结构光测量中的高精度相位误差补偿算法

在模拟分析投影仪伽马非线性对相位误差影响的基础上,提出一种直接分析投影光栅特征并建立相位误差查找表的算法.对相位误差进行补偿.该算法通过分析一组投射到标准白色平板上的光栅图像,确定光栅相位值与相位误差的对应关系,并量化存储在一个查找表中,测量过程中使用查找表对相位误差进行补偿.实验结果表明.该方法可大大降低由投影仪伽马非线性引起的相位误差,系统测量精度达到0.043 mm,比误差补偿前提高了5.6倍.     

提高多频条纹投影相位提取精度的反向误差补偿法

为了减小光栅投影三维测量系统中数字投影仪的非线性响应引起的相位误差,提出了一种提高物体相位测量精度和速度的多频条纹反向相位误差补偿方法。该方法通过投影与最高频率相同且具有特定相移量的补偿相移条纹图,获取相位误差大小相等,符号相反的两幅主值相位图,二者运算后误差得以抵消,与多频法相结合从而得到精确的绝对相位值。采用标准平面对提出的方法进行实验验证,并与最近提出的希尔伯特变换补偿方法以及典型相位补偿方法进行比较。实验结果表明,所提方法能有效提高相位测量的精度和速度。通过对自由曲面以及表面不连续物体进行相位误差补偿,进一步验证了该方法的可行性和有效性。     

光栅投影测量系统中的相位误差校正算法

依据光栅图像的相位单调增长特性,提出了一种基于最小二乘法的单调光顺算法进行光栅相位误差校正。详细分析了光栅图像的相位单调特性和相位误差,以光栅图像相位的单调性为约束条件,改进了传统的最小二乘拟合方法,对绝对相位图进行相位误差校正。实验结果表明,该方法降低了投影仪伽马非线性造成的相位误差,误差减少60%以上,校正后的绝对相位具有光滑性良好的优点。     

转镜式傅里叶变换光谱仪光程差非线性的拟合法补偿

转镜式傅里叶变换光谱仪中光程差非线性带来了干涉图周期变化和相位误差,对非线性的拟合法补偿利用采样得到的干涉图进行重构,得到不含相位和周期误差的干涉图,再按照均匀采样函数对干涉图进行二次采样,消除非线性光程差对复原光谱的影响.经过二次采样的干涉图不含周期和相位误差,可以直接使用现有的软件进行光谱复原.计算机仿真实验证明这种方法行之有效,且利用4次多项式拟合干涉图效果更好.     

单周期条纹双四步相移投影仪的标定方法

针对投影仪标定方法中存在畸变及倾斜投影引起条纹周期、条纹级数变化的问题,提出一种单周期条纹双四步相移投影仪的标定方法.设计生成横向和纵向各两组单周期条纹图像,经投影仪投影到带有圆形标识的标定板上,相机同步采集标定板图像,叠加由双四步相移获得的两幅相位主值图,对叠加相位主值图相位展开,利用展开的绝对相位值计算投影仪像素坐标值,最终将投影仪标定转换为成熟的相机标定.实验结果表明:仿真投影仪标定实验准确度的最大重投影误差约为0.4pixel,均方根误差为0.132 96pixel;实际投影仪标定实验准确度的最大反投影误差约为0.46pixel,均方根误差为0.143 12pixel;实验结果与仿真结果的最大反投影误差相差15%,均方根误差相差7.6%.与现有的采用三频相位展开进行投影仪标定的方法相比,投影光栅图像数可减少8幅.该方法改善了现有投影仪标定方法的不足,标定准确度和标定效率均得到提高.     

数字显微全息中二次项相位误差的补偿

针对预放大式数字离轴显微全息技术中二次项相位误差,提出一种基于单幅全息再现的补偿方法.该方法是在显微物镜后焦平面处实现该相位误差的数学补偿,相位误差补偿过程具有调整参量单一化的优点.同时,利用显微物镜的后焦平面上物点特征实现了最佳再现距离的自动判断.该文以相位光栅为实验样本,对该方法进行了实验例证,实验结果验证了本文所提出二次项相位误差的补位误差的补偿方法的正确性和有效性.     

离焦投影三维测量的二值光栅生成方法

投影仪离焦技术克服了光栅投影三维测量中的投影仪非线性问题,但二值光栅存在的高次谐波会降低生成光栅的正弦性,从而引入相位误差。提出一种二值光栅图像生成方法,基于正弦光栅图像所具有的对称性和周期性,选取了尺寸较小的二值块。然后对所选块进行随机初始化,再采用多像素跳变方法对二值块进行优化,根据优化所得的二值块生成整幅二值光栅图像。最后结合相移算法,在投影仪离焦测量环境下,计算得到三维测量所需的相位信息。相较于传统二值光栅图像生成方法,所提方法能够保证高质量相位信息获取,且适用于不同程度的离焦测量环境。仿真及实验验证了所提方法更加适用于离焦投影三维测量。     

一种非线性相位误差的全场补偿方法

提出了一种非线性相位误差的全场补偿方法,通过对参考面相位的多次测量,提高了参考面的解相精度,提取出一个逼近理想值的期望相位面,并用该期望相位面检测非线性相位误差。重构待测物体时,可直接用物体展开相位值在查找表中查找对应的非线性相位误差,并以此对物体的展开相位进行全场补偿。采用所提方法重构已知具体高度的平面,平均绝对误差的最大值从0.48 mm减小到0.06 mm,方均根误差的最大值从0.55 mm减小到0.07 mm。     

合成孔径激光雷达非线性啁啾克服的匹配滤波算法

合成孔径激光成像雷达发射的信号一般为线性啁啾激光信号,然而激光器的实际扫频特性会产生二次及高次等非线性项.非线性项的存在会造成相位误差,导致距离向成像聚焦模糊,降低距离向成像分辨率,因此必须对光源的非线性啁啾进行补偿.因此提出非匹配参考通道匹配滤波算法对非线性啁啾相位误差进行克服.仿真结果显示,距离向图像可以得到较好压缩.同时,还分析了参考通道长度及非线性贡献比例等不同参数环境下的距离向压缩效果.     

基于系统非线性校正与滤波的相位测量

相位测量系统中投影仪输出与CCD输入变形条纹光强之间的非线性导致频谱混叠,影响测量精度,为了消除他们之间的非线性关系,提高相位测量精度,分析了系统非线性产生的基本原因,提出了一种基于系统非线性校正与滤波的相位测量方法.首先校正由投影仪中伽马畸变产生的系统非线性,然后采用低通滤波器对校正后输出的频谱进行滤波,只保留频谱中的基频成份,最后采用四步相移法对变形条纹进行相位测量.MATLAB仿真与实际实验结果表明,所提方法的系统非线性校正与滤波效果较好,有助于提高相位测量精度.     

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.