激光通过不同厚度的强散射介质的聚焦

通常情况下,激光光束经过散射介质后会产生无序的散斑。基于波前相位调制技术,利用连续序列算法结合四步相移法对入射激光的波面进行相位调制,使其通过散射介质在目标位置处形成聚焦。实验着重研究激光经过不同厚度的强散射片样品的聚焦,讨论散射介质的厚度与聚焦点光强增长因子的关系。实验结果表明,透过的散射介质越厚,聚焦光斑尺寸越小,目标位置处聚焦点的光强增长因子也越小。     

相干光经过不透明散射介质的聚焦

由于散射介质的散射特性,入射光通过散射介质后相干性受到破坏形成散斑场,使得传统光学成像系统无法聚焦探测内部物体的信息。根据波前整形原理,通过反馈优化算法调控入射光束的相位,在目标位置处将散斑调制为聚焦光斑,光强增强。基于此方法,在实验上搭建实验平台,对空间光调制器(SLM)的每个像素加载系列等间隔相位,一次采集多幅散斑图像,应用局部最大优化算法实现对光束任意点聚焦,并提出了一种得到不同大小区域的均匀的聚焦光斑的新方法,为实现聚焦平面内的目标成像提供了新思路。     

基于黄金分割法改进反馈波前整形技术实现散射光的聚焦EI北大核心CSCD

通过对入射光束的相位进行调控,可以将激光经过散射介质的散斑调制为聚焦光斑。提出了基于黄金分割法改进反馈波前整形技术的方法,实现了光束经散射介质的聚焦。理论分析了黄金分割法相比现有的顺序算法在搜索单峰函数极大值时的优势,并对散射聚焦过程进行了数值模拟。结果表明,当改变总调制单元数和单个调制单元的相位调制精度时,采用黄金分割法调制相位相比于顺序算法,在调制速度上具有一定优势。     

基于黄金分割法改进反馈波前整形技术实现散射光的聚焦

通过对入射光束的相位进行调控,可以将激光经过散射介质的散斑调制为聚焦光斑。提出了基于黄金分割法改进反馈波前整形技术的方法,实现了光束经散射介质的聚焦。理论分析了黄金分割法相比现有的顺序算法在搜索单峰函数极大值时的优势,并对散射聚焦过程进行了数值模拟。结果表明,当改变总调制单元数和单个调制单元的相位调制精度时,采用黄金分割法调制相位相比于顺序算法,在调制速度上具有一定优势。     

高斯光束经表面有缺陷的厚非线性介质的传输特性

光学元件上存在的缺陷会对传输光束产生局域振幅和位相调制. 基于衍射理论模型和分步傅里叶算法,模拟分析了高斯光束经过表面有缺陷的非线性介质的传输过程中于介质内及从介质出射后在自由空气的传输特性, 并详细研究了在厚介质前表面有缺陷的情况下,介质中和自由空气中的光强分布演化规律. 研究表明,介质厚度越长、介质的非线性折射率越大,光束整体聚焦越厉害, 聚焦点离介质后表面越近.光束受调制点的位置离中心越近,光束分裂成丝产生的局部光强越大, 且介质表面存在缺陷将使通过的光束在介质后表面处产生一个很大的光强, 相位调制型缺陷产生的这一光强点比振幅调制型缺陷产生的光强点更强.     

调制散射光束的振幅实现聚焦

光束通过生物组织白色涂料等散射介质后将发生散射,实现对散射光束进行整形和控制具有重要的意义。使用振幅调制实时反馈的方法,对入射到散射样品的相干光的振幅进行预调整,探测器实时读取散射光斑数据作为反馈依据,以此调制空间光调制器上的对应区域,进一步补偿由散射介质引起的振幅失调。实验结果表明,目标位置光强得到显著增加,其能量平均值是调制前散射区域能量平均值的11倍。     

四元裂解位相调制实现相干光通过散射介质聚焦

光在不均匀介质中传播会受到散射的干扰,在这些散射材料中,例如粉末、生物组织、亚波长颗粒对入射光多次散射使得出射光无法聚焦,从而在接收平面形成散斑.本文提出四元裂解位相调制方法对入射相干光场进行调制,使其通过散射介质聚焦.此方法利用入射光场全场调制,充分考虑光场单元之间的干涉作用,从整个空间光调制器的调制面开始,逐层进行四元裂解及位相优化.运用此方法在实验中实现了相干光的前向散射和后向散射有效聚焦,这为生物医学领域中通过散射介质成像提供了新的思路和方法.     

基于数字微镜器件超像素法实现散射介质传输矩阵的自参考干涉测量

散射介质的传输矩阵系统描述了散射介质对输入与输出光场之间的变换关系,是研究与控制光在无序介质中传输特性的重要工具.本文使用数字微镜器件实现散射介质传输矩阵的自参考干涉测量,首先利用超像素法实现对入射光的复振幅调制获得同时包含参考光和信号光的复合场,进而基于四步相移法分别测量了散射介质在Hadamard基和轨道角动量(OAM)基下的传输矩阵.进一步,根据相位共轭原理实现了光透过散射介质后的单点聚焦、多点聚焦以及涡旋聚焦,验证了传输矩阵测量的准确性.该方法能够有效提高光场调制的自由度,实现散射介质传输矩阵的测量,对散射环境下的光学成像和光通信等具有潜在应用价值.     

基于纯相位调制的散射介质传输矩阵测量与光波聚焦

提出了基于纯相位调制的传输矩阵通用测量方法,搭建了传输矩阵测量的实验装置。分别利用单位矩阵调制和哈达玛矩阵调制方式进行了传输矩阵测量,并实现了穿透散射介质的光波聚焦。实验结果表明,基于单位矩阵测得的传输矩阵,其聚焦点光强为背景光强的19倍;而基于哈达玛矩阵测得的传输矩阵,其聚焦点光强为背景光强的16倍。     

4pi聚焦系统中振幅和相位调制的径向偏振涡旋光束聚焦特性的研究

基于Richards-Wolf矢量衍射积分公式, 研究了径向偏振涡旋光束在振幅和相位调制下的4pi聚焦特性.振幅调制是通过振幅滤波实现, 即改变入射光束起始积分值达到调节,相位调制是通过添加相位延迟角δ 的液晶相位延迟器来改变入射光束的偏振态.模拟结果显示,随着振幅的减小, 4pi聚焦系统焦点附近的光轴上呈现出多光球结构; 而相位调制对焦点附近的光强分布产生拉伸作用, 即调节入射光束的拓扑核m和相位延迟器的延迟角δ,可以得到特殊的光强分布. 随着相位δ增大, m=0产生的多光球结构慢慢向光链结构转变,最终变成暗通道;而m=1产生的光链结构慢慢变成光球结构; m=2产生的暗通道变成光球和光链叠加的结构, 这种特殊聚焦光束在光学微操纵领域具有潜在的应用价值. 关键词： 物理光学 偏振 光链 4pi聚焦系统     

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.