湍流大气中斜程传输光场的相位特性

运用功率谱反演法对斜程传输大气湍流扰动相位屏进行了数值模拟。通过建立激光束初始畸变相位模型,从波前峰谷值和波前功率谱密度函数两个方面,对激光束通过大气湍流后的相位特性进行了研究,重点分析了湍流传输距离和天顶角对激光束波前相位分布的影响。研究结果表明:激光束在湍流大气斜程传输后,其波前相位会发生明显畸变,且空间高频相位较低频相位所占比例明显增加;通过湍流后的波前相位与传输距离及天顶角密切相关,传输距离越长,天顶角越大,相位畸变程度越大,高频相位所占比例越多。     

湍流大气中斜程传输部分相干光的光束扩展

运用维格纳分布函数与广义惠更斯菲涅耳原理,推导出部分相干光束在任意折射率起伏功率谱模型的大气湍流中斜程传输时,其束宽传输的一般解析表达式。在此基础上,以部分相干平顶(PCFT)光束为例,给出了PCFT光束在大气湍流中斜程传输的束宽扩展解析表达式。在合肥地区大气折射率结构常数高度分布模式下开展了数值计算和分析,并与水平传输时的结果进行了比较。研究结果表明,光束在斜程传输时的束宽不仅与光束阶数、空间相干度、束腰半径、湍流的强弱和传输距离有关,还与天顶角密切相关。光束的阶数越高,空间相干性越差,天顶角越小,则其束宽扩展受湍流的影响就越小。当天顶角小于60°时,大气湍流对斜程传输光束束宽的影响明显小于水平传输的情况。     

特殊函数在湍流大气传输光束漂移中的应用

研究特殊函数在激光湍流大气传输光束漂移效应中的应用,为提高跟瞄系统的瞄准精度提供理论依据。基于特殊函数和修正Von Karman谱,对斜程湍流大气传输光束漂移方差表达式进行了数值仿真。数值模拟表明,特殊函数高次项的近似舍去对激光湍流大气传输光束指向精度影响甚微。在外尺度较小时,光束漂移方差变化较快;随着外尺度的增加,光束漂移增加缓慢且趋于饱和。在相同的外尺度下,发射孔径（初始光束束径）与发射天顶角的增大对光束漂移具有抑制作用,且天顶角接近π/2,光束漂移角方差快速减小。     

斜程大气中聚焦J0相关部分相干光束的传输特性

采用部分相干光交叉谱密度理论,给出了适用于任意大气湍流条件的斜程湍流大气传输J₀相关部分相干光束在接收面内的长期平均光强分布、光束长期扩展和质量因子的解析表达式,分析了天顶角、传输距离、光源相干性以及湍流外尺度对接收面光强分布特性和光束扩展的影响.研究结果表明：在天顶角和传输距离一定的条件下,通过选择合适的光源相干性可控制焦面光强为平顶分布或中心光强为最大;在传输距离给定的条件下,随着天顶角或大气湍流外尺度的增加,焦斑光强分布均由中央凹陷分布逐渐变为高斯分布.焦面附近光强的中央凹陷比焦面的中央凹陷浅.J₀相关部分相干光束实际焦斑位置随天顶角、湍流外尺度的增加以及相干性减弱而移向发射端. 关键词： 部分相干束 大气湍流 0相关')" href="#">J₀相关 斜程传输     

部分相干厄米高斯光束在斜程大气湍流中的扩展

基于广义惠更斯-菲涅耳原理,推导出部分相干厄米高斯（H-G）光束通过斜程大气湍流传输的均方根束宽和角扩展的解析表达式,并用以研究了传输路径和光束阶数对部分相干H-G光束传输的影响。引入相对束宽和相对角扩展来定量描述光束抗拒大气湍流的能力。结果表明:在相同条件下,部分相干H-G光束斜程传输受大气湍流的影响要小于水平传输,与水平传输相比斜程传输更有利于部分相干光束在大气湍流中的传输,光束阶数越大部分相干H-G光束受大气湍流的影响越小。     

Partially coherent vortex beams propagating through slant atmospheric turbulence and coherence vortex evolution

Using the extended Huygens-Fresnel principle and the quadratic approximation of the phase structure function, and taking the Gaussian Schell-model (GSM) vortex beam as a typical example of partially coherent vortex beams, the explicit expressions for the cross-spectral density function and average intensity of GSM vortex beams with topological charge m=+1 propagating through slant atmospheric turbulence are derived, and used to study the propagation properties of GSM vortex beams in atmospheric turbulence along a slant path and evolution behavior of coherence vortices. It is shown that the spreading of GSM vortex beams along a horizontal path is larger than that along a slant path in the long atmospheric propagation. The propagation through horizontal atmospheric turbulence can be treated as a special case of the altitude-independent structure constant. The position of coherence vortices in slant atmospheric turbulence does not coincide with that in horizontal atmospheric turbulence, and the dependence of position of coherence vortices on the zenith angle, wavelength and reference point is illustrated by numerical examples. A comparison with the previous work is also made.     

Pulsed single-photon Bessel beams propagation in non-Kolmogorov turbulent atmosphere

The analytic expressions are derived for the turbulent broadening, the long-term temporal broadening, the acquisition probability of single-pulse and the transmittance probability density of a pulsed space-time Bessel photon-beam propagating along a slant path in weak non-Kolmogorov atmospheric turbulence, based on the assumption of a pulsed Bessel beam with the initial Gaussian temporal shape of pulse and diffraction-free spatial distribution. It is shown that the turbulent broadening and the long-term temporal broadening are the nolinear-increase functions of the index of non-Kolmogorov turbulence and the effect of non-Kolmogorov index on the acquisition probability of single-pulse can be approximatively described by a Gaussian function with a peak value at non-Kolmogorov index close to 3.7 for the case of the input half-pulse width greater than picosecond (ps). The transmittance of probability density is decreasing as the increasing of the structure constant of the index of refraction, the zenith angle of communication channel, the propagation path and the pulse broadening. There is turbulent diffraction for Bessel beam propagation in turbulent atmosphere, but its free-space diffraction-free characteristic is reservation.     

Polarization fluctuations of single photon states in a slant channel with non-Kolmogorov turbulence

Polarization properties of single photon states propagating through the non-Kolmogorov turbulence in a slant channel are studied based on the degree of polarization of quantum field. The degree of polarization of single photon states for linearly polarized quantum beam propagation in a slant turbulent channel are developed. Our results show that the effects of the outer scale fluctuations of atmospheric turbulence and the wavelength difference of the beams on the polarization can be ignored and the smaller inner scale of turbulence and larger zenith angle of communication channel will lead to larger fluctuations of the polarization of single-photon states. The effects of the inner scale of turbulence on polarization of single photon states are more significant for bigger refractive-index power and bigger zenith angles.     

Depolarization characteristics of incompletely polarized and partially coherent laser beams in slant atmospheric turbulence

Based on the cross-spectral density function of transmission theory and the unified theory of coherence and polarization, the depolarization characteristics of incompletely polarized and partially coherent laser propagation in slant atmospheric turbulence are investigated. According to extended Huygens–Fresnel principle, the analytical expressions for intensity and degree of polarization in the slant path are derived. The effects of the wavelength, the initial spot size and the transmission distance on the intensity and degree of polarization are described. The results show that a more stable distribution of the degree of polarization at the receiver is obtained with increasing wavelength for a certain receiver height. The conclusions play an important role in optical communications and target recognition.     

Self-transformation of a powerful femtosecond laser pulse into a white-light laser pulse in bulk optical media (or supercontinuum generation)

We present experimental and theoretical results on white-light generation in the filamentation of a high-power femtosecond laser pulse in water and atmospheric air. We have shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of the light field on the whole transverse scale of the beam, including its edges. We found that the sources of the supercontinuum blue wing are in the rings, surrounding the filament, as well as at the back of the pulse, where shock-wave formation enhanced by self-steepening takes place. We report on the first observation and demonstration of the interference of the supercontinuum spectral components arising in the course of multiple filamentation in a terawatt laser pulse. We demonstrate that the conversion efficiency of an initially narrow laser pulse spectrum into the supercontinuum depends on the length of the filament with high intensity gradients and can be increased by introducing an initial chirp. PACS 42.65.Jx; 42.65.Re; 42.25.Bs; 42.50.Hz     

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     

大气环境下用于瞄准的激光波束传播特性研究

针对激光瞄准过程中的光斑偏移现象,结合修正的Von Karman谱的折射指数起伏和Hufnagel-Vally湍流模型上的近似积分,研究了高斯脉冲光束在湍流大气中远场水平以及斜程传播时的脉冲展宽和闪烁指数,分析了强湍流条件下1.06 μm准单色光斜程大气传输光强分布与脉冲展宽的关系,对数值结果进行比较,发现远距离传输中波长和距离对波束瞄准偏差影响较大.从理论和实验上对大气环境下激光光斑瞄准偏差进行了分析研究,研究结果表明:将激光光斑全场数据及分析结果应用到现有偏差补偿算法中,可以实现激光瞄准偏差的有效补偿,在大气能见度1 km～3 km范围内,激光瞄准偏差测量误差σA≤0.1 mrad.     

Propagation of partially coherent beams carrying an edge dislocation through atmospheric turbulence along a slant path

This paper derives the explicit expressions for the average intensity, beam width and angular spread of Gaussian Schell-model (GSM) beams with edge dislocation propagating through atmospheric turbulence along a slant path. The propagation of GSM beams with edge dislocation through horizontal atmospheric turbulence can be treated as a special case through a slant one. The propagation properties of GSM beams with edge dislocation through slant atmospheric turbulence are studied, where the influence of edge dislocation parameters including the slope p and off-axis distance d on the spreading of GSM beams with edge dislocation in atmospheric turbulence is stressed. It shows that the spreading of the intensity profile of GSM beams with edge dislocation along a slant path is smaller than that along a horizontal path in the long-distance atmospheric propagation. The larger the slope vert pvert and the smaller the off-axis distance vert dvert are, the less the beam-width spreading and angular spread of GSM beams with edge dislocation are affected by turbulence. The GSM beams with edge dislocation is less affected by turbulence than that of GSM beams without edge dislocation. The results are illustrated numerically and their validity is interpreted physically.     

Continuous-wave supercontinuum laser based on an erbium-doped fiber ring cavity incorporating a highly nonlinear optical fiber

We experimentally demonstrate a novel erbium-doped fiber based continuous-wave (cw) supercontinuum laser. The laser has a simple ring-cavity structure incorporating an erbium-doped fiber and a highly nonlinear dispersion-shifted fiber (HNL-DSF). Differently from previously demonstrated cw supercontinuum sources based on single propagation of a strong Raman pump laser beam through a highly nonlinear fiber, erbium gain inside the cavity generates a seed light oscillation, and the oscillated light subsequently evolves into a supercontinuum by nonlinear effects such as modulation instability and stimulated Raman scattering in the HNL-DSF. High quality of the depolarized supercontinuum laser output with a spectral bandwidth larger than 250 nm is readily achieved.     

飞秒平顶光束经微透镜阵列在熔融石英中的成丝及其超连续辐射

实验研究了平顶激光光束经微透镜阵列在熔融石英中成丝的演化以及超连续辐射的产生,并进一步与高斯光束的成丝和超连续辐射进行了对比研究.分别对这两种光束的多丝传输进行了横向和纵向成像.结果表明,使用平顶光束可以获得更为均匀的多丝分布,成丝的起点也更为一致;尤其重要的是,相对于高斯光束,平顶光束可以使用更高的入射激光脉冲能量而不会造成介质的损伤,从而可以获得更高脉冲能量和更高转换效率的超连续辐射.     

Echo-wave scattering characteristics of the diffuse target in the slant atmospheric turbulence

The extended Huygens–Fresnel principle is utilized to make analysis of the received intensity for laser beam propagation through the atmospheric turbulence in the slant path. For a diffuse target, the effects of the turbulence on the statistical parameters such as the mutual coherence function and the mean intensity are studied in detail. The influence of the incident wave radius and the distance of the incident beam on these parameters are discussed.     

Numerical simulation of super-continuum laser propagation in turbulent atmosphere

Considering the atmospheric extinction and turbulence effects,we investigate the propagation performances of supercontinuum laser sources in atmospheric turbulence statistically by using the numerical simulation method,and the differences in propagation properties between the super-continuum(SC)laser and its pump laser are also analyzed.It is found that the propagation characteristics of super-continuum laser are almost similar to those of the pump laser.The degradation of source coherence degree may cause the relative beam spreading and scintillation indexes to decrease at different propagation distances or different turbulence strengths.The root-mean-square value of beam wandering is insensitive to the variation of source correlation length,and less aperture averaging occurs when the laser source becomes less coherent.Additionally,from the point of view of beam wandering,the SC laser has no advantage over the pump laser.Although the pump laser can bring about a bigger aperture average,the SC laser has a lower scintillation which may be due to the multiple wavelength homogenization effects on intensity fluctuations.This would be the most important virtue of the SC laser that can be utilized to improve the performance of laser engineering.     

半导体激光光栅外腔光谱合束压窄光谱的理论研究

介绍了一种减少半导体激光器阵列光栅外腔光谱合束(SBC)整体光谱展宽的方法,通过加入一组变形棱镜对传统光谱合束结构进行了改善。变形棱镜的作用实现减小半导体激光线阵输出光斑宽度,减小入射到光栅上的入射角度进而减小整体的光谱线宽。采用发光单元宽度为100 m、周期为500 m、由19个发光单元组成的常规CM-Bar条进行光栅-外腔光谱合束技术的理论推导及软件模拟,得到了光谱线宽为3.2 nm。与通过增大柱透镜焦距来减小光谱线宽的方法相比,此方法的优势是保证了整体光谱合束的整体结构在500 mm以内,使得各个发光单元有足够反馈量,抑制光束间串扰,保证合束后的光束质量和效率。     

飞秒光脉冲在光子晶体光纤中的非线性传输和超连续谱产生

采用分步傅里叶方法数值模拟了飞秒光脉冲在光子晶体光纤中非线性传输和超连续谱的产生. 计算和分析了高阶色散和非线性效应对超连续谱形状和带宽的影响. 结果表明在光子晶体光纤中产生了孤子自频移现象. 同时也发现脉冲内拉曼散射和自相位调制的联合作用导致了超连续谱中精细结构的出现. 另外，还发现高阶色散和初始光脉冲的峰值功率对超连续谱的带宽和平滑也有直接影响. 关键词： 光子晶体光纤 孤子自频移 超连续谱     

基于拉盖尔-高斯光束的通信系统在非Kolmogorov湍流中传输的系统容量

涡旋波束在大气湍流中的传输有非常重要的理论研究和实际应用意义. 本文基于利托夫近似和广义惠更斯-菲涅耳原理, 推导出拉盖尔-高斯(LG)光束在非Kolmogorov湍流中斜程传输时的螺旋谱, 并进一步推导出系统的容量. 对基于LG光束的通信系统容量进行了数值计算, 并对指数参数、光束波长、天顶角、湍流内尺度、外尺度、结构常数对系统容量的影响进行了分析比较. 本文的结论能够为LG光束在非Kolmogorov湍流中的通信提供一定的参考价值. 关键词： 拉盖尔-高斯光束 非Kolmogorov湍流 平均容量