非相干光源无衍射光的自重建

基于Hankel 波理论分析了非相干光源产生Bessel光束的自重建特性, 利用光学设计软件ZEMAX模拟了Bessel光束经过轴上圆形障碍物后的截面光强分布. 由于发光二级管(LED) 具有一定的频谱宽度且不像激光具有很高的相干度, 因此我们采用一定频宽范围的连续谱来描述. 从模拟结果可以直观地看出Bessel光束被轴上圆形障碍物遮挡后逐步完成自重建, 说明用LED非相干光作为光源具有自重建特性.实验上采用LED和轴棱锥元件产生Bessel光束, 然后通过轴上圆形障碍物、轴上方形障碍物, 并拍摄了不同位置处的截面光强分布图, 证实了非相干光源产生Bessel光束的自重建特性.实验结果和模拟结果相符合. 关键词： 无衍射光束 LED光源 轴棱锥 自重建     

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     

热光源产生贝塞尔光束的理论与实验

利用热光源获得了近似贝塞尔光束。对多波长光波同时入射轴棱锥的情形进行分析和数值模拟,结果显示轴棱锥后会形成近似贝塞尔光束,但受光场非相干叠加的影响截面光强对比度降低。实验采用卤素灯杯作为产生贝塞尔光束的热光源,针对热光源相干性差的特点设计出一套光学系统,提高光波的空间相干性,再使光波平行透过轴棱锥得到了近似贝塞尔光束。将实验结果与数值计算对比,发现实验所得贝塞尔光束的特性与理论基本吻合,但最大无衍射距离较理论计算所得的短,并对此进行了分析。     

基于轴棱锥产生近似无衍射Mathieu光束的新方法

提出了一种基于轴棱锥产生零阶近似无衍射Mathieu光束的新方法,利用轴棱锥聚焦具有椭圆高斯振幅调制的平面波,得到近似零阶无衍射Mathieu光束.根据椭圆高斯平面波经轴棱锥衍射的衍射积分公式,对光强分布进行了数值模拟,依据几何光学模型计算了近似无衍射Mathieu光束的最大无衍射距离,并设计了实验对理论模拟的结果进行了验证.实验采用柱透镜和准直扩束系统变换圆高斯光束产生具有椭圆高斯振幅调制的平面波,用轴棱锥聚焦该平面波后得到近似无衍射Mathieu光束,实验结果与理论模拟和计算相符.     

光源相干度对Bessel光最大无衍射距离的影响

理论分析和实验研究了光源相干度对其产生的Bessel光最大无衍射距离的影响.以He-Ne激光器为光源,用旋转毛玻璃法产生部分相干光,经过扩束后再用轴棱锥法产生Bessel光束.通过移动旋转毛玻璃改变部分相干光的相干度,测量其产生的Bessel光束的最大无衍射距离,得到光源相干度与产生Bessel光束最大无衍射距离的关系.结果表明,随着光源相干度的降低,Bessel光束最大无衍射距离明显变短,当相干度g≈1下降为g=0.8时,最大无衍射距离缩短了一半.用高斯-谢尔模型进行分析和模拟,理论模拟结果与实验观察结果基本吻合.     

体视显微镜与CCD成像系统在高阶贝塞尔光束光斑测量中的应用

主要对光学测量仪器体视显微镜与CCD成像系统在高阶贝塞尔光束光斑测量中的应用进行分析,通过以非相干LED绿光经过螺旋相位板和轴棱锥产生的高阶贝塞尔(Bessel)光束为例,模拟出沿轴向不同距离处的截面光强分布图,并与相应的实验参数(如:最大无衍射距离、中心暗斑直径等)进行比较。实验结果表明,体视显微镜与CCD成像系统的测量结果相吻合,但是体视显微镜测量误差较大会影响测量结果的准确度。     

轴棱锥对无衍射光束的线聚焦特性

基于汉克尔波理论和衍射积分理论详细分析了轴棱锥对无衍射光束的线聚焦特性, 提出了一种产生周期性局域空心光束的新方法, 即无衍射贝塞尔光束经过轴棱锥聚焦后产生具有塔尔博特效应的局域空心光束. 数值模拟了无衍射贝塞尔光束照射轴棱锥后, 沿传输距离变化的光强分布及一个周期内光强的演变和局域空心光束的形成过程. 设计实验系统, 由He-Ne激光经过一套光学系统后透过轴棱锥, 产生近似无衍射贝塞尔光束, 再由第二个轴棱锥对产生的无衍射贝塞尔光束进行线聚焦, 在第二个轴棱锥后面由显微镜观测到周期性局域空心光束, 并用CCD照相机拍摄了两个周期内的光斑图, 实验结果和理论分析相符合. 研究结果可用于多层面微粒的操控, 对周期性局域空心光束在光学微操控领域的应用具有重要的指导意义. 关键词： 贝塞尔光束 轴棱锥 线聚焦 局域空心光束     

自成像局域空心光束产生的新方法及粒子俘获

本文提出了产生自成像局域空心光束(self-imaged bottle beams)的一种光学元件------液体轴棱锥.从衍射理论结合几何光学对经过轴棱锥后的光场进行了分析, 得出注入液体折射率小于轴棱锥材料折射率时可产生自成像局域空心光束.并通过软件MathCAD模拟, 得到一个完整周期光束的变换过程和局域空心光束的演变过程.研究发现液体轴棱锥产生的自成像局域 空心光束具有周期及相干长度可调的特点.分析了如何利用自成像局域空心光束对粒子进行俘获, 讨论了用自成像局域空心光束进行多层面粒子俘获的优势.     

空心光束尺寸的精确调控

提出一种利用环缝精确调控空心光束尺寸的新方法.首先用环缝和旋转毛玻璃产生非相干准单色的环形光源,再经过薄透镜聚焦后产生空间相干度按第一类零阶贝塞尔函数分布的贝塞尔-谢尔模型光场,最后通过轴棱锥对贝塞尔-谢尔模型光场的线聚焦,产生空心光束.通过改变环缝大小控制非相干环形光源尺寸,对贝塞尔-谢尔模型光场的空间相干度分布进行调制,从而精确控制空心光束尺寸.应用广义衍射积分理论以及贝塞尔-谢尔模型描述该光学系统所产生的光场分布.设计相关实验进行验证,理论分析、数值模拟与实验结果相符合.该研究成果对不同尺寸粒子的微操控提供有力的手段.     

双锥面轴棱锥产生长无衍射距离的太赫兹贝塞尔光束

为了克服传统轴棱锥产生太赫兹贝塞尔光束无衍射距离短的局限,设计了一种双锥面轴棱锥结构。基于几何光学理论对双锥面轴棱锥产生太赫兹贝塞尔光束的原理进行了分析,并对太赫兹贝塞尔光束的无衍射特性和双锥面轴棱锥顶点对齐误差对无衍射特性的影响进行了理论和仿真计算分析。同时,采用透射式太赫兹时域光谱系统对双锥面轴棱锥的功能进行了实验验证。仿真和实验结果表明：太赫兹波经过锥面底角为γ₂=20°、γ₁=15°的双锥面轴棱锥后能够产生110 mm无衍射距离的太赫兹贝塞尔光束,相较于传统轴棱锥,无衍射距离增加了82.63 mm;无衍射距离随着锥形底面角度的增加而增大。     

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.