阵列抽样衍射成像中记录孔径对再现像的影响

从标量衍射理论出发,对基于阵列抽样的相干衍射成像中物波抽样针孔大小和图像传感器有效记录孔径对重现物波的影响进行了理论分析,给出了描述抽样孔径和记录孔径对波前再现像影响的数学公式。理论分析表明,在抽样波前的重现过程中记录孔径的有限大小会导致相邻抽样孔径间发生串扰;这种串扰效应是产生波前重现误差及由此引起的成像噪声的主要来源。通过数值分析串扰次函数极大幅度随记录孔径的变化情况,发现这种串扰效应随着记录孔径的增大振荡减弱。基于该理论分析,给出了最佳记录孔径的定义及其计算公式。并通过一个实验测量实例进一步验证了当记录孔径大小等于最佳记录孔径时得到的衍射像的信噪比最大。     

菲涅耳数字全息成像系统的焦深

焦深是表征成像系统性能的重要参数.基于透镜相干成像的焦深判据,利用数值积分和计算机模拟的方法,分析和推导了菲涅耳数字全息成像系统的焦深,给出了相应的焦深表达式,并与利用理想成像四分之一波像差的判据得到的焦深进行了比较.结果表明:菲涅耳数字全息成像系统的焦深不同于透镜相干成像系统的焦深.在记录距离和CCD参数一定的条件下,离轴菲涅耳数字全息成像系统的焦深略大于同轴菲涅耳数字全息的焦深;离轴对称偏置情况下的焦深略大于非对称情况下的焦深.利用像强度判据得到的焦深,能够较好地表征数字全息成像系统的性能.     

图像传感器像素化效应对菲涅耳非相干关联全息分辨率的影响

作为复光场显微成像的一种新技术,菲涅耳非相干关联全息术(Fresnel incoherent correlation holography,FINCH)因其非相干光记录的特点在近年来受到关注.FINCH作为一种新型非相干全息系统,如何设计光路实现其最佳的分辨率是一个关键问题.然而,针对这个问题的讨论,目前已有文献存在不同的观点,有关FINCH最佳分辨率的成像条件仍有待研究.全息图有效孔径大小是决定全息成像系统分辨率的重要因素,在FINCH系统中,全息记录距离的变化则会引起全息图有效孔径发生变化,全息图的有效孔径大小不仅与光路各元件的孔径有关,还与相干光波相互干涉叠加区域的面积以及图像传感器的像素间距等因素有关.本文基于波动光学理论,结合FINCH全息图的波带结构特征,研究了FINCH全息图的有效孔径.研究发现数字全息记录相机的像素化特性是影响FINCH成像分辨率的决定性因素,并进一步通过数值模拟和光学实验验证了理论分析结果:全息图记录距离(Z_h)等于空间光调制器加载的衍射透镜焦距(f_d)时,FINCH系统的再现像将会达到最佳横向分辨率,且分辨率随成像距离|Z_h-f_d|的增大而降低.     

衍射微透镜列阵在Shack-Hartmann波前传感器中的应用

菲涅耳衍射微透镜列阵是目前广泛应用的衍射光学元件之一,它是基于衍射原理,由计算机设计,并通过微细加工技术制作成的.本文介绍了8相位台阶菲涅耳衍射微透镜列阵的制作方法,并描述了一个用16×16微透镜列阵形成的小型 Shack-Hartmann 波前传感实验系统,用该系统可对入射波前进行测量和重建.     

显微数字全息中物光波前重建方法研究和比较

根据全息理论和线性系统理论,采用离轴无透镜傅里叶变换全息记录光路,对利用菲涅耳近似法、基于瑞利—索末菲衍射积分的卷积法以及角谱理论方法数值重建全息图进行了比较研究,并做了计算机模拟验证.结果表明：菲涅耳近似法和角谱方法重建像质比较好,且菲涅耳方法重建速度快；在记录距离极小的情况下,尽管记录距离不满足通常的菲涅耳近似条件,菲涅耳近似公式仍然成立；自由空间脉冲响应的快速傅里叶变换的性质与距离有关,由卷积方法得到的再现像只在某一特定距离下比较理想；对于极小物场、大孔径显微数字全息来说,菲涅耳近似重建方法是较为有效的方法.     

波带片的衍射(一)

一、引言 菲涅耳波带片①的衍射,揭示了利用衍射原理制造成像元件的可能性.然而由于制作上的困难、衍射效率低及存在多级像等原因,长期以来它没有得到很快的发展及实际应用.60年代以来全息透镜的研制和应用,使波带片在光学教学中占有了新的地位.但是,菲涅耳波带片为什么存在多个焦点?各焦点的位置和强度怎样?波带片与普通透镜的成像原理有何不同?这些问题在一般的光学教材中尚未见有严密而易于为大学生所接受的数学论证.本文从菲涅耳-基尔霍夫衍射积分公式出发,经过恰当的近似处理,求出了波带片沿轴线上的衍射场分布,然后对上述问题进行了讨…     

单色点源圆孔菲涅耳衍射光场的计算与模似

点源圆孔衍射在光束的传输与变换、光学盛像系统的衍射成像及光信息处理等方面有着重要的作用．本文探讨单色点源照射圆孔产生的菲涅耳衍射光场的计算．     

轴棱锥产生无衍射光束自再现特性的几何光学分析

由几何光学方法分析轴棱锥产生无衍射光束自再现特性,很好地解释了无衍射光束自再现的形成原理.在菲涅耳近似条件下,利用菲涅耳衍射理论可以对光传输进行很好地描述;而光束经过障碍物后的很小距离内,菲涅耳衍射近似条件已不满足,衍射理论不再适合描述光束的传输特性,这时可以利用几何光学分析光束传输特性.首先从几何光学角度对轴棱锥产生无衍射光束的自再现特性进行了详细的描述,并对光束传输进行仿真,最后通过实验验证轴棱锥产生无衍射光束的自再现特性,实验结果与理论分析相符合. 关键词： 无衍射光束 几何光学 轴棱锥 自再现     

相移同轴无透镜傅里叶数字全息的分析与实验

应用菲涅耳衍射和全息理论，详细分析了无透镜傅里叶变换数字全息图的记录、再现方法和再现像的特点，分析了相移数字全息图的记录和再现方法，并进行了相应的实验验证。结果表明：直接对无透镜傅里叶数字全息图进行傅里叶逆变换可同时得到与物体完全相同的再现像及其共轭像；同轴无透镜傅里叶数字全息术能最大程度满足CCD对采样条件的要求，从而可以增大记录物体的尺寸，减小记录距离，明显提高再现像的清晰度和分辨率；相移数字全息术能有效地消除数字再现光场中的零级光场和共轭像，显著提高再现像的信噪比。条件许可时，相移同轴无透镜傅里叶数字全息术是目前解决数字全息术中再现像的分离与满足采样条件之间矛盾的最佳方法。     

线宽误差对菲涅耳透镜衍射效率的影响

基于标量衍射理论,首先得出存在线宽误差情况下4阶菲涅耳透镜衍射效率的解析式;并以 4阶、8阶和16阶菲涅耳透镜为例,进行计算机模拟计算并系统分析线宽误差对菲涅耳透镜衍射效率的影响。通过实验测量16阶菲涅耳透镜的线宽误差,分析其对16阶菲涅耳透镜衍射效率的影响。模拟计算结果对多阶菲涅耳透镜的制作提供重要的理论指导。     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.