光的偏振对热光关联成像的影响

根据完全偏振光的不同偏振态和部分偏振光的不同偏振度对二阶关联函数的影响,利用统计光学理论研究了光的不同偏振性质对关联成像的可见度和信噪比的影响.研究表明,光的偏振对热光关联成像的影响体现在部分偏振光的不同偏振度上,关联成像的可见度是偏振度的二次函数,由二阶关联函数方法得到的信噪比随着偏振度增大而提高,而利用减去背景项的二阶关联函数以及二阶相干度方法得到的信噪比均与偏振度无关. 关键词： 关联成像 偏振 可见度 信噪比     

基于光场一阶关联的时域成像

与通常利用二阶强度关联测量实现时域鬼成像不同,本文利用时域热光源借助干涉仪通过一阶关联实现时域成像.基于空域光束的近轴衍射和时域窄带脉冲在色散介质中色散之间的空间-时间二象性,在时域脉冲响应函数的基础上得到了表征一阶关联时域成像的强度表达式,分析研究了光源脉冲宽度和相干时间对成像可见度和分辨率的影响.结果一方面表明基于热光场一阶关联的时域成像在不需要额外色散补偿或消除条件下可以实现时域物体信号的再现,另一方面表明当光源脉冲宽度一定时,成像可见度随光源脉冲相干时间的增加而增加,但是成像分辨率逐渐降低,其中当光源脉冲宽度约为100 ps,相干时间约为0.5 ps时,间隔为20 ps,宽度为8 ps的时域矩形波型物体的成像质量(兼顾可见度和分辨率)较好.该结果对于基于热光一阶关联的时域成像在时序信号测量中的应用具有重要意义.     

大气湍流对航空光电侦察图像质量影响因素分析北大核心CSCD

大气湍流效应是严重影响航空光电侦察系统图像质量的重要因素之一。从大气湍流参数描述出发,研究了大气湍流对光学系统成像质量的影响机理,分析了大气湍流影响光学系统调制传递函数(modulation transfer function,MTF)的影响因素,建立了大气湍流影响光学系统成像MTF的理论模型。仿真结果表明,在大气湍流影响下,光学系统光瞳口径与大气相干直径的比值对光学系统成像MTF影响较大。通过光学系统地面外场实验图像测试对比,验证了实际光学系统受到大气湍流影响后的成像MTF理论模型。研究结果可为大气湍流影响下的航空光电侦察系统设计及提升其成像质量提供理论支持。     

强度关联三维衍射层析的实验研究

提出了一种基于强度关联成像的新型三维衍射层析技术.利用强度关联成像技术(鬼成像)可实现无透镜傅里叶变换的特点,并结合衍射层析技术和二步相位恢复算法,使用波长为650 nm的赝热光实现了强度关联三维衍射层析.详细描述了强度关联三维衍射层析的基本原理以及具体实验结果,为在第三代同步辐射光源实现非相干X光衍射成像积累了经验.     

大气湍流畸变对空间目标清晰干涉成像仿真研究

本文利用功率谱反演法分别展开对符合 Kolmogonov 统计规律和修正后的Von Karmen 统计规律的大气湍流畸变波前相位屏进行了数值模拟研究. 得到各种模型下相位屏对该成像系统干涉成像图. 仿真结果表明, 采用闭合相位原理, 基本可以消除大气湍流对光束波前的影响, 验证了采用相位闭合技术的优势.     

傅里叶望远镜大气湍流模拟实验

傅里叶望远术是一种能对深空暗弱目标进行高分辨率成像的技术.为了验证大气湍流对傅里叶望远镜系统的影响,进行了实验室环境下大气湍流模拟实验研究.在三束光傅里叶望远镜实验系统上,通过控制射频驱动器的输出功率来模拟光强抖动,改变射频驱动器的瞬时频率来模拟相位抖动.给出了实验理论依据,推导了湍流强度与实验变量的关系.实验在弱湍流闪烁和相位抖动两种情况下,分别给单束光和三束光加随机扰动并计算其Strehl比.结果表明,只在单束光上加扰动时重建图像影响不大;在三束光上加扰动时,弱湍流光强抖动对傅里叶望远镜系统的成像效果影响具有较大的随机性,而相位抖动会严重影响系统成像质量.因此,消除光强抖动和相位抖动影响是图像重建算法改进应该考虑的一个关键因素.     

激光雷达测量大气湍流廓线

 介绍了一种利用成像激光光斑测量大气湍流廓线的激光雷达原理。通过对分层大气湍流的光束波面变化的测量,获取各分层大气湍流的相干长度,据此利用平面波近似算法反演湍流强度廓线。通过搭建的大气湍流廓线激光雷达实验系统获得了湍流廓线的实验数据,并且与系留飞艇搭载的温度脉动仪在同时段固定高度进行了对比实验,验证了近似递推算法的可行性。最后对实验中出现的误差进行了讨论。     

部分相干Airy光束在大气湍流中的光强演化

为了利用Airy光束的无衍射、自恢复和自弯曲特性抑制大气湍流效应,实现远距离无线光通信,对部分相Airy光束在大气湍流中传输时的光强演化进行了研究.利用高斯-谢尔模型的交叉谱密度函数、广义惠更斯-菲涅尔原理以及Rytov相位近似法,推导了部分相干Airy在湍流大气中平均光强的表达式.分别从传播距离、湍流强度等方面对光强分布的影响进行了模拟仿真,并对光束自身参数对光强分布的影响进行了相关实验验证.结果表明:随着传播距离的增加,部分相干Airy光束的旁瓣逐渐衰减,主瓣逐渐扩散;在传播足够远时,其旁瓣逐渐消失,主瓣逐渐演化为高斯分布.仿真和实验结果一致表明光束的截断因子越小、特征长度和相干长度越长,光束的光强分布保持越完整.     

非均匀拉盖尔-高斯关联光束及其传输特性

提出了一种新型特殊关联部分相干光束即非均匀拉盖尔-高斯关联光束,其在x和y方向上的关联结构函数分别为非均匀关联和拉盖尔-高斯关联函数.基于广义惠更斯-菲涅耳衍射积分公式,推导了这种光束交叉谱密度函数在自由空间以及大气湍流中的传输式,并计算了该光束经过自由空间和湍流大气传输的光强分布和关联结构函数分布演化特性.研究发现,该光束光强分布在传输过程中展现出自聚焦、自偏移和自分裂等奇异特性,同时发现控制关联结构函数参数可以有效地降低湍流大气的影响.关联结构调控为操控光束传输行为提供了一种新颖而有效的手段,在大气激光通信、微粒操控等领域具有重要的应用前景.     

基于空间调制的星体观测张角强度关联干涉测量

强度关联干涉测量利用光场的高阶关联特性获取星体空间角度信息,有望实现脉冲星角位置的高精度测量。然而常规的强度关联测量需要满足相干探测条件,这对探测器的时间分辨提出了极高的要求。提出基于空间调制的星体观测张角强度关联干涉测量方法,在探测器前放置调制屏以对光场进行空间调制,通过旋转调制屏获取二阶干涉条纹。理论推导了当两条光路的调制屏存在角度差时二阶关联函数的表达式,并基于理论推导结果设计双反射镜实验方案并进行相关可见光实验验证,所得实验结果与理论分析结果相符。该方法大幅降低了探测器的时间分辨要求,对于我国未来实现航天器的自主导航具有重要意义。     

Visibility enhancement in two-dimensional lensless ghost imaging with true thermal light

We report an experimental demonstration of two-dimensional(2D) lensless ghost imaging with true thermal light. An electrodeless discharge lamp with a higher light intensity than the hollow cathode lamp used before is employed as a light source. The main problem encountered by the 2D lensless ghost imaging with true thermal light is that its coherence time is much shorter than the resolution time of the detection system. To overcome this difficulty we derive a method based on the relationship between the true and measured values of the second-order optical intensity correlation, by which means the visibility of the ghost image can be dramatically enhanced. This method would also be suitable for ghost imaging with natural sunlight.     

Reply to: “Comment on: ‘Exposed-key weakness of αη’ [Phys. Lett. A 370 (2007) 131]” [Phys. Lett. A 372 (2008) 7091]

We present simulations countering the claims in [R. Nair, H.P. Yuen, Phys. Lett. A 372 (2008) 7091] that the approximation given in [C. Ahn, K. Birnbaum, Phys. Lett. A 370 (2007) 131, quant-ph/0612058] for the eavesdropper's entropy on the encrypted key cannot be valid, and additionally discuss ways in which our views on security requirements differ from those given in [R. Nair, H.P. Yuen, Phys. Lett. A 372 (2008) 7091].     

Shape Coexistence for 179Hg in Relativistic Mean-Field Theory

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Spatial longitudinal coherence length of a thermal source and its influence on lensless ghost imaging

The spatial longitudinal coherence length (SLCL), which is determined by the size of and the distance from the source, is introduced to investigate the longitudinal resolution of lensless ghost imaging. Its influence is discussed quantitatively by simulation. The discrepancy of position sensitivity between Scarcelli et al. [Appl. Phys. Lett.88, 061106 (2006)] and Basano and Ottonello [Appl. Phys. Lett.88, 091109 (2006)] is clarified.     

Pair-Breaking Critical Current Density of Two-Band Superconductor MgB2

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强度涨落在热光鬼成像中的作用

热光鬼成像的图像质量在实际应用中具有重要作用. 通过理论分析和数值模拟, 发现光场的强度涨落程度会影响热光鬼成像的对比度, 基于此, 提出可以通过调节热光场的平均强度和强度波动的方差来提高成像对比度, 并且研究了这一方法对成像信噪比的影响. 将这种方法与另一种提高成像对比度的方法——高阶鬼成像进行了对比, 所得结果将有助于提高对热光鬼成像的理解. 关键词： 鬼成像 强度涨落 对比度 信噪比     

The effects of the N atom collective Lamb shift on single photon superradiance

The problem of single photon collective spontaneous emission, a.k.a. superradiance, from N atoms prepared by a single photon pulse of wave vector k₀ has been the subject of recent interest. It has been shown that a single photon absorbed uniformly by the N atoms will be followed by spontaneous emission in the same direction [M. Scully, E. Fry, C.H.R. Ooi, K. Wodkiewicz, Phys. Rev. Lett. 96 (2006) 010501; M. Scully, Laser Phys. 17 (2007) 635]; and in extensions of this work we have found a new kind of cavity QED in which the atomic cloud acts as a cavity containing the photon [A.A. Svidzinsky, J.T. Chang, M.O. Scully, Phys. Rev. Lett. 100 (2008) 160504]. In most of our studies, we have neglected virtual photon (“Lamb shift”) contributions. However, in a recent interesting paper, Friedberg and Mannassah [R. Friedberg, J.T. Manassah, Phys. Lett. A 372 (2008) 2514] study the effect of virtual photons investigating ways in which such effects can modify the time dependence and angular distributions of collective single photon emission. In the present Letter, we show that such virtual transitions play no essential role in our problem. The conclusions of [M. Scully, E. Fry, C.H.R. Ooi, K. Wodkiewicz, Phys. Rev. Lett. 96 (2006) 010501; M. Scully, Laser Phys. 17 (2007) 635; A.A. Svidzinsky, J.T. Chang, M.O. Scully, Phys. Rev. Lett. 100 (2008) 160504] stand as published. However, the N atom Lamb shift is an interesting problem in its own right and we here extend previous work both analytically and numerically.     

基于统计光学的正负热光非定域成像

本文, 基于经典统计光学, 建立符合热光特性的统计模型, 通过数值模拟证明了吴令安和Meyers提出的图像重构算法, 并进行了定性的理论分析. 在关联成像获得的数据样本中, 根据桶探测器的光强涨落进行分组, 分别以某个阈值作为下限和上限, 再将分组后的独立样本和相应的面探测器信号进行强度关联, 则可以得到物体的正像或负像. 然而, 不经过关联运算, 直接对分组后的面探测器信号进行算数平均也可以得到物体的正像或负像, 同时成像的对比度得到较大提高. 这种分组对应的非定域成像进一步说明强度涨落在热光成像中的重要性. 最后以字符掩膜版作为成像物体, 分别运用关联成像和分组对应正负成像算法重构物体的图像, 实验结果证明这种新的正负算法可以提高非定域成像的对比度. 关键词： 统计光学 热光 关联成像 正负非定域成像     

Log-Poisson statistics and extended self-similarity in driven dissipative systems

The Bak–Chen–Tang forest fire model [Phys. Lett. A 147 (1990) 297] was proposed as a toy model of turbulent systems, where energy (in the form of trees) is injected uniformly and globally, but is dissipated (burns) locally. We review our previous results on the model [Phys. Rev. E 62 (2000) 1613; Phys. Rev. Lett. 86 (2000) 4215] and present our new results on the statistics of the higher-order moments for the spatial distribution of fires. We show numerically that the spatial distribution of dissipation can be described by Log-Poisson statistics which leads to extended self-similarity [Phys. Rev. E. 48 (1993) R29; Phys. Rev. Lett. 73 (1994) 959]. Similar behavior is also found in models based on directed percolation; this suggests that the concept of Log-Poisson statistics of (appropriately normalized) variables can be used to describe scaling not only in turbulence but also in a wide range of driven dissipative systems.     

Ultimate state of thermal convection

The ultimate regime of thermal convection, the so-called Kraichnan regime [R. H. Kraichnan, Phys. Fluids 5, 1374 (1962)]], hitherto has been elusive. Here numerical evidence for that regime is presented by performing simulations of the bulk of turbulence only, eliminating the thermal and kinetic boundary layers and replacing them with periodic boundary conditions.