云南高美古湍流大气中激光短曝光光斑统计特性研究

Statistical Properties of Short-Exposure Laser Patterns in Turbulent Atmosphere of Gaomeigu,Yunnan Province

为了探究激光在自由高空湍流大气中传输的短曝光光斑特性,在云南高美古3200 m的高海拔地区,选择远离下垫面的地势条件,开展了532 nm固体激光1,2,3,4,4.7 km大气传输光斑成像探测实验。利用4000 frame/s采样速率下的短曝光光斑数据,定性地描述了湍流大气中短曝光远场激光光斑的网纹状形态结构以及零星亮点的移动、消散和重构过程,并对激光光斑灰度值进行了定量的统计分析。结果表明:利用二维的光斑图像数据,可以较为方便地获得光强闪烁的孔径平均因子和理想的点闪烁指数,进而得到包括湍流强度和湍流内尺度在内的关键光学湍流参数;同时,也可对激光闪烁频谱、截止频率以及光斑的空间相关特性进行定量的判断。相关数据有助于研究高原或高空长程传输条件下的激光能量在目标靶面上的空间分布及其时间变化特性,为研究激光长程大气传输后光电系统的变化提供了一定的参考。

Objective When laser propagates in the atmosphere,it is inevitable that a random change of the refractive index of the atmospheric turbulence causes laser spot broken and distorted,which seriously damages the laser beam quality.Under the conditions of weak-and strong-fluctuating media,the turbulence model has relatively rich theoretical research results.In view of the complexity of the turbulence itself,the current theory of light transmission in random media is not enough to perfectly present the far-field transmission characteristics of the laser spot in the strong-fluctuation region.As for the high-altitude laser transmission engineering applications,the spatial distribution of laser energy on the target surface and its temporal variation characteristics are vital for photoelectric system evaluation.However,the research of long-range high-altitude laser atmospheric transmission faces the following problems:first,there are few transmission tests in high-altitude areas;second,short-exposure laser spot data at high frame-rate sampling are rare;third,two-dimensional imaging methods to obtain more comprehensive laser statistical parameters are still lacking.In this study,we use a high-speed camera to perform laser spot tests at different distances in high-altitude areas.Using the laser short-exposure image data at sampling rate of 4000 frame·s-1,the change characteristics of short-exposure far-field laser spots in the turbulent atmosphere are qualitatively described,and the optical turbulence parameters such as ideal point scintillation indexes,aperture average factors,and inner scales are calculated.Our aim is to use the external field laser transmission test data to understand the spatial laser energy distribution on the target surface as well as its temporal variation characteristics under highaltitude conditions and to simultaneously determine its impact on the laser photoelectric system.Methods The experiment is performed at an altitude of 3200 m in Gaomeigu,far from the underlying surface.The transmitting terminal is a 532 nm solid-state laser,while the receiving system is consisted of a high-speed CCD camera and a diffuse reflection screen.The camera is placed obliquely in front of the screen at~9 m,the transmitter is located in the balcony in the third floor of the Gaomeigu radar station at~10 m above the ground,and the receiver is located on the open ground at distances of 1,2,3,4,and 4.7 km from the transmitting terminal.The selected geographical location can ensure that most of the transmission paths are longer than 15 m from the underlying surface and the maximum height is larger than 100 m.In addition,the test is also performed in the canyon,far away from the underlying surface.The straight-line distance between two ends is~2 km,and the maximum link distance from the underlying surface is~300 m.The sampling rate of laser spot images is unified to4000 frame·s-1,each image is 320 pixel×240 pixel,and the image accuracy is 8 bit.For the acquired twodimensional data of the laser spot,using the imaging method,the scintillation index under different receiving apertures is calculated,and subsequently polynomial fitting is used to obtain the ideal point scintillation index.With the former results,the refractive index structural parameters such as the path,aperture average factor,and inner scale can also be derived.Moreover,this method based on the calculation of the gray value of the light spot can also display the correlation coefficient between the power spectrum and its scaling rate as well as the spatial light intensity distribution.Results and Discussion At a distance of 1 km,with the playback of a short-exposure spot image at a sampling rate of up to 4000 frame·s-1,the broken laser spot shows a clear mesh-like morphological structure,and a few sharp bright spots are scattered.There are irregular shapes and different size holes inside the spot(Fig.3).As time elapses,these flowing mesh-like structures and scattered bright spots undergo deformation and reconstruction.There is a certain unclear relationship between size of the reticulated texture and the scale of the turbulence.Using the light spot gray value data,the aperture average factor of different receiving radii on the target surface is obtained.The variation of the laser spot aperture average factor with the receiving aperture shows that the aperture average factor is not only a function of the aperture but also is related to the turbulence state(Fig.6).The daily variation of Cn2 in high-altitude areas is relatively stable,showing a different trend from the"Mexican hat"trend near the ground at low altitudes(Fig.7).The power spectral distributions of laser scintillation under different transmission distances show that the cutoff frequency of the power spectrum of a single pixel is generally higher than the average cutoff frequency of the circular domain,which indicates that the aperture smoothing effect of light intensity scintillation still exists in the frequency domain.The scaling rate obtained from the power spectrum shows a positive correlation trend with the ideal scintillation index(Figs.9 and 11).Taking the spot centroid as the point of origin,the normalized spatial correlation coefficient of the light intensity fluctuations at the distance from the origin point shows that the spatial correlation radius increases with the increase of the scintillation index(Fig.12).It is a completely different concept from the coherence length.Conclusion The main purpose of designing this experiment is to find the transmission conditions far away from the underlying surface in high-altitude areas and to obtain the characteristics of the laser spot transmitted in high-altitude free atmospheric turbulence.Judging from the fact that there is no obvious"Mexican hat"structure in the daily variation trend of most transmission paths,the underlying surface on the transmission path has little influence,but there are still big differences between the real test conditions and high-altitude"free"atmosphere.In the future,we hope that we will have the opportunity to conduct high frame-rate laser spot image testing on higher and farther platforms to promote the understanding of laser transmission in free turbulent atmospheres.