Cramer-Rao lower bound for the estimation of the degree of polarization in active coherent imagery at low photon levels

The degree of polarization (DOP) is an important tool in many optical measurement and imaging applications. We address the problem of its estimation in images that are perturbed with both speckle and photon noise, by determining the Cramer-Rao lower bounds (CRLBs) when the illuminated materials are purely depolarizing. We demonstrate that the CRLBs are simply the sum of the CRLBs due to speckle noise and Poisson noise. We use this result to analyze the influence of different optical parameters on DOP estimation.     

低太阳高度角条件下的天空偏振模式模拟及大气折射影响研究

为模拟低太阳高度角条件下的天空偏振模式,自主开发了考虑大气球形几何及大气折射效应的辐射传输模式VSPART,并将其运用于漫射光偏振特性仿真.在模式中,基于射线追踪法实现了光线传播轨迹的追踪和入射光偏振态及透过率的计算,基于矩阵算法实现了辐射传输方程的求解.将VSPART模拟结果与基准值、SPDISORT模拟值进行了比较,验证了模型的准确性.在瑞利散射大气和含气溶胶大气条件下,模拟并分析了漫射光偏振度及偏振方向的分布特征,讨论了大气球形几何及折射效应对天空偏振度的影响.结果表明,低太阳高度角条件下,随着波长增加,瑞利散射大气对应的偏振度整体随之增强,中性点向大天顶角方向移动;气溶胶的存在并不改变天空偏振度分布特征,但对偏振方向影响显著,随着光学厚度的增加,天空偏振度值迅速降低;中性点的偏移可能与低阶散射过程紧密相关;大气球形几何和折射效应的主要影响区域为地平线区域、两中性点附近及天顶区域;瑞利散射大气条件下,随着波长增加,大气球形几何及折射效应的影响逐步减弱,特别在中性点附近及天顶区域,其影响逐步消失;随着气溶胶光学厚度的增加,其影响随之增强.     

水下目标物偏振成像特性研究

偏振图像比传统强度图像包含更丰富的物体表面反射及散射信息。用萨尔萨(SALSA)相机在自然光照下获取水下偏振图像，研究不同材质物体、放置深度、牛奶浓度及波段因素对水下目标物偏振成像的影响。结果表明:蓝色波段偏振成像能够较好地获取水下物体的边界轮廓等信息; 不同材质的目标物在水下呈现不同的偏振特性, 紫铜偏振度最高达0.69;在1.40 mg/L牛奶浑浊度的水下，偏振图像仍能通过比较目标物的偏振度(degree of polarization，DOP)信息来检测出水下目标物，瓷片的DOP仅降低0.31;此外，在水下约40 cm深度下，偏振成像获取的图像比强度图像轮廓更为清晰，如铁的偏振对比度比强度对比度高5.26%。     

Precision of degree of polarization estimation in the presence of additive Gaussian detector noise

We address the problem of degree of polarization (DOP) estimation in images limited by additive Gaussian detector noise. We derive and analyze the probability density function (PDF) of the pixelwise DOP estimate, which is shown to have significantly different statistical properties than when noise is Gamma distributed (speckle). We then determine the Cramer-Rao Lower Bound and the maximum likelihood estimator of the DOP. We deduce from this study practical solutions for characterizing and reducing the noise in these images.     

Coherent recovery of the degree of polarization of light propagating in random anisotropy media

We report on the study of light polarization behavior in random anisotropy scattering media. It is shown that, in the case of low scattering events, the degree of polarization (DOP) demonstrates oscillatory behavior due to the coherent character of light scattering. A strong increase in the DOP is demonstrated by using electro-optic fine tuning of the refractive index modulation depth of the scattering media.     

一种基于旋转1/4波片法的激光偏振度测量仪

为了满足光的偏振特性测量要求,设计了一套激光偏振度测量仪。该装置利用旋转1/4波片对光波进行调制,通过对调制信号进行傅里叶分析获得光源的偏振度。利用测量光功率的方式对激光偏振度测量仪的偏振度进行校准,实验结果表明,所研制的激光偏振度测量仪测量精度优于0.5%,满足高精度激光偏振度测量需求。     

Polarization properties of Rayleigh backscattering with a high degree of coherence in single-mode fibers

The state and degree of polarization (SOP, DOP) of Rayleigh backscattering (RB) are analyzed by using Jones calculus for the case the source coherent length is larger than the fiber length. It shows that the SOP of RB is the same as the SOP of input light, and the fiber disturbance does not influence the SOP of RB. The DOP of RB is also the same as that of input light. The corresponding experiments have been carried out, and the results agree well with all the predictions.     

椭球粒子烟雾浓度对激光偏振度影响的实验研究

以斯托克斯矢量理论为基础,通过搭建偏振传输半实物模拟装置进行模拟实验,观测激光在模拟环境(不同浓度的椭球粒子在非均匀分布状态)下偏振度(degree of polarization, DOP)的变化。实验选取灵芝孢子碳化粉末作为椭球粒子的实验对象,通过烟雾机将椭球粒子形成不同浓度的烟雾,研究入射波长为532 nm、671 nm的激光在0°、+45°、90°的线偏振光以及左旋圆偏振光经过模拟环境后偏振度(DOP)的变化情况。实验结果表明:入射偏振光随着烟雾浓度的增大,偏振度呈下降趋势;3种入射线偏振光随浓度增大降幅不尽相同,没有明显的规律可循;不同波长低浓度烟雾时,线偏振光与圆偏振光的偏振度下降幅度大抵相同,大约为3%左右;随着烟雾浓度的增大,线偏振光的偏振度表现出不同程度的下降,可达20%,圆偏振光的偏振度仅下降5%,表现出了较好的保偏能力。     

基于偏振度的偏振模色散补偿中检测信号对不同归零码型的响应研究

从理论上数值分析和比较了一阶和二阶偏振模色散对各种光调制码型（RZ、CSRZ、RZ-DPSK、CSRZ-DPSK）的偏振模色散的影响,结果表明:不同码型的偏振度大不相同,RZ码受偏振度影响最大,CSRZ-DPSK码受二阶偏振模色散的影响最小,在补偿系统中更适合缓解偏振模色散.     

表面粗糙度对黑云斜长片麻岩不同探测角偏振度波谱影响研究

以黑云斜长片麻岩为例,探讨了表面粗糙度因子在350～2 500nm波段对岩石不同天顶角及方位角偏振度探测的影响,并在特征波段进行了定量分析。表明不同粗糙度的岩石表面,入射天顶角一定时,在镜面反射方位,偏振度随探测天顶角先是上升,到达最高值后又开始下降。(520±10)nm波段的偏振度在探测天顶角方向达到的最大峰值与表面粗糙度进行相关分析,发现他们呈幂函数关系。回归方程为:y=0.604x-0.297,可决系数R2为0.985 4。峰值所在天顶角度与表面粗糙度相关模型为,y=3.419 4x+51.584,y<90°,模型可决系数R2为0.817 7。偏振度随探测方位角变化的规律为以180°方位时偏振度最大,向两侧随探测方位离180°越远逐渐越低,并趋于0。(520±10)nm波段的偏振度在探测方位角180°方向达到的最大峰值与表面粗糙度进行相关分析,发现他们呈幂函数关系。回归方程为:y=0.582 2x-0.333,可决系数R2为0.984 3。对以上回归模型进行F检验,发现偏振度峰值及所对应角度与粗糙度之间存在显著的相关关系。     

Polarization-Sensitive Optical Coherence Tomography using only Linearly Polarized Light

This work presents polarization-sensitive optical coherence tomography (PS-OCT) using only linearly polarized light. The main characteristics of linear PS-OCT include the following. (1) In a turbid medium with linear birefringence, the degree of polarization (DOP) of backscattered linearly incident polarized light does not change significantly with the increase of birefringence value; (2) the phase retardation of the sample can be obtained from only the phase difference between the orthogonal polarization heterodyne signals, so a common phase noise rejection mode is introduced. These advantages may not only support the immunity of background phase noise and laser frequency noise, but also increase the sensitivity of phase retardation measurements; (3) the dynamic range of phase retardation in this system is between 0° and 180°     

Numerical study on the wavelength-dependent polarization of light scattered by aqueous droplets for RGB spectrums

Based on Lorenz-Mie scattering theory, the transformation matrix of an aqueous droplet is derived analytically. By employing single scattering approximation and log-normal size distribution function, the DOP (degree of polarization) of the light scattered by a unit volume of aqueous droplets for RGB (red, green and blue) spectrums is studied. In our study, the wavelength-dependent complex refractive index for water with fine resolution is novelly obtained by using Piecewise Interpolation method based on Segelstein's refractive index data. The analytical results reveal that extrema of the DOP which is dependent on the relative intensities of the orthogonal polarizations are associated with scattering angles, and, for RGB spectrums, increasing wavelength can alleviate the influence of the resonance on the orthogonal polarizations and the DOP.     

模拟溢油样品激光诱导荧光光谱的偏振特性研究

激光诱导荧光是海洋溢油探测的有效手段之一,但该技术的应用易受自然水体中叶绿素、CDOM等物质荧光信号的干扰。为了寻求排除自然水体荧光干扰的方法,基于532 nm连续激光器搭建了激光诱导偏振荧光实验装置,并针对六种不同密度的模拟溢油样品和自然水体开展了荧光光谱偏振特性研究。实验结果发现,与自然水体的荧光光谱不具有明显的偏振特性不同,所有模拟溢油样品的诱导荧光均具有明显的偏振特性,这一结论说明激光诱导荧光光谱的偏振特性可以作为排除叶绿素、CDOM等物质荧光干扰的依据。实验还发现,溢油样品的荧光偏振性质因样品种类而异。在线偏振光激发下,原油样品荧光偏振度随波长逐渐降低,其中重质原油样品偏振度降低幅度最大,轻质原油样品幅度最小,而柴油样品荧光偏振度没有明显变化;当周期性改变激发光的偏振状态时,所有模拟溢油样品的荧光偏振度随之发生趋势相同的周期性波动,中质样品荧光偏振度波动的幅度低于重质样品,但明显超过轻质样品。这一结果说明,模拟溢油样品诱导荧光偏振度的波长变化特性及对激发光偏振态的响应特性均与样品密度存在一定关联,其偏振特性可以作为辅助油种识别的重要参量。     

Maximal degree of polarization for visible light scattered by an aqueous droplet

The maximal degree of polarization (DOP) for the visible solar light scattered by an aqueous droplet at a given scattering angle is found in this paper. Based on the Lorenz-Mie theory, the numerical expression for the DOP of the light scattered by any spherical particle is deduced. For the given scattering angle and aqueous droplet, the DOP dependent on wavelength is calculated according to familiar solar spectral colors. The maximal DOP of each spectral color is exhibited and some analyses are given. This analysis may contribute to applications of indicating atmospheric conditions and polarized light compass navigation.     

光源偏振对水体颗粒后向散射系数测量的影响

颗粒后向散射系数是水体主要的固有光学特性,也是海洋水色的决定因子和海洋水色卫星遥感反演的基础参数。当前,利用光学仪器现场原位测量是获取水体颗粒后向散射系数的主要方法。由于光源自身和仪器光路中镜面反射和折射,其出射光源可能存在一定的偏振,进而会影响水体后向散射系数测量的精度。目前,关于水体后向散射系数测量仪器的光源偏振性及其对颗粒后散射系数测量精度影响的研究尚为空白。针对该问题,以应用广泛的水体后向散射测量仪HydroScat6(HS-6)为例,对其出射光源的偏振特性进行了系统测量,并进一步开展了光源偏振对水体颗粒后向散射系数测量精度影响的实验研究。结果表明,HS-6六个光学通道除590 nm通道出射光源偏振度略低外(~15%),其他通道出射光源中心波长偏振度均在20%~30%。因此,HS-6出射光源具有显著的偏振特性。光源偏振对颗粒后向散射系数测量具有不可忽略的影响,且影响程度随着波段,线偏振角度及悬浮泥沙浓度而变化。不同悬浮泥沙浓度下,光源偏振引起的420,442,470,510,590和670 nm六个波段的平均偏差可达15.49%,11.27%,12.79%,14.43%,13.76%,12.46%。因此,利用光学仪器现场测量颗粒后向散射系数需要考虑光源偏振的影响,并需尽可能降低出射光源的偏振度。     

Computational polarization imaging from a single speckle image

We experimentally demonstrate that imaging of the degree of polarization (DOP) can be achieved from a single intensity image acquired under coherent illumination. This computational technique is based on the analysis of the speckle characteristics in a statistically homogeneous neighborhood of the pixel. Over a variety of samples obtained experimentally, we show that a simple calibration step allows this method to quickly and simply provide correct estimated values of the DOP, with precision in agreement with theoretical predictions. It is shown that unlike linear polarimetric contrast imaging, this method remains valid on birefringent samples.     

一种全局参数估计的水下主动偏振去雾算法

为改善水下主动光照明条件下后向散射光对成像的影响,通过分析水下主动偏振成像模型,提出一种基于拟合函数的全局参数估计的水下主动偏振去雾算法。该算法结合图像增强作为图像预处理,再设定二元多项式函数,利用最小二乘法进行后向散射光偏振度变量的拟合求解,得到对比度更高、信息更丰富的水下复原图像。实验结果表明:该算法可有效改善主动光照明条件下的水下图像质量,提高图像对比度,复原被淹没的细节信息,复原图像的图像增强测量值较以往算法相比提升70%,且能适用于不同浓度介质的情况。     

不同波长的散射介质后向散射光偏振度特性

基于Stockes矢量,通过测量以线偏振光和圆偏振光入射时脂肪乳剂后向散射光的偏振度,研究了532nm、650nm和780nm三个波长的光与散射粒子粒径为325nm的脂肪乳剂溶液作用后,其后向散射光的偏振度特性.研究结果表明,对于入射线偏振光,780nm波长后向散射光中的线偏振光成分多于圆偏振光成分,而532nm波长则相反;对于入射圆偏振光,三个波长后向散射光中的圆偏振光成分均多于线偏振光成分;532nm波长的总偏振度高于650nm和780nm两个波长各自的总偏振度;线偏振光的保偏性优于圆偏振光的保偏性,但偏振光在散射介质中的穿透深度较小.因此,后向散射成像技术适用于物体表层成像,而且选择波长略大于粒径的线偏振光可以提高成像质量.     

不同波长的散射介质后向散射光偏振度特性

基于Stockes矢量,通过测量以线偏振光和圆偏振光入射时脂肪乳剂后向散射光的偏振度,研究了532 nm、650 nm和780 nm三个波长的光与散射粒子粒径为325 nm的脂肪乳剂溶液作用后,其后向散射光的偏振度特性.研究结果表明,对于入射线偏振光,780 nm波长后向散射光中的线偏振光成分多于圆偏振光成分,而532 nm波长则相反;对于入射圆偏振光,三个波长后向散射光中的圆偏振光成分均多于线偏振光成分;532 nm波长的总偏振度高于650 nm和780 nm两个波长各自的总偏振度;线偏振光的保偏性优于圆偏振光的保偏性,但偏振光在散射介质中的穿透深度较小.因此,后向散射成像技术适用于物体表层成像,而且选择波长略大于粒径的线偏振光可以提高成像质量.     

Accuracy of Gaussian Approach for the Performance Evaluation of Direct-Detection Receiver with Partially Polarized Noise

We investigate the accuracy of a Gaussian approach (GA) developed to estimate the performance of a direct-detection optical receiver with arbitrary optical and electrical filtering and in the presence of partially polarized noise due to polarization-dependent loss (PDL). The accuracy is assessed by comparison of the performance estimates obtained from the GA with the estimates obtained from a rigorous method (RM) based on the calculation of the moment-generating function of the current at the optical receiver output.

We show that the GA has a good accuracy when considering the variation of the optical filter bandwidth, extinction ratio, degree of polarization of the noise (DOP), and angle between signal and noise polarizations. However, it fails to predict the receiver sensitivity within 2 dB of the RM when DOP is greater than 0.7 and signal and noise polarizations are orthogonal in the Jones space.

Nevertheless, it is shown that the GA provides receiver sensitivity estimates with good accuracy in most cases of long-haul optical communication systems influenced by PDL, where the typical average DOP is below 0.15. Due to its simplicity, shorter computation time, and good accuracy, the GA is a good tool to assess the performance of such optical systems.