因子。研究结果表明,涡旋光束和高斯光束的闪烁因子随着传输距离的增大而增大,并且随着水下湍流强度增大,涡旋光束和高斯光束对应的闪烁因子也越大。在12.6m的传播距离内,拓扑电荷m=2的涡旋光束的闪烁因子远大于高斯光束的闪烁因子。另外,在不同强度的水下湍流中,拓扑荷数m=6的涡旋光束传播到5.4m时,其径向闪烁因子都先减小然后再增大。此外,拓扑荷数m=6的涡旋光束经过一定距离的传播后,其闪烁因子低于拓扑荷数m=4的涡旋光束的闪烁因子。本文研究结果对探索涡旋光束在海洋湍流中的应用具有重要价值。     

水下拉盖尔-高斯涡旋光束及其叠加态传输特性

通过实验研究了拉盖尔-高斯涡旋光束及其叠加态在水下湍流中的传输特性,充分考虑了不同温度差和盐度差的水流扩散产生的湍流对4种光束（高斯光束,阶数为0、拓扑荷数为6的拉盖尔-高斯涡旋光束,阶数为1、拓扑荷数为2与阶数为0、拓扑荷数为6的拉盖尔-高斯涡旋光叠加,阶数为1、拓扑荷数为2与阶数为0、拓扑荷数为10的拉盖尔-高斯涡旋光叠加）传输的影响,并对4种光束的漂移方差和闪烁指数进行深入讨论与分析。实验结果表明：随着湍流强度的增大,4种光束的漂移方差和闪烁指数都增大,相比其他3种光束,拉盖尔-高斯涡旋光束的漂移方差和闪烁指数较小;在较弱的湍流强度下,两种涡旋光叠加态的漂移方差和闪烁指数与拉盖尔-高斯涡旋光束相近。     

完美涡旋光束在大气湍流中的斜程传输特性

完美涡旋（POV）光束具有光束半径与拓扑荷数无关的特点,与其他涡旋光束相比具有更加稳定的空间强度分布特性。利用多相位屏法和傅里叶变换法,分析了POV光束在大气湍流中的斜程传输特性。采用光束漂移和孔径平均闪烁指数作为大气湍流影响光束质量的评价参数,对比了POV光束与高斯涡旋光束在相同传输条件下的光束质量。结果表明：相比于高斯涡旋光束,POV光束的光束稳定性更好。当拓扑荷数增大或天顶角减小时,POV光束抵抗大气湍流的能力增强。在不改变POV光束拓扑荷数的前提下增大其光束半径,也能提高POV光束对大气湍流的抵抗能力。     

湍流大气中无衍射涡旋光束的展宽及相位奇点的演化

以经过圆形孔径截断的Bessel涡旋光束和Bessel-Gauss涡旋光束为例,数值模拟了近似无衍射涡旋光束在湍流大气中传输时引起的光束扩展和畸变光场中相位奇点的变化。仿真结果表明,与Bessel涡旋光束相比,Bessel-Gauss涡旋光束由大气湍流引起的束宽扩展较小,且在一定条件下其相位奇点代数和与入射涡旋光束的拓扑荷数保持一致;在远距离传输时,Bessel-Gauss涡旋光束相位奇点代数和的起伏偏差远小于Bessel涡旋光束。Bessel-Gauss涡旋光束在自由空间光通信中作为信息载体具有较大的优势。     

部分相干离轴涡旋光束在大气湍流中的光强分布

根据广义惠更斯-菲涅耳原理以及交叉谱密度函数,利用Kolmogorov湍流谱推导得到部分相干离轴涡旋光束在大气湍流中传输时光束的光强分布表达式,讨论了离轴距离、拓扑荷数、束腰宽度、相干长度、传输距离、湍流强度参数对光强分布的影响。仿真结果表明,拓扑荷数的大小不会改变暗核的位置;离轴距离的正负号决定了暗核移动的方向和光强集中区域的方向;暗核向上移动的距离随相干长度、束腰宽度的增大而减小,随传输距离、湍流强度的增大而增大;光斑展宽随相干长度、束腰宽度、传输距离、湍流强度的增大而增大。     

涡旋光束在湍流大气中的光束扩展

涡旋光束在湍流大气中传输时,其振幅和相位会发生随机起伏,导致在接收平面处的光强起伏及光束扩展等。以低阶拉盖尔-高斯涡旋光束为例,利用激光大气传输四维程序数值模拟了不同条件下的涡旋光束在湍流大气中传输时引起的光束扩展。由模拟结果可知,传输距离越长或湍流越强,涡旋光束在大气中传输时的束宽扩展受湍流的影响越大;涡旋光束的拓扑荷数越高、光束的束腰越小或光波的波长越长,其束宽扩展受大气湍流的影响越小。湍流的内尺度和外尺度也会影响涡旋光束的光束扩展,但影响程度相对较小。另外,通过计算仿真还比较了涡旋光束和普通高斯光束在湍流大气中传输时引起的光束扩展的差异。     

超高斯涡旋光束在空间中的传输特性

研究了超高斯涡旋光束光强最大值、光斑半径以及环围能量半径等参数随传输距离和拓扑荷数的变化规律,并与高斯涡旋光束做了比较,结果表明:超高斯涡旋光束的光斑半径和环围能量半径随拓扑荷数及传输距离呈近似线性关系;对同一拓扑荷数和传输距离,高斯涡旋光束的能量较超高斯涡旋光束要发散;当拓扑荷数较大时,超高斯涡旋光束的光斑半径比高斯涡旋光束更大。针对光束质量研究了广义光束质量因子随传输距离和拓扑荷数的变化,结果表明传输距离足够远时,拓扑荷数较小的超高斯涡旋光束具有更好的光束质量。     

贝塞尔高斯光束在各向异性湍流中的传输特性

采用随机相位屏仿真方法模拟了各向异性大气湍流及贝塞尔高斯涡旋光束在其中的强度分布、在轴闪烁指数和抖动效应,分析了各向异性湍流参数和波源参数对涡旋光束传输质量的影响.结果表明,在各向异性大气湍流中,贝塞尔高斯涡旋光束的强度分布随传输距离的变化情况与离轴距离有关,仅一级圆环处强度值单调递减,其余次级圆环处强度值均呈现先增后降的趋势.在近距离处,贝塞尔高斯涡旋光束的在轴闪烁指数随波形参数的增大而减小,随光束宽度的增大呈现先上升后下降再上升的趋势,该现象与贝塞尔高斯光束的光斑尺寸大小相关;其抖动效应随波形参数、拓扑荷数量、波长和束腰半径的增大而减弱.但在远距离处贝塞尔高斯涡旋光束的闪烁效应和抖动效应随波形参数的影响与近距离处相反,这与贝塞尔高斯光束的展宽突然增大的现象一致.贝塞尔高斯涡旋光束在各向异性湍流大气中的抖动效应小于在各向同性湍流大气中的情况,并且在远距离处大于拉盖尔高斯涡旋光束的抖动效应.     

相位屏法模拟高斯阵列光束海洋湍流传输特性

基于功率谱反演法产生海洋湍流相位屏,对多次传输过程进行统计平均,仿真分析不同海洋湍流参量下不同高斯阵列光束(矩形分布、径向分布及单束)长曝光光斑半径、光斑质心漂移特性及光强闪烁特性。结果表明:光束长曝光光斑半径、光斑质心漂移标准差及轴上闪烁系数均随湍流效应(湍流强度或传输距离)的增强而增大;同时,阵列光束与单束高斯光的光斑半径趋于一致,当传输距离继续增大时,单束高斯光束长曝光光斑半径略大。相对于单束高斯光,阵列光束在相同湍流条件下具有更小的漂移标准差,但轴上闪烁系数较大。相对于大气湍流而言,海洋湍流具有较强的闪烁效应。     

贝塞尔高斯涡旋光束在大气湍流中的传输特性

基于广义惠更斯-菲涅耳原理,推导了贝塞尔高斯涡旋光束在湍流大气中传输时系统平均光强的解析表达式,研究了贝塞尔高斯空心涡旋光束在湍流大气中的光强传输特性,同时分析了大气湍流的强弱、涡旋光束的拓扑荷等对光束质量的影响.结果表明:贝塞尔高斯涡旋光束在大气湍流中传输时,光强分布经历几个连续的变化,相位奇异性也会在传输过程中消失,该过程与涡旋光束拓扑荷的数目、光束的束腰宽度以及大气湍流的强弱等因素密切相关.拓扑荷数目高的涡旋光束在湍流大气中传输时,其奇异性的保持较拓扑荷数目低的涡旋光束要好.另外,基于桶中功率理论,分析研究了涡旋光束的拓扑荷数目、大气湍流强弱和束腰宽度对贝塞尔高斯涡旋光束在大气湍流中传输时的光束质量的影响.     

Propagation Characteristics of a Partially Coherent Gaussian Schell-model Array Vortex Beam in the Joint Turbulence Effect of a Jet Engine and Atmosphere

This work investigates the joint effects of jet engine exhaust-induced turbulence and atmospheric turbulence on the propagation of a partially coherent Gaussian Schell-model Array (GSMA) vortex beam. Using the two-process propagation method, analytical formulae are derived for the cross-spectral density, spectral density, degree of coherence, and beam width of the considered beam. The results show that the considered beam takes different shapes; when the spatial coherence is large, the spectral density of the GSMA vortex beam takes an elliptical shape, whereas when the spatial coherence is smaller, the spectral density remains a Gaussian shape. The evolution profile of the degree of coherence weakens gradually when the propagation distance, topological charge, and turbulence strength increase. Moreover, the profile of the degree of coherence takes the Gaussian profile when the propagation distance is longer or turbulence atmospheric is stronger. Furthermore, the results reveal that the corresponding beam spreads faster with a larger propagation distance, lower spatial coherence, and high-strength turbulence. This study also concludes from the results that the beam is affected more when its propagation is near the jet engine exhaust, which means that this latter has a significant impact.     

涡旋光束在湍流大气中的传输特性

根据广义的惠更斯-菲涅耳原理, 研究了涡旋光束在湍流大气中的传输特性。研究结果表明, 涡旋光束在湍流大气中传输时, 截面光强会从空心分布转化为高斯分布。光束所带的拓扑电荷数以及大气湍流均会影响光强分布的变化。研究结果还表明, 涡旋光束能够抑制大气湍流对光束扩展的影响, 这一现象得到了实验上的证实。通过杨氏双缝干涉的方法, 还研究了涡旋光束经过湍流大气传输后的拓扑电荷数。研究发现, 涡旋光束经过湍流大气后, 拓扑电荷数将发生波动。     

Intensity distribution properties of Gaussian vortex beam propagation in atmospheric turbulence

By using wave optics numerical simulation, the intensity-hole effect, beam spreading and wandering properties of Gaussian vortex beam propagation in atmospheric turbulence are investigated quantitatively. It is found that an intensity hole in the center of the beam pattern appears gradually as a Gaussian vortex beam propagates. The size of the intensity hole increases with the increase of the topological charge of the vortex phase. However, the intensity hole could to some extent be filled with optical energy by atmospheric turbulence, especially in strong turbulence. The radius of the intensity hole first decreases and then increases with the growth of turbulence strength. The effective radius of vortex beam with larger topological charge is greater than with a smaller topological charge. But the topological charge has no evident influence on beam wandering.     

The transformation of an edge dislocation in atmospheric turbulence

The explicit expression for the cross-spectral density of partially coherent elliptical Gaussian beams carrying an edge dislocation propagating through atmospheric turbulence along a slant path is derived, and used to study the transformation of the edge dislocation in atmospheric turbulence. We find that the edge dislocation disappears and transforms to a noncanonical coherence vortex, when partially coherent elliptical Gaussian beams with edge dislocation propagate through atmospheric turbulence. The inversion of the topological charge of the coherence vortex may take place. The ellipticity of the beam and slope of the edge dislocation play a dominant role in the evolution of the coherence vortex. In the coherent limit the coherence vortex in turbulence becomes an intensity vortex, however, differing from the case in free-space propagation, the position of the intensity vortex depends on the choice of the reference point. The results are illustrated analytically and numerically.     

Power coupling of Gaussian vortex beam propagation through an optical system with the Cassegrain-telescope receiver in turbulent atmosphere

Model of Gaussian vortex beam propagation through an optical system with the Cassegrain-telescope receiver in turbulent atmosphere is established. With this model, the analytical formulas of the average intensity distribution at the receiver plane are derived, and the influences of the optical topological charge, the propagation distance and the turbulence strength are numerically analyzed. These studies show that optical power at the receiver plane concentrates in an annular area, which is suitable for power coupling by the Cassegrain-telescope receiver; the optical topological charge of the vortex source need to be optimized to access the most power coupling. Under the H-V 5/7 turbulence model, power coupling efficiencies of the optical system with different parameters are calculated. Results show that in comparison with the Gaussian beams, Gaussian vortex beams have great advantages in power coupling of optical systems with the Cassegrain-telescope receivers in turbulent atmosphere, which can be a new attractive application of the vortex beams.     

Propagation of Gaussian background vortex beams diffracted at a half-plane screen

Taking the Gaussian background vortex beam with topological charge +2 as a typical example, a closed-form expression for vortex Gaussian beams passing through a half-plane screen is derived and used to study the propagation dynamics of on-axis and off-axis vortex diffracted beams, and to compare with the case of the free-space propagation. It is shown that there may exist many phase singularities or no phase singularity of vortex diffracted Gaussian beams in the diffraction field. Number and position of phase singularities are dependent on the vortex position at the source plane and propagation distance. The creation, motion and annihilation of phase singularities in the diffraction field may appear by varying the vortex position and propagation distance.     

Experimental investigation on the scintillation index of vortex beams propagating in simulated atmospheric turbulence

The scintillation index of vortex beam in simulated atmospheric turbulence is experimentally investigated. The fluctuation of the intensity can be effectively reduced by vortex beams. In particular, the reduction of scintillation is more pronounced for vortex beams with larger topological charge.     

Propagation properties of anomalous hollow beams in a turbulent atmosphere

Propagation of circular and elliptical anomalous hollow beams in a turbulent atmosphere is investigated in detail. Based on the extended Huygens–Fresnel integral, analytical formulae for the average irradiance of circular and elliptical anomalous hollow beams propagating in a turbulent atmosphere are derived. The irradiance and spreading properties of circular and elliptical anomalous hollow beams in a turbulent atmosphere and in free space are studied numerically. It is found that circular and elliptical anomalous hollow beams at short propagation distance in turbulent atmosphere have similar propagation properties to those of free space, while at long propagation distance, circular and elliptical anomalous hollow beams eventually become circular Gaussian beams in a turbulent atmosphere, which is much different from their propagation properties in free space. The conversion from an anomalous hollow beam to a circular Gaussian beam becomes quicker and the beam spot spreads more rapidly for a larger structure constant, a shorter wavelength and a smaller waist size of the initial beam.     

部分相干涡旋光束通过大气湍流上行和下行传输的比较研究

基于广义惠更斯-菲涅耳原理,以高斯-谢尔模型(GSM)涡旋光束作为典型的部分相干涡旋光束,推导出GSM涡旋光束通过大气湍流斜程传输的平均光强、均方根束宽和交叉谱密度函数的解析表达式,并用以研究了大气湍流中上行和下行对GSM涡旋光束传输和对相干涡旋的影响.结果表明,在相同条件下,GSM涡旋光束下行传输受大气湍流的影响要小于上行传输,下行传输时相干涡旋拓扑电荷守恒距离要长于上行传输.对所得结果做了物理解释. 关键词： 部分相干涡旋光束 相干涡旋 大气湍流 上行和下行传输