Invariant polarimetric contrast parameters of coherent light

Many applications use an active coherent illumination and analyze the variation of the polarization state of optical signals. However, as a result of the use of coherent light, these signals are generally strongly perturbed with speckle noise. This is the case, for example, for active polarimetric imaging systems that are useful for enhancing contrast between different elements in a scene. We propose a rigorous definition of the minimal set of parameters that characterize the difference between two coherent and partially polarized states. Indeed, two states of partially polarized light are a priori defined by eight parameters, for example, their two Stokes vectors. We demonstrate that the processing performance for such signal processing tasks as detection, localization, or segmentation of spatial or temporal polarization variations is uniquely determined by two scalar functions of these eight parameters. These two scalar functions are the invariant parameters that define the polarimetric contrast between two polarized states of coherent light. Different polarization configurations with the same invariant contrast parameters will necessarily lead to the same performance for a given task, which is a desirable quality for a rigorous contrast measure. The definition of these polarimetric contrast parameters simplifies the analysis and the specification of processing techniques for coherent polarimetric signals.     

Visible-regime polarimetric imager: a fully polarimetric,real-time imaging system

A fully polarimetric optical camera system has been constructed to obtain polarimetric information simultaneously from four synchronized charge-coupled device imagers at video frame rates of 60 Hz and a resolution of 640 x 480 pixels. The imagers view the same scene along the same optical axis by means of a four-way beam-splitting prism similar to ones used for multiple-imager, common-aperture color TV cameras. Appropriate polarizing filters in front of each imager provide the polarimetric information. Mueller matrix analysis of the polarimetric response of the prism, analyzing filters, and imagers is applied to the detected intensities in each imager as a function of the applied state of polarization over a wide range of linear and circular polarization combinations to obtain an average polarimetric calibration consistent to approximately 2%. Higher accuracies can be obtained by improvement of the polarimetric modeling of the splitting prism and by implementation of a pixel-by-pixel calibration.     

Low-noise bimodal all-fibre sensor using depolarized light

Abstract

A low-noise scheme for an all-fibre bimodal sensor is presented. Depolarized excitation of the sensor from a lower coherency source is used to eliminate polarimetric noises which inevitably arise due to polarizer/analyser and fibre-to-fibre misalignments. The sensor features simplicity and simultaneous detection of the X- and Y-polarized interference signals. Overall noise level as low as 0.8% is achieved.     

Target segmentation in active polarimetric images by use of statistical active contours

We address the problem of target segmentation in active polarimetric images, which can reveal contrasts that do not appear in standard intensity images. However, these images are perturbed by strong specklelike noise. For the purpose of segmentation we thus use statistical active contours, which are known to possess noise robustness properties. The polarimetric imagers we consider acquire two different images of the same scene so as to form a two-channel image (TCI). These two images can be combined to form the orthogonal state contrast image (OSCI), which represents the degree of polarization of the backscattered light if its coherency matrix is diagonal. We characterize the segmentation performance of the statistical active contour procedure on the TCI and on the OSCI. In particular, we show that if the illumination beam is spatially nonuniform, it is more efficient to perform the segmentation on the OSCI, which is independent of the spatial variations of the illumination.     

Inverse source problem for partially polarized beams in turbulent media

The propagation of a class of partially polarized beams though atmospheric turbulence is treated using the coherent-modes-decomposition approach. This approach allows specifying the properties of sources generating radiation with a narrow polarization distribution that could be employed in active polarimetric remote sensing applications.     

Statistical algorithms for target detection in coherent active polarimetric images.

We address the problem of small-target detection with a polarimetric imager that provides orthogonal state contrast images. Such active systems allow one to measure the degree of polarization of the light backscattered by purely depolarizing isotropic materials. To be independent of the spatial nonuniformities of the illumination beam, small-target detection on the orthogonal state contrast image must be performed without using the image of backscattered intensity. We thus propose and develop a simple and efficient target detection algorithm based on a nonlinear pointwise transformation of the orthogonal state contrast image followed by a maximum-likelihood algorithm optimal for additive Gaussian perturbations. We demonstrate the efficiency of this suboptimal technique in comparison with the optimal one, which, however, assumes a priori knowledge about the scene that is not available in practice. We illustrate the performance of this approach on both simulated and real polarimetric images.     

General state contrast imaging: an optimized polarimetric imaging modality insensitive to spatial intensity fluctuations

In active polarization imaging, one frequently needs to be insensitive to noninformative spatial intensity fluctuations. We investigate a way of solving this issue with general state contrast (GSC) imaging. It consists in acquiring two scalar polarimetric images with optimized illumination and analysis polarization states, then forming a ratio. We propose a method for maximizing the discrimination ability between a target and a background in GSC images by determining the optimal illumination and analysis states. A further advantage of this approach is to provide an objective way of quantifying the performance improvement obtained by increasing the number of degrees of freedom of a GSC imager. The efficiency of this approach is demonstrated on simulated and real-world images.     

Imaging multispectral polarimetric sensor: single-pixel design,fabrication, and characterization

A discrete-component approach was taken to establish the operational feasibility of a novel, imaging, midinfrared, multispectral, polarimetric sensor for remote-sensing application. The sensor is designed to exploit the spectral and polarimetric characteristics of the scene as discriminants. Pixelated multispectral filters and polarization filters were designed and fabricated on sapphire and Si substrates, respectively, and both were characterized. A single-pixel spectropolarimetric composite filter was characterized by use of a Fourier transform infrared spectrometer and a Pt-Si thermal-imaging camera. The experimental results show excellent agreement with theoretical predictions.     

Polarimetric optical imaging of scattering surfaces

A polarimetric optical specular event detector (OSED) has been developed to provide spatially and temporally resolved polarimetric data of backscattering in the visible from water wave surfaces. The OSED acquires simultaneous, two-dimensionally resolved images of the remote target in two orthogonal planes of polarization. With the use of plane-polarized illumination the OSED presently can measure, in an ensemble of breaking waves, the equivalent four-element polarization matrix common to polarimetric radars. Upgrade to full Stokes parameter state of polarization measurements is straightforward with the use of present single-aperture, multi-imager CCD camera technology. The OSED is used in conjunction with a coherent pulse-chirped radar (PCR), which also measures the four-element polarization matrix, to provide direct time-correlated identification of backscattering mechanisms operative during wave-breaking events which heretofore have not been described theoretically. We describe the instrument and its implementation, and examples of spatially resolved polarimetric data are displayed as correlated with the PCR backscatter cross section and polarization ratio records.     

极化合成孔径雷达图像船舶目标检测算法

结合区域划分和结构检测模板提出了改进极化白化滤波(IPWF)算法,利用IPWF算法融合极化合成孔径雷达(POL-SAR)中各极化通道图像,同时抑制相干斑,然后利用双参数恒虚警率(CFAR)检测方法对融合后的图像进行船舶目标检测.本文利用香港地区SIR-C全极化单视复数据进行了实验,结果表明IPWF算法更好地降低了相干斑因子,提高了船舶目标的检测率、控制了虚警率,同时可以更好地保持船舶目标的结构信息.     

手性近晶C相串型液晶共聚物的合成与性能

以对苯二甲酰氮,2,5－二[4-((S)-2-甲基丁氧基）苯甲酰氧基]对苯二酚和三缩四乙醇为单体,采用低温溶液缩聚的方法,合成了一系列新的手性近晶C相串型液晶共聚物。通过DCS,偏光显微镜,WAXD和旋光分析等方法,发现所有的手性共聚物都有光活性,当加热至熔融温度以上时形成近晶C相,共聚物的熔融温度（Tm)和各负同性温度（Ti)随共聚物分子中三缩四乙二醇用量的改变呈规律性变化。     

Measurement of ultralow supermirror birefringence by use of the polarimetric differential cavity ringdown technique

We demonstrate a novel technique for measuring ultralow linear birefringence of supermirrors (high-reflectivity dielectric mirror coatings). The polarimetric cavity ringdown technique is used in conjunction with the differential detection scheme with circular polarization to enhance the measurement sensitivity. The technique could, in principle, provide the convenience and reliability of linear detection signals and a reasonable tolerance to experimental imperfections. Phase retardation and orientation of each cavity mirror can be determined separately without the influence of the other mirror. The minimum detectable phase retardation achieved experimentally with this technique is ~6 x 10(-8) rad.     

Polarimetric imaging method for target enhancement in haze based on polarimetric retrieval

Polarimetric imaging has been proven to be an effective way in detecting the targets of interest in complicated surroundings by analyzing the polarization property, instead of the intensity, of the light emanating from the objects. Unfortunately, polarimetric imaging encounters difficulty when the surroundings are very scattered, where on the one hand the polarization property of the object light usually becomes very faint after a strong depolarized scattering process; on the other hand, the object light will be blended with the atmospheric light scattered by haze particles (airlight). In this paper, we propose a polarimetric imaging retrieval method that can be used for such challenging conditions. Firstly, the airlight radiance is estimated precisely. Then, the airlight is removed from the hazy images. Finally, the residual polarization property of the object light is regained, which ensures the validity of the polarimetric imaging method in these conditions. The experiments confirm that the proposed method is useful in enhancing polarimetric imaging detection in haze.     

General adaptive-neighborhood technique for improving synthetic aperture radar interferometric coherence estimation

A new method for filtering the coherence map issued from synthetic aperture radar (SAR) interferometric data is presented. For each pixel of the interferogram, an adaptive neighborhood is determined by a region-growing technique driven by the information provided by the amplitude images. Then pixels in the derived adaptive neighborhood are complex averaged to yield the filtered value of the coherence, after a phase-compensation step is performed. An extension of the algorithm is proposed for polarimetric interferometric SAR images. The proposed method has been applied to both European Remote Sensing (ERS) satellite SAR images and airborne high-resolution polarimetric interferometric SAR images. Both subjective and objective performance analysis, including coherence edge detection, shows that the proposed method provides better results than the standard phase-compensated fixed multilook filter and the Lee adaptive coherence filter.     

Multifusion, Multispectral, Optical Polarimetric Imaging Sensing Principles

The goal of this study is to present novel, multifusion optical imaging sensing principles, based on active-multispectral polarimetric imaging of targets surrounded by scattered media. Specifically, the novelty of this study consists in the fusion of multispectral images, with polarimetric imaging principles, forming image differences. The experimental results indicate clearly that, high-contrast multispectral Mueller polarimetric image differences, as well as Degree of Linear Polarization (DOLP) images can be obtained from transmitted or backscattered photons, from targets embedded in turbid media.     

Minimization of temperature effects of high-birefringent elliptical fibers for polarimetric optical-fiber sensors

The temperature dependence of polarization-maintaining fibers is a problem in polarimetric optical-fiber sensors. We report a novel method for making a temperature-insensitive, polarization-maintaining fiber, which may be used for the sensing part in a polarimetric strain sensor. The fiber has a double-clad elliptical core with built-in stresses in the core and cladding regions. To minimize the temperature sensitivity, the built-in stresses are balanced with the refractive-index differences and the core ellipticity properly chosen. The temperature and strain sensitivities of the fiber are calculated. A practical design and some potential applications of such a temperature-insensitive fiber with a high strain sensitivity are presented.     

Fractional Order Linear Active Disturbance Rejection Control for Linear Flexible Joint System

A linear flexible joint system using fractional order linear active disturbance rejection control is studied in this paper. With this control scheme, the performance against disturbances, uncertainties, and attenuation is enhanced. Linear active disturbance rejection control (LADRC) is mainly based on an extended state observer (ESO) technology. A fractional integral (FOI) action is combined with the LADRC technique which proposes a hybrid control scheme like FO-LADRC. Incorporating this FOI action improves the robustness of the standard LADRC. The set-point tracking of the proposed FO-LADRC scheme is designed by Bode's ideal transfer function (BITF) based robust closed-loop concept, an appropriate pole placement method. The effectiveness of the proposed FO-LADRC scheme is illustrated through experimental results on the linear flexible joint system (LFJS). The results show the enhancement of the robustness with disturbance rejection. Furthermore, a comparative analysis is presented with the results obtained using the integer-order LADRC and FO-LADRC scheme.     

Scattering by layered structures with rough surfaces: comparison of polarimetric optical scatterometer measurements with theory

A laboratory model of a layered structure with a rough upper surface (a glass microscope slide cut with a diamond saw) is used to obtain optical polarimetric data. Scatterometer measurements were made of all the Mueller matrix elements associated with light scattered in arbitrary directions. (A preliminary measurement of scattering from a smooth opaque gold film on a silicon wafer was used to validate the calculation of the Mueller matrix elements.) These measurements are compared with corresponding analytical solutions based on the full-wave approach. Physical interpretations of the analytical solutions that account for scattering upon reflection and transmission across rough interfaces are given in a companion paper. The agreement between calculations and measurements suggests that the full wave, polarimetric solutions can provide a reliable database for electromagnetic detection of rough surfaces in remote-sensing applications.     

A complete scheme of built-in self-tests (BIST) structure for faultdiagnosis in analog circuits and systems

The implementation of BIST in analog circuits is investigated, and a complete BIST scheme is proposed. This scheme can be included in any analog or mixed analog-digital circuit and can check its responses by following selected testing procedures. A CMOS chip supporting the proposed BIST structure is designed to facilitate the application of the scheme in a variety of analog circuits. Results from the application of the BIST scheme on active circuits are given, showing its effectiveness and its convenience     

Noise distribution of Mueller matrices retrieved with active rotating polarimeters

Two methods used to retrieve Mueller matrices from intensity measurements are revisited. It is shown that with symmetry or orthogonality considerations, numerical inversions of polarimetric equations can be avoided. With the obtained analytical formulas, noise propagation can be analyzed. If the intensity noise is a Gaussian white noise, the noise of Mueller matrices features remarkable properties. Mueller components are mutually correlated according to a scheme that involves decomposition into four blocks of 2x2 matrices. Variances are unequally distributed: the middle 2x2 block has the highest variance, the element on the bottom right has the lowest. These characteristics have been validated on experimental Mueller images of the free space.