新型相干结构光场调控及应用研究进展

结构光场的空域调控包括振幅、相位、偏振、相干度等丰富自由度,对其自由度的单一或联合调控引发了一系列新奇物理效应,在新型结构光场构建及多种领域中具有重要应用。相比于完全相干光场,部分相干光场在抵抗散斑噪声和大气湍流扰动等方面具有独特优势。近年来,具有新型相干结构的部分相干光束在大气传输、光学加密与成像、信息鲁棒传输、高质量光束整形等领域有着重要研究价值。本文详细综述了具有新型相干结构部分相干光场的理论构建与实验合成的研究进展,并重点介绍了新型相干结构光场在复杂环境中的鲁棒传输特性及其在光学加密、成像、鲁棒信息传输及光束整形中的应用研究进展。研究表明,新型相干结构光场调控不仅提供了一种有效抵抗复杂环境扰动的有效手段,而且丰富了光场调控技术在多种领域中的应用。最后,对新型相干结构调控技术发展趋势及潜在应用前景进行了展望。

     

Multi-receivers and sparse-pixel pseudo-thermal light source for compressive ghost imaging against turbulence

For remote sensing with computational ghost imaging, the atmospheric turbulence will influence much on the performance since the transmitted patterns from the pseudo-thermal source will be distorted. As the distribution of refractive index on the propagation path varies with time, reducing the time consumption of data acquisition will be an effective way for mitigating the influence. We propose using fewer pixels of spatial light modulator to increase the modulation rate of the pseudo-thermal source, with the positions of pixels being randomly distributed. Multi-bucket detectors are employed for further reducing of the data acquisition time. The sparse recovery algorithm from compressed sensing is employed to enhance the quality of image. The performance of this system is demonstrated by comprehensive numerical analyses.     

Composite coherence vortices in coherent and incoherent superpositions of two off-axis partially coherent vortex beams

The composite coherence vortices by coherently and incoherently superimposing two parallel, off-axis partially coherent vortex beams and their evolution in free space are studied. It is shown that the superposition scheme, off-axis distance, coherence parameter and propagation distance affect the position and number of composite coherence vortices. The motion, creation and annihilation of composite coherence vortices appear by varying the off-axis distance, coherence parameter and propagation distance. The coherent and incoherent superpositions result in the different position and number of composite coherence vortices and their different evolution behavior in the coherent limit.     

Topology optimization for optical microlithography with partially coherent illumination

This article revisits a topology optimization design approach for micro‐manufacturing and extends it to optical microlithography with partially coherent illumination. The solution is based on a combination of two technologies, the topology optimization and the proximity error correction in microlithography/nanolithography. The key steps include (i) modeling the physical inputs of the fabrication process, including the ultraviolet light illumination source and the mask, as the design variables in optimization and (ii) applying physical filtering and heaviside projection for topology optimization, which corresponds to the aerial image formulation and the pattern development processes, respectively. The proposed approach results in an effective source and a binary design mask, which can be sent directly to fabrication without additional post‐processing steps for proximity error correction. Meanwhile, the performance of the device is optimized and robust with respect to process variations, such as dose/photo‐resist variations and lens defocus. A compliant micro‐gripper design example is considered to demonstrate the applicability of this approach. Copyright © 2016 John Wiley & Sons, Ltd.     

多束部分相干光通过强湍流对光强闪烁的影响

在Rytov方差的基础上,利用Andrews的唯像闪烁模型,推导出部分相干光通过强大气湍流后其对数光强起伏方差的公式,并用此公式对部分相干光通过强湍流后给光强闪烁造成的影响进行了仿真。其结果表明,当光源的相干性变差,即变为部分相干光后,对数光强起伏方差变小;当采用多束部分相干光时,接收面上光强起伏方差得到明显改善,而且光束越多,改善越明显。     

Optical image encryption scheme with multiple light paths based on compressive ghost imaging

An optical image encryption method with multiple light paths is proposed based on compressive ghost imaging. In the encryption process, M random phase-only masks (POMs) are generated by means of logistic map algorithm, and these masks are then uploaded to the spatial light modulator (SLM). The collimated laser light is divided into several beams by beam splitters as it passes through the SLM, and the light beams illuminate the secret images, which are converted into sparse images by discrete wavelet transform beforehand. Thus, the secret images are simultaneously encrypted into intensity vectors by ghost imaging. The distances between the SLM and secret images vary and can be used as the main keys with original POM and the logistic map algorithm coefficient in the decryption process. In the proposed method, the storage space can be significantly decreased and the security of the system can be improved. The feasibility, security and robustness of the method are further analysed through computer simulations.     

Optical trapping Rayleigh dielectric particles with focused partially coherent dark hollow beams

The focusing properties of coherent and partially coherent dark hollow beams (DHBs) through a paraxial ABCD optical system are theoretically investigated. It is found that the evolution behavior of the intensity distribution of focused partially coherent DHBs is closely related to their spatial coherence. The radiation forces (RFs) of focused coherent and partially coherent DHBs acting on a Rayleigh dielectric particle are also theoretically investigated. Numerical results show that the coherent and partially coherent DHBs can be focused into a tight focal spot, which can be used to stably trap a Rayleigh dielectric particle with high refractive index at the focus point. The influences of different beam parameters, including the spatial coherence, beam waist width, beam order, and hollow parameter of partially coherent DHBs, on the RFs and the trap stiffness are analyzed in detail. Finally, the stability conditions for effective trapping particles are also discussed.     

部分相干平顶光束的实验研究

本文利用对入射光进行相干度调制的方法,在实验上研究了聚焦光场中的部分相干平顶光束,用CCD记录了光束的产生和演变过程,验证了聚焦光场中部分相干平顶光束的存在.此外,还研究了对于不同的聚焦透镜焦距和环形光阑中心拦截比,不同空间相干度对平顶光束的影响.研究表明,两处平顶光束的大小及两处平顶光束之间的距离随着空间相干长度和环形光阑拦截比的增大而减小,随聚焦透镜焦距的增大而增大.     

Propagation properties of partially coherent anomalous hollow beams in uniaxial crystals

Analytical propagation formulae for partially coherent anomalous hollow beams in uniaxial crystals are derived. Paraxial propagation of partially coherent anomalous hollow beams in uniaxial crystals orthogonal to the optical axis is investigated based on the beam propagation equations. The propagation properties of partially coherent anomalous hollow beams in uniaxial crystals and in free space are studied numerically and comparatively. It is found that the propagation properties of partially coherent anomalous hollow beams in uniaxial crystals behave very differently from those in free space and are closely determined by the parameters of the uniaxial crystals and the initial coherence width. The uniaxial crystals provide an effective way for generating astigmatic beams.     

Effects of atmosphere on spectrum of modified partially coherent flat-topped beams

The analytical formulas for the spectrum of modified partially coherent flat-topped (MPCFT) beams propagating in a turbulent atmosphere have been derived under strong fluctuation condition of turbulence. The spectral properties of MPCFT beams propagating in a strong turbulence have been investigated, and the changes of on-axis and off-axis spectral distributions have been analyzed quantitatively. The results show that the on-axis relative spectral shift of the modified partially coherent flat-topped beam exhibits spectral switch for the beam order M > 1, and with the increasing propagation distance the blue-shift gradually decreases in atmospheric turbulence. A rapid transition for the off-axis spectrum appears at one critical position in turbulence, and the position decreases with the increasing beam order for the lower beam order and the nearer propagation distance. The spectral properties and the spectral shifts of MPCFT beams also depend on the inner scale, the spatial coherence parameter.     

光谱相关能级匹配遥感目标提取算法

光谱相关能级波形匹配算法是在利用光谱相关系数确定谱带强度相似度,能级波形匹配技术确定波形特征相似度的基础上,将二者的乘积作为图像光谱与参考光谱间的总相似度来进行的高光谱遥感和多光谱遥感精确目标提取算法.在阐述基本原理的基础上,首先模拟了大气、噪声等边界条件对算法相似度的影响;其次,利用400个图像测试光谱对算法进行了精度评价;最后,对4种端元进行了目标提取.边界条件模拟结果表明,算法对大气效应具有较好的抑制作用,当大气效应小于模拟数值的50倍时,可以省略大气订正.虽然算法对噪声较为敏感,但通过噪声消除算法可以将该影响减小到最低,甚至将其消除掉;精度评价结果表明,算法的平均精度为85%.     

遥测技术在机动车排气检测中的应用研究

针对机动车保有量快速增长而传统排气监测效果与实际路况排放差异大、检测效率低、社会经济成本高等现状,探讨机动车排气遥测技术的优点,并以实际道路测试数据为例分析筛选高排车、比功率分布区间等实际应用问题。结果表明:遥测技术适用于机动车排气监测技术,对筛选高排车尤其出色。     

Propagation properties of partially coherent radially polarized beam in a turbulent atmosphere

Based on the Huygens–Fresnel principle and the unified theory of coherence and polarization of partially coherent beams, we investigate the propagation characteristics of a partially coherent radially polarized doughnut (PCRPD) beam in a turbulent atmosphere. It is found that, after propagating through a turbulent atmosphere, the doughnut beam spot is changed into a circular Gaussian beam. Moreover, the degree of coherence, the degree of polarization and the degree of cross-polarization of the beam will change on propagation, and this change is dependent upon the degree of coherence of the source and atmospheric turbulence.     

Propagation of a partially coherent higher-order cosh-Gaussian beam through a paraxial ABCD optical system

Based on the generalized Huygens–Fresnel integral, analytical expressions for the mutual coherence function, the spatial complex degree of coherence, and the effective size of a partially coherent higher-order cosh-Gaussian beam through a paraxial ABCD optical system have been derived. As a numerical example, the propagation of a partially coherent higher-order cosh-Gaussian beam through an optical Fourier-transforming system with a limiting aperture is illustrated. The normalized intensity distribution, the spatial complex degree of coherence, and the effective beam size for the partially coherent higher-order cosh-Gaussian beam are numerically demonstrated in the observation plane. The influences of the spatial coherence length and the limiting aperture on the normalized intensity distribution, the spatial complex degree of coherence, and the effective beam size are also examined in detail.     

Subnanosecond switching, plasma electrode pockels cell for high-contrast imaging with partially coherent light

A high-contrast optical switch for imaging partially coherent light (~150 times the diffraction limit) requires that it have a large angular acceptance. We describe the development of a high-speed Pockels cell that uses a thin crystal to simultaneously achieve high-contrast (greater than 1800:1) and large-angular acceptance (greater than 7 mrad for a 5-mm aperture). A KD*P crystal was used in a longitudinal-mode configuration with plasma discharges forming low-resistance, high optical transmission electrodes to couple the switching voltage. Rise times of the switched optical pulse of the order of 500 ps were observed. Characterization of the device in the near and far fields was also performed.     

Forest survey and management using remote sensing

Some of the important work done in India and other parts of the world in the application of remote sensing for forest survey and management has been reviewed in this paper. The account of work has been given under three main headingsviz airborne remote sensing, satellite remote sensing and multistage approach.     

Focal shift and focal switch of partially coherent flat-topped beams passing through an alignment and misalignment lens system with aperture

The focal shift and focal switch of partially coherent flat-topped (PCFT) beams passing through a lens system with an aperture are studied in detail. We have shown that a focal shift is also present for beams propagating through an aligned optical system and the amount of the focal shift depends not only on the radius aperture of the focusing system, but also on the spatial coherence and order of the flat-topped beam of the incident partially coherent light. A new phenomenon called ‘focal switch’ occurs for misaligned optical systems, i.e. the focal shift experiences a sudden transition as the aligned optical system becomes misaligned, and the influence of the spatial coherence, order of flat-topped beam and aperture size reduction, on the focal switch is investigated in detail. Finally, the necessary conditions for the focal switch to take place are investigated.     

On an approach for improving the range resolution of pulsed coherent Doppler lidars

As a result of an analysis of the autocovariance of the complex heterodyne lidar signal, some general-enough inverse techniques (algorithms) are derived for recovering with high range resolution, below the sensing pulse length, of Doppler-velocity profiles in the atmosphere. Unlike our preceding works, it is assumed here that the laser pulses can have arbitrary fluctuating shape. The presence is also supposed of possible regular, arbitrary in form, intrapulse frequency deviations (chirp) and random frequency, phase and radial (Doppler)-velocity fluctuations. The algorithm performance and efficiency are studied and illustrated by computer simulations, taking into account the influence of the chirp and various random factors such as additive noise, pulse-shape fluctuations and radial-velocity fluctuations. It is shown that the algorithms developed allow the Doppler-velocity profiles to be determined with a considerably shorter resolution interval compared with the pulse length, at a reasonable number of signal realizations (laser shots) and appropriate data processing to reduce the statistical error due to the random factors.     

遥感影像在地外文明探索中发挥作用

六十年代以来,已向月球和水星、金星、火星、木星、土星、天王星,海王星等太阳系行星发去数十个空间遥感器,并已向地球上的地面站发回大量信息,使科学家们得以发现很多有趣的事物,显示为地球外生物高度文明的作为。本文着重介绍了在月球上发现的异常景物,显示可能在多年以前,曾有高度文明的生物在月球着陆。     

Shedding light on electrodeposition dynamics tracked <Emphasis Type="Italic">in situ</Emphasis> via soft X-ray coherent diffraction imaging

The in situ physicochemical analysis of nanostructured functional materials is crucial for advances in their design and production. X-ray coherent diffraction imaging (CDI) methods have recently demonstrated impressive potential for characterizing such materials with a high spatial resolution and elemental sensitivity; however, moving from the current ex situ static regime to the in situ dynamic one remains a challenge. By combining soft X-ray ptychography and single-shot keyhole CDI, we performed the first in situ spatiotemporal study on an electrodeposition process in a sealed wet environment, employed for the fabrication of oxygen-reduction catalysts, which are key components for alkaline fuel cells and metal-air batteries. The results provide the first experimental demonstration of theoretically predicted Turing–Hopf electrochemical pattern formation resulting from morphochemical coupling, adding a new dimension for the in-depth in situ characterization of electrodeposition processes in space and time.

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