基于激光偏振特性的水下声信号探测方法研究

水下航行体辐射的声波传播到水-空气分界面上时,会在水表面形成微扰,使水表面元产生微倾角.提出一种利用激光偏振特性对水下声信号进行探测的方法,不直接利用散射光强信息,而是利用激光在布儒斯特角附近入射水表面时,反射光偏振特性随入射角剧烈变化的特点对水表面微扰进行高精度探测.建立了该方法的理论模型,得到了入射波、出射波垂直分...     

模拟溢油样品激光诱导荧光光谱的偏振特性研究

激光诱导荧光是海洋溢油探测的有效手段之一,但该技术的应用易受自然水体中叶绿素、CDOM等物质荧光信号的干扰。为了寻求排除自然水体荧光干扰的方法,基于532 nm连续激光器搭建了激光诱导偏振荧光实验装置,并针对六种不同密度的模拟溢油样品和自然水体开展了荧光光谱偏振特性研究。实验结果发现,与自然水体的荧光光谱不具有明显的偏振特性不同,所有模拟溢油样品的诱导荧光均具有明显的偏振特性,这一结论说明激光诱导荧光光谱的偏振特性可以作为排除叶绿素、CDOM等物质荧光干扰的依据。实验还发现,溢油样品的荧光偏振性质因样品种类而异。在线偏振光激发下,原油样品荧光偏振度随波长逐渐降低,其中重质原油样品偏振度降低幅度最大,轻质原油样品幅度最小,而柴油样品荧光偏振度没有明显变化;当周期性改变激发光的偏振状态时,所有模拟溢油样品的荧光偏振度随之发生趋势相同的周期性波动,中质样品荧光偏振度波动的幅度低于重质样品,但明显超过轻质样品。这一结果说明,模拟溢油样品诱导荧光偏振度的波长变化特性及对激发光偏振态的响应特性均与样品密度存在一定关联,其偏振特性可以作为辅助油种识别的重要参量。     

湿度影响下的气溶胶粒子的偏振特性

气溶胶散射特性对大气辐射和气候研究具有重要意义。为研究气溶胶在不同相对湿度下的散射特性,根据Mie散射理论建立了湿度偏振模型,在0.55μm的可见光下,分析比较了3种气溶胶的单粒子和粒子群在不同相对湿度下的偏振特性。结果表明,相对湿度变化对气溶胶偏振度有规律性的影响,尤其在120°～150°的后向散射角区域,3种粒子偏振度都随相对湿度的增大而增大,说明了部分后向散射角上散射光偏振度能有效反映气溶胶粒子随相对湿度变化的信息。     

尾流气泡幕光学特性的数理模型

对实验室模拟的尾流以及真实尾流气泡幕中气泡的运动规律进行了分析和计算,并提出了气泡幕中气泡随半径分布规律的数学物理模型,引入了最可几半径的概念从单个气泡对激光散射的影响推导出了气泡幕对激光的散射效应的数学表达式,并利用所得到的表达方式在实验室条件下进行了相关计算所建立的理论模型对于气泡分布的计算结果同实验结果相比,一致性相当好在气泡幕对激光散射的影响中,首次对激光偏振化特性的影响进行了区别计算计算结果表明:激光的两种偏振化状态对气泡幕散射的影响从粗糙结构来看没有明显差异,但其精细结构有所不同对不扩束和扩束两个条件下的计算明确显示出,扩束条件下散射光的强度是不扩束条件下的二倍以上,但二者的精细结构并没有可见的差别.     

基于蒙特卡罗法的水下激光光幕探测性能研究

为了提高水下激光光幕的探测性能, 根据水下光束传播规律, 构建了水下激光光幕探测模型, 基于蒙特卡罗模拟方法对水下激光光幕探测性能进行研究。基于水下激光光幕探测模型, 利用MATLAB软件进行仿真, 分析海水衰减系数、初始功率及传输距离对水下激光光幕传输的影响。仿真结果表明:海水的衰减系数越小, 水下激光光幕传输率受到的影响越小。海水参数和传输距离一定时, 随着初始功率的增加, 只会影响到达探测端的最终功率, 但对传输率影响不大。当海水衰减系数一定, 传输距离为1 m时, 其传输率约为15%且变化稳定; 当传输距离增加到30 m时, 传输率在5%以下。     

水云条件下大气偏振特性研究及其模拟分析

研究了水云光学特性对大气偏振特性的影响,对水云条件下大气偏振模式进行了仿真。使用基于蒙特卡罗法的矢量辐射传输模式,模拟350~700nm波长下大气偏振特性随水云的光学厚度、有效半径的变化趋势。通过计算450nm波长处不同太阳高度角下的全天空离散点的Stocks矢量,对水云下大气的偏振分布模式进行研究,并与晴朗大气下的天空光偏振度和偏振方位角模式进行分析比对。结果表明,波长越大,偏振度随水云光学厚度增大而减小的趋势越明显。随着有效半径的减小偏振度有一定程度的减小,但对短波波段影响较小。随着太阳高度角的增大,天空可探测区域整体偏振度下降。天空光的偏振度近0区域,会对其附近偏振方位角的准确性产生一定影响。水云天气下的大气偏振分布研究为偏振光导航传感器的实际应用提供了理论基础。     

不同气溶胶环境中相干激光雷达回波特性

基于Mie散射理论,给出了利用半解析蒙特卡罗法模拟气溶胶粒子对水平线偏振激光后向散射的主要过程;基于气溶胶和云的光学特性库,选择了4种典型环境、5种典型气溶胶组分和3种湿度条件,分别计算了1.5μm和2.0μm波段处后向散射回波的退偏度、偏振度和回波光子数,得到了后向散射光的偏振特性随各组分粒径的变化规律以及各气溶胶环境的后向散射激光回波特性,并分析了不同环境下达到相同探测性能时所需的激光功率比,为远距离高精度相干激光雷达系统的设计和分析提供了参考。     

基于多重散射强度和偏振特征的舰船尾流气泡激光探测方法

研究多重散射效应对舰船尾流气泡群光散射强度和偏振特征的影响是舰船光尾流探测以及新型光自导鱼雷研究的基础. 基于矢量Monte Carlo方法建立了舰船尾流气泡群激光后向探测仿真模型, 重点研究了尾流气泡群的多重散射机理,分析了多重散射效应、尾流气泡群密度对回波信号强度和偏振特征的影响规律. 基于粒子碰撞重要性抽样的基本思想, 在传统能量接收方法的基础上, 提出了回波光子偏振贡献接收方法和回波信号偏振信息统计方法, 解决了小视场系统光子返回概率低无法形成回波能量的难题. 构建了模拟尾流气泡群激光散射强度和偏振探测实验平台, 从实验的角度验证了模拟结果的准确性. 实验和模拟结果的一致性表明, 利用回波强度、偏振信息可表征气泡群距离、密度信息, 从而可对舰船尾流特别是低密度尾流进行高精度的探测和辨识. 关键词： Monte Carlo 偏振 多重散射 气泡     

石榴石型偏振调节器温度效应研究

采用磁控溅射法制备了石榴石型偏振调节器并研究了温度对其的影响.利用斯托克斯偏振仪和CCD,研究激光透过石榴石及石榴石型偏振调节器的偏振极化性质和光斑变化.实验结果表明:在25～75℃温度范围内,激光穿过不同材料时,斯托克斯参量变化趋势不同;但是因方位角不改变,激光的偏振度、线偏振度、圆偏振度基本不变.     

表面纳米粒子缺陷的偏振散射特性区分

为了区分纳米量级的表面上方颗粒物灰尘与表面下方气泡粒子这两种表面缺陷,且获得该方法的适用环境与最佳观测条件,根据瑞利散射理论结合偏振双向反射分布函数,建立了两种表面缺陷的偏振散射模型并进行了验证。在此基础上,通过仿真分析得到不同缺陷环境、不同观测条件对两种表面缺陷粒子偏振散射特性的影响。结果表明:利用p偏振光入射表面,而后探测p偏振光的双向反射分布函数值随散射方位角的变化趋势可区分两种表面缺陷;无论表面下方气泡粒子位置如何改变,均不影响该趋势的变化情况;不同光学元件表面材料、缺陷粒子种类、缺陷粒子大小对两种表面缺陷的偏振散射模型有一定影响,但整体趋势不变。实验中,针对本文所述两种表面缺陷进行区分时,可选取入射角度和探测散射角度均为45°,采用较小波长入射光进行实验。     

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.