A simplified holography based superresolution system

In this paper we are proposing a simple idea based on holography to achieve superresolution. The object is illuminated by three fibers which maintain the mutual coherence between the light waves. The object in-plane rotation along with fiber-based illumination is used to achieve superresolution. The object in a 4f optical system is illuminated by an on-axis fiber to make the central part of the object׳s spectrum to the pass through the limiting square-aperture placed at the Fourier plane and the corresponding hologram of the image is recorded at the image plane. The on-axis fiber is switched off and the two off axis fibers (one positioned on the vertical axis and the other positioned on diagonal) are switched on one by one for each orientation of the object position. Four orientations of object in-plane rotation are used differing in angle by 90°. This will allow the recording of eight holographic images in addition to the one recorded with on-axis fiber. The three fibers are at the vertices of a right angled isosceles triangle and are aligned toward the centre of the lens following the fiber plane to generate plane waves for object illumination. The nine holographic images are processed for construction of object׳s original spectrum, the inverse of which gives the super-resolved image of the original object. Mathematical modeling and simulations are reported.     

The Detection of Defects in Optical Fibers Using a Hybrid Opto-electronic Correlator

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The Detection of Defects in Optical Fibers Using a Hybrid Opto-electronic Correlator

A hybrid opto-electronic correlator for detecting defects in optical fibers is proposed. After the light from a He-Ne laser being expanded and filtered it is not collimated but directly passes a Fourier transform lens and illuminates a reference fiber and a test fiber at the same input plane. The Fourier transform spectrum of the two fibers is therefore obtained at the rear focal plane of the lens, where it is sampled via a CCD array connected with a computer through a frame grabber. The computer performs filter, inverse Fourier transform and setting threshold operation on classification. The system is an equivalent of joint transform correlator with a Fourier lens of long focal length. The experiment results for optical fibers having incoordinate defects are presented. The results indicate that the system can be used for fiber defect detection, and has the advantages of high identification, compact configuration, easy adjustment and flexible manipulation.     

An engineering prototype of Hadamard transform spectral imager based on Digital Micro-mirror Device

An engineering prototype of Hadamard transform spectral imager for object detection is presented. The programmable mask, using Digital Micro-mirror Device, is set on the second image plane to implement the function of spectral domain encoding. This method can significantly increase the detecting possibility with improved SNR of the relatively weak spectrum signature. A refractive configuration designed for visible spectrum application is proposed, and the optical design strategies of each functional unit are described in detail in order to meet both the image quality and the structural feasibility. The method of mask positioning is studied and the promising spectral recovery results based on the laboratory experiments are presented.     

Fiber optic bundle array wide field-of-view optical receiver for free space optical communications

We propose a design for a free space optical communications (FSOC) receiver terminal that offers an improved field of view (FOV) in comparison to conventional FSOC receivers. The design utilizes a microlens to couple the incident optical signal into an individual fiber in a bundle routed to remote optical detectors. Each fiber in the bundle collects power from a solid angle of space; utilizing multiple fibers enhances the total FOV of the receiver over typical single-fiber designs. The microlens-to-fiber-bundle design is scalable and modular and can be replicated in an array to increase aperture size. The microlens is moved laterally with a piezoelectric transducer to optimize power coupling into a given fiber core in the bundle as the source appears to move due to relative motion between the transmitter and receiver. The optimum position of the lens array is determined via a feedback loop whose input is derived from a position sensing detector behind another lens. Light coupled into like fibers in each array cell is optically combined (in fiber) before illuminating discrete detectors.     

二维圆周光综合孔径阵的优化排列及其成像特性研究

光综合孔径成像的原理是应用干涉原理在空间频率域中进行采样,并通过傅里叶反变换或其他数值变换方法得到空间分辨力远高于单个孔径成像系统的目标图像。由若干个相同的小孔径在二维圆周上优化排列组成的综合孔径成像系统可以在二维空间频率域中实现较为均匀分布的、具有无冗余度的采样点覆盖,为高质量实时成像提供了一个有效的途径。运用模拟退火算法对由7～16个子孔径组成的二维圆周综合孔径阵列进行优化排列。并依据光学衍射成像原理,从空域和频域两个方面详细分析了二维圆周上优化排列与均匀排列光综合孔径阵的成像特性。对7～16个子孔径组成的光综合孔径的仿真结果表明：无论是在空域还是频域上,子孔径直径增大、孔径数目增多以及综合孔径阵的优化排列都是有利于提高成像质量的。但综合后的子孔径的直径的增大,虽然能获得极高的角分辨力,却并不利于成像质量的提高。     

Simultaneous measurement of whole in-plane displacement using phase-shifting ESPI

A novel method for simultaneously measuring whole field in-plane displacements by using optical fiber phase-shifting electronic speckle pattern interferometry (ESPI) is presented in this paper. A 1 × 4 single mode optical fiber beamsplitter is employed to split the laser beam into four beams of equal intensity. One pair of fibers is utilized to illuminate the diffuse target at equal angles in the horizontal plane so it is sensitive only to horizontal in-plane displacement. Another pair of optical fibers is set to be sensitive only to vertical in-plane displacement. The polarization directions of light emitted by fibers are the same for each pair, but are at a right angle between pairs. The optical fibers are equal in length for each pair, but are not equal between two pairs. In this case the speckles are interference between each pair of fibers, thus the horizontal and vertical displacement components can be obtained simultaneously. By means of a fiber phase shift technique we can obtain the quantitative data of whole field displacements. This method has made it possible to study the in-plane displacement of a thin metal plate, and the examples of the results are shown to demonstrate the novel method.     

联合变换相关器在光纤检测中离焦影响的讨论

对联合变换相关器在光纤检测中离焦采集谱问题进行了理论分析,计算机模拟以及光纤缺陷检测实验。得出了离焦距离与相关峰值的关系曲线,示出了对不同相似程度的输入物的离焦谱和焦面谱两种情况下进行相关的计算机模拟结果;提供了在焦面上和离焦一定范围两种情况下缺陷程度不同的光纤的相关检测结果。     

Investigation of influence of tilt angle on mode dispersion in step-index plastic optical fibers

Effect of numerical aperture on mode dispersion and bandwidth is reported for commercially available step-index plastic optical fibers. For the first time, the functional relationship is given between dispersion and the “tilt-angle” describing the slant of the input/output face for a fiber terminated by a plane not perpendicular to the fiber axis. This tilt of a non-squarely terminated fiber may be intentional as in some biomedical spectroscopic sensors, or otherwise when exploiting the quick-interconnectivity potential of plastic fibers.     

电光调制在被动综合孔径成像探测中的应用

介绍了一种新型被动综合孔径成像探测方法:视场辐射信号被接收和放大后,通过电光幅度调制将其幅度和相位信息加载到光波上,经光纤传输在末端形成阵列,通过光学系统直接成像,将视场实时恢复出来.该方法可实现工作在微波、毫米波和太赫兹波段的高分辨力实时成像探测的目的.深入分析了电光调制器在综合孔径成像探测中的应用,建立电光调制模型,讨论了在小信号调制下的电光幅度调制近似理论.通过数值计算与仿真分析,得到综合孔径成像探测中电光调制器的调制信号强度限制的有关结论.结果表明,利用上变频电光调制技术和光信息处理,所得到的成像仿真图的半峰全宽和信噪比性能都优于传统的基于下变频技术的成像仿真结果.     

Effects of End-Face Tilt Angle on Numerical Aperture for Straight and Bent Plastic Optical Fibers

Numerical aperture (NA) is an important parameter of optical fibers. The larger it is, the greater the amount of light input into the fiber and, hence, the greater the achievable transmission distance. In this article, analytical functions are given for numerical aperture of an angle-ended plastic optical fiber in terms of the tilt angle of its input/output face. This tilt may be intentional, as in biomedical spectroscopic sensors, or otherwise when exploiting the quick interconnectivity potential of plastic fibers. The influence of fiber curvature on its numerical aperture has also been quantified as the functions given cover both straight and curved fiber configurations.     

空-频域中自成像分析和解释

基于魏格纳变换和魏格纳分布函数,分析讨论一维物体在空域-频域空间的自成像及其形成过程。从成像过程中各衍射频谱分量的光程差出发,给出Talbot效应和Montgomery效应的统一解释。对于周期物的Talbot效应,得到了用杨氏双缝干涉解释自成像现象的理论依据。周期物的自成像是物平面上间距为2倍周期,光程差为波长的整数平方倍的各衍射频谱分量同相相干迭加的结果。Montgomery效应是物平面上间距为抛物线关系,光程差为波长整数倍的各衍射频谱分量同相相干迭加的结果。     

Piecewise spectrally band-pass for compressive coded aperture spectral imaging

Coded aperture snapshot spectral imaging(CASSI) has been discussed in recent years. It has the remarkable advantages of high optical throughput, snapshot imaging, etc. The entire spatial-spectral data-cube can be reconstructed with just a single two-dimensional(2D) compressive sensing measurement. On the other hand, for less spectrally sparse scenes,the insufficiency of sparse sampling and aliasing in spatial-spectral images reduce the accuracy of reconstructed threedimensional(3D) spectral cube. To solve this problem, this paper extends the improved CASSI. A band-pass filter array is mounted on the coded mask, and then the first image plane is divided into some continuous spectral sub-band areas. The entire 3D spectral cube could be captured by the relative movement between the object and the instrument. The principle analysis and imaging simulation are presented. Compared with peak signal-to-noise ratio(PSNR) and the information entropy of the reconstructed images at different numbers of spectral sub-band areas, the reconstructed 3D spectral cube reveals an observable improvement in the reconstruction fidelity, with an increase in the number of the sub-bands and a simultaneous decrease in the number of spectral channels of each sub-band.     

光学稀疏孔径系统的成像及其评价方法

对典型阵列结构的光学稀疏孔径系统成像特性进行了数值仿真分析,并采用基于光学实验测量的调制传递函数（MTF）完成了光学稀疏孔径系统成像实验的图像复原处理.针对复杂目标成像,为了评价光学稀疏孔径系统最终成像的整体质量,不仅考虑系统的调制传递函数指标,还提出了一种基于相关系数的成像质量客观评价方法.数值仿真结果和光学实验结果均表明,基于相关系数的成像质量客观评价方法是可行的,实验说明光学稀疏孔径系统成像质量可以达到其等效单个大孔径成像系统的成像效果.     

绝对探测大气温度的纯转动拉曼激光雷达系统

纯转动拉曼激光雷达是探测大气温度廓线的重要手段之一,其正常工作需要配置其他并行校正设备,制约其在气象及环境监测领域中的实用化进程.基于大气氮气分子的纯转动拉曼谱型对温度的依赖性,提出并设计了绝对探测大气温度廓线的纯转动拉曼激光雷达系统.系统采用波长532 nm且脉冲能量300 m J的激光激励源和口径250 mm卡塞格林望远镜的接收器,设计了衍射光栅和光纤Bragg光栅结合的多通道并行纯转动拉曼光谱分光系统;仿真分析氮气和氧气分子的纯转动拉曼散射谱线间关系,优化选择了6条氮气分子的纯转动拉曼谱线以直接反演大气温度,设计了两级滤光器间转接光纤阵列的结构;基于最小二乘原理推导了绝对探测大气温度的反演算法,并结合标准大气模型,分析了纯转动拉曼激光雷达绝对探测大气温度的探测性能.结果表明,所设计纯转动拉曼激光雷达系统可直接反演大气温度廓线,在测量时间17 min内,温度偏差小于0.5 K的探测高度达2.0 km.     

Single-exposure two-dimensional superresolution in digital holography using a vertical cavity surface-emitting laser source array

We present a new implementation capable of producing two-dimensional (2D) superresolution (SR) imaging in a single exposure by aperture synthesis in digital lensless Fourier holography when using angular multiplexing provided by a vertical cavity surface-emitting laser source array. The system performs the recording in a single CCD snapshot of a multiplexed hologram coming from the incoherent addition of multiple subholograms, where each contains information about a different 2D spatial frequency band of the object's spectrum. Thus, a set of nonoverlapping bandpass images of the input object can be recovered by Fourier transformation (FT) of the multiplexed hologram. The SR is obtained by coherent addition of the information contained in each bandpass image while generating an enlarged synthetic aperture. Experimental results demonstrate improvement in resolution and image quality.     

基于法向映射法的二维光纤光谱弯曲校正

二维光纤光谱图是天文望远镜系统中的光谱仪的观测结果,由此又经过一系列的后续处理,才能生成普遍意义上的一维光谱。由于光谱仪和CCD相机的光学畸变,使得二维光纤光谱图像普遍出现了弯曲现象,尤其是在光纤两端表现得尤为明显,这个问题一直以来没有很好的解决办法,也没有在任何参考文献中看到相关工作。这种弯曲给后续的抽谱工作造成困扰,处理不好会很大程度上影响波长定标,从而影响一维谱的准确性。给出的一种法向映射法可以对这种二维光纤光谱图的弯曲进行有效的校正。该法把二光纤光谱图中的每一条光纤光谱进行单独处理。首先做预处理抽取每根光纤中心线,然后把该中心线当作光滑曲线求取每个点的法线方向,以整条中心线的某个点（一般是最凸点）为基点作理想竖直线,光纤中心线所有点沿自身法线方向投影到这条竖直线上,从而实现了光纤中心线本身的校直。因光纤宽度在二维光纤光谱图中一般是7个像素,因此将光纤中心线向左向右各依次平移3个像素,分别实现上述流程,即可得到整条光纤光谱的校直结果。该法实施过程中有两个问题需要注意：一个是坐标点的均匀化问题,一个是像素灰度值的精度保持问题。坐标点的均匀化问题是由于在该法中使用了法向映射,从而造成形成后的校直线的点的疏密程度不均匀,这对后续处理不利。解决办法是在实际操作中采用三次样条插值的方法进行直线上点的密度均匀化,保证获得一系列的整数点坐标,以利于后续的处理。而像素灰度值的精度保持问题,在插值计算过程中始终保持像素灰度值的64位高精度数,最终也得到同样精度的结果,避免造成像素值损失。该法实施的最后,还需要进行头尾一致性的截取,把延伸出图像高度范围的像素去掉,仅保留图像范围内的像素点。如果没有这个过程,由于光纤本身是有宽度的,映射出来的直线长短不一,从而给后续处理造成困难。实验完整处理了整幅的二维光纤光谱图,用曲线拟合的方法较好地解决了光纤中心线提取过程中的个别地方的亮度偏差问题,用法向映射法得到了校直后的二维光谱,并做了后续的抽谱对比。在一维谱的抽谱对比中可以看到,校正前后的谱线差值在有弯曲现象存在的光谱两端变化较为明显。这个实验结果证明了该方法对光谱的弯曲情况可以完全改善--在两端的变化较大,在中间的变化很小,这完全符合观测的基本认知。不仅如此,由于像素点的错位和像素值的插值计算,也会造成流量值叠加的效果发生显著变化。因此,通过观察抽谱后的谱线在校正前后的移动情况,验证了对于特征谱线的波长位置的精确获取具有重要影响。本文创造性地设计的二维光纤光谱的自动弯曲校正的方法。     

Microscopic scattering imaging measurement and digital evaluation system of defects for fine optical surface

According to ISO10110-7 and the engineering standards of Inertial Confinement Fusion (ICF), this paper presents a microscopic scattering imaging and analyzing system which allows one to automatically evaluate defects in random distributions and shapes on the surface of fine optical components of large aperture. The annularly disposed multi-beam fiber light sources illuminate the target surface with a special angle. The image, which has bright defects on black background, is suitable for digital image processing. With XY-scanning system, the defect information of full aperture can be obtained by stitching the sub-aperture image array, according to the algorithms of template matching. The full aperture image is divided into N × N sub-apertures, each of which has view field of approximate 3 mm × 4 mm. Image processing software for image recognition has been established using mathematical morphology with high computing efficiency and friendly graphics user interface. A group of standard reticules fabricated by binary optics can scale defects for calibration. As a result the lateral resolution of the system is better than sub-micrometer while the total view field can be hundreds of millimeters. The comparison quantitative data results between the experiment and standard demonstrate the system is competent for the digital evaluation of defect characterization of fine optical surfaces, especially for the ones with large aperture.     

Antiresonant guiding microstructured optical fibers for sensing applications

A novel refractometric sensor utilizing unique spectral properties of antiresonant-guiding microstructured optical fibers is proposed. The sensor operation is based on the wavelength shift of the transmission spectrum in response to the refractive index change of a sample loaded in the air-holes of the microstructured optical fiber. Refractive index changes on the order of 0.1% can be detected using less than a nanoliter of a sample.     

Image encryption based on gyrator transform and two-step phase-shifting interferometry

A novel optical image encryption method is proposed, based on gyrator transform and phase-shifting interferometry. The input two-dimensional image to be encrypted is gyrator transformed two times, and two random phase masks are placed at the input plane and the output plane of the first gyrator transform. Two-step phase-shifting interferometry is used to record the digital holograms of the input image encrypted by use of double-random phase encoding technique in gyrator transform domain. The rotation angles of gyrator transform, the random phase mask in the gyrator plane and the arbitrary phase shift used for recording form the keys for decryption of the input image. Numerical simulations are presented to verify its validity and efficiency.