Effect of chromaticity on quarter-wave retarder performance

When a polarized polychromatic beam passes through a birefringent medium, the constituent spectral components suffer different change of state of polarization. As a result when the beam passes through an analyzer, the intensity of the resultant beam changes depending on the orientation of the analyzer, state of polarization of the input beam, spectral and spatial intensity distribution of the source and the polarizing properties of the birefringent medium. In the present study the intensity variation of the resulting beam passing through a quarter-wave retarder plate is observed theoretically with the variation of the azimuthal angle of the analyzer for monochromatic and polychromatic source of light. A simple experiment is conducted to observe the intensity variation for an incandescent lamp and HeNe laser as polychromatic and monochromatic source of light passing through a birefringent polymer-based quarter-wave retarder plate. The experimentally obtained values are compared with theoretical values and a good agreement is observed.     

一种基于电光调制光频梳光谱干涉的绝对测距方法

提出了一种基于电光调制光学频率梳的光谱干涉测距方法.理论分析了电光调制光学频率梳的数学模型和光谱扩展原理,并分析得出了光谱干涉测距方法的非模糊范围和分辨力的影响因素.在实验中,使用三只级联的电光相位调制器调制单频连续波激光生成了40多阶高功率梳齿状边带,并通过单模光纤和高非线性光纤对电光调制器输出的激光进行光谱扩展,得到重复频率为10 GHz,光谱宽度达30 nm的光学频率梳.将该光频梳作为光谱干涉测距装置的光源,可以实现无"死区"的绝对距离测量.另外,使用等频率间隔重采样和二次方程脉冲峰值拟合算法对测量结果进行数据处理,可以修正系统误差,提升测距精度.实验结果表明,在1 m的测量范围内,使用该装置可以在任意位置达到±15μm以内的绝对测距精度.     

基于长光程技术的痕量海水营养盐自动分析仪的设计与测试

海水中可溶性的营养盐浓度是海洋监测的常规项目之一;大洋水体中的营养盐含量很低,使用传统的分光光度计难于定量测量。文章基于液芯波导长光程技术,设计了高灵敏度的痕量海水营养盐光谱分析仪。分析仪以1.02 nm的光谱分辨率实现350～900 nm光谱范围的水样吸光度连续光谱测量,并由嵌入式PC104微机自动实现了水样采集、吸光度测量和数据分析的功能。针对亚硝酸盐的测试实验结果表明,仪器检出限达到nmol·L-1的量级,比传统分光光度计的检测限提高了三个量级,且显色反应时间可缩短到3 min,能适用于大洋水体中营养盐的现场痕量分析。     

利用空间光调制器的偏振特性实现图像边缘增强

讨论了光电混合型范德卢格特相关器中起偏器和检偏器对场景空间光调制器的影响。提出了一种通过设置起偏器和检偏器相对于场景空间光调制器入射表面液晶指向矢方向的角度来实现图像边缘增强的预处理方法。通过对XGA2L11型电寻址薄膜晶体管驱动透射式液晶空间光调制器的实验测试表明,该方法减少了计算机图像预处理的运算量,有效地提高了目标识别的速度,实现了场景图像边缘增强的实时预处理。     

Characterisation of light emitting diodes (LEDs) for application in digital holographic microscopy for inspection of micro and nanostructured surfaces

Partial coherent light sources open up prospects for phase noise reduction in digital holographically reconstructed phase distributions by suppressing multiple reflections in the experimental setup. Thus, light emitting diodes (LEDs) are investigated for application in digital holographic microscopy. First, the spectral properties and the resulting coherence length of an LED are characterised. In addition, an analysis of dispersion effects and their influence on the hologram formation is carried out. The coherence length of LEDs in the range of a few micrometers restricts the maximum interference fringe number in off-axis holography for spatial phase shifting. Thus, the application of temporal phase-shifting-based digital holographic reconstruction techniques is compared to spatial phase-shifting-based methods. It is demonstrated that LEDs are applicable for digital holographic microscopy in connection with both spatial and temporal phase-shifting-based techniques for reduction of noise in comparison to a laser-light-based experimental setup.     

Spectral anomalies and spectral switches of partially coherent and polychromatic light diffracted at an aperture

Inrecentyearsanewbranchofmodernphysicalopticscalledsingularoptics[1]hasbeendeveloped.In2002theworkinsingularopticswasextendedfrommonochromaticlighttospatiallyfullycoherentandpolychromaticlightbyGburetal.[2—4].Itwasfoundthat,whenaconverging,spatiallyfullycoherentandpolychromaticsphericalwaveisdiffractedatanaperture,anomalousspectralchangestakeplaceclosetozerosoftheintensityinthefarfield;i.e.thespectrumisredshiftedatsomepoints,blueshiftedatothers,andsplitintotwolineselsewhere.Thepredictionsofsp…     

Time-resolved spectral analysis of transient processes in Fabry-Perot interferometers

Parameters of a spectrochronograph that represents a Fabry-Perot interferometer combined with a spatial-time analyzer (a streak camera) are analyzed. Using numerical simulation, time scans of interferograms for bandwidth-limited and phase-modulated pulsed optical signals are obtained. The conditions are determined under which a Fabry-Perot interferometer represents a high-accuracy instrument for measuring the timeresolved spectral dynamics of short optical signals. In particular, an adequate representation of relatively smooth and monotone changes in amplitude and phase characteristics of an optical signal in a spectrochronograph requires that the mode lifetime in a Fabry-Perot interferometer be matched to the time scale of changes in the spectral structure of a pulse being studied. The character of distortions formed in the time-resolved spectral patterns in the case where these conditions are not fulfilled is qualitatively determined, and quantitative estimates of the distortions are presented. The results of the analysis and the numerical calculations show that a spectrochronogram obtained using a Fabry-Perot interferometer and a time analyzer (a streak camera) with parameters corresponding to the criteria formulated in the paper enable one to obtain information on the initial amplitude-phase pattern of a light pulse.     

空心阴极元素灯光谱的空间分布测量方法

提出利用成像光谱仪研究空心阴极元素灯光谱空间分布的检测方法。自制了基于Offner成像系统的推扫式凸面光栅成像光谱仪,覆盖波长范围400~1 000 nm,视场角22°,光谱分辨率2~3 nm;利用该光谱仪第一次得到得到Hg元素灯光谱的空间分布,给出了不同波长下的空间图像和不同空间位置的光谱分布信息,具有较高的空间分辨率和光谱分辨率;并同时获取了不同工作电流条件下的元素灯高光谱数据,对比分析了处于不同工作状态下光谱空间分布的差异。这为空心阴极元素灯光谱的空间分布等性能研究提供了一个很好的工具,相关方法也可以用来研究其他类型的光源。     

Acousto-optic filter of nonpolarized electromagnetic radiation

An acousto-optic tunable filter that is insensitive to the polarization of incident light is studied both theoretically and experimentally. The fundamental possibility of designing an acousto-optic filter of nonpolarized light is demonstrated. The filter operates in the visible range of electromagnetic waves with λ=480–750 nm and provides a high spatial and temporal resolution. The number of resolvable spots of the filtered image reaches a value of 400×400 with the spectral bandwidth of the device being Δλ=1.7 nm. The results of the processing of optical images formed by arbitrarily polarized light beams are discussed. An acousto-optic filter based on a TeO₂ crystal can be used in optics and spectroscopy in the processing of light beams with an arbitrary polarization, as well as in analyzing optical images formed by light beams whose polarization varies with time.     

Bidimensional near-field sampling spectrometry

We report on a concept of compact optical Fourier-transform spectrometer based on bidimensional (2D) spatial sampling of a confined interferogram. The spectrometer consists of a nanostructured glass surface on which two light beams interfere in total internal reflection. Subwavelength spatial sampling of the interferogram near field is achieved by introducing a tilt angle between a 2D array of optical nanoantennas and the interferogram pattern. The intensity distribution of the scattered light is recorded on a 2D CCD camera, and a one-dimensional Fourier transform of the interferogram is used to recover the input light spectrum. Experimental results show a wide spectral bandwidth in the visible range, down to 380 nm, with spectral resolution of 1.6 nm around 780 nm.     

Spectral density of ion acoustic plasma turbulence

Plasma wave spectral density has been measured as a function of wavenumber and frequency by a two-step technique: an analog analyzer measures the cross-power spectral density, and then a digital computer performs the spatial Fourier transform.     

Detection of the X-ray spectra of imploding neon Z-pinch with elliptically bent mica crystal spectrometer

A wide variety of X-ray and extreme ultraviolet diagnostics are being developed to study on Yang acceler- ator. An elliptically bent crystal spectrometer is designed with a focal length of 1350 mm. A mica crystal with an interplanar spacing of 1.984 nm bent onto an elliptical substrate with eccentricity of 0.9485 is used. The crystal analyzer covers the Bragg angle range from 30° to 60°. The mica crystal can efficiently reflect radiation in multiple orders, covering the entire spectral range from 0.1 to 1.73 nm except for a gap from 0.86 to 1.0 nm. The application experiment is performed on Yang accelerator using the bent mica crystal analyzer. Spectra of neon-puff Z-pinch plasmas are recorded with a X-ray film, showing the H-like and the He-like lines of neon. Each spectrum has been identified and used for the wavelength calibration, and most of the line radiation is contained in the He-α and the L-α lines. The experimental results have demonstrated that the spectral resolution approximates 379.     

杨氏实验远场的光谱位移和光谱开关

从部分相干光的传输定律出发，研究了杨氏双缝实验远场的光谱变化。指出杨氏实验中光谱开关在远场也会出现.缝的衍射和光的空间相干性是产生光谱开关的物理原因，增大缝参量ε和源的空间相关度△光谱开关效应越显著。作了详细的数值计算以说明远场光谱位移和光谱开关的特性，并与已有的工作进行了比较。     

On-axis spectral shifts and spectral switches of Gaussian Schell-model beams focused by an astigmatic aperture lens

Starting from the propagation law of partially coherent light, the on-axis spectral shifts and spectral switches of Gaussian Schell-model (GSM) beams focused by an astigmatic aperture lens are studied. It is shown that, as compared with an aberration-free case whose spectral shifts and spectral switches are induced by spatial correlation and aperture diffraction, the spectral shifts and spectral switches of GSM beams also depend upon the astigmatism of the lens for an astigmatism case. Detailed numerical calculations are made to illustrate the behavior of spectral shifts and spectral switches of GSM beams focused by an astigmatic aperture lens.     

Reflecting phase volume holographic grating (PVHG) in LiNbO3

The spectral-angular selectivity of reflecting PVHG in LiNbO 3 and electric control of light diffraction are investigated. It is shown that a PVHG with approximate 7000 lines/mmand thickness spatial frequency 3.7mmhas a spectral selectivity 0.023 nm and angular selectivity that varies in the range from 2 to 50 ang. min, depending on the angle between the beams that record the grating. Application of an 850 V electric field decreases the diffraction effectiveness by one-half for an extraordinary wave and a transverse scheme (the transverse dimension of the PVHG is 5 mm). It is established that the spectral-angular characteristics of reflecting PVHG and electric control of light diffraction can be described within the framework of the theory of coupled wave and the electrooptical effect.Translated from Lazernye Sistemy, pp. 112–120, 1982.     

空间振幅调制光谱偏振测量技术研究

空间振幅调制光谱偏振测量技术通过由光楔、偏振片组成的调制模块,将目标的偏振信息调制到与光谱维垂直的空间维,单次测量即可同时获取目标的偏振信息和光谱信息。首先阐述空间振幅调制光谱偏振测量技术的基本原理,然后利用空间调制光谱偏振仿真软件进行理论仿真,并且给出了具体的实验研究方案,通过对比理论模拟图像和实验图像,验证实验的可靠性。以Q=0,U=1,V=0和Q=1,U=0,V=0作为入射偏振光进行实验,在波长范围500～600 nm采样10组数据,利用最小二乘法对实验输出的偏振光进行偏振解调。结果表明：解调后入射光的线偏振度与参考入射光的误差值分别小于2.5%和4%,偏振角误差值小于1.8%和2%,验证了实验和算法的可行性,并对两组复合光楔中心误差对偏振测量的影响作了简要分析,结果表明中心误差对偏振分量Q和V的测量影响较大。     

正交偏振光谱微循环成像方法及装置

鉴于微循环图像成像质量的提高是微循环检测仪器实用化的关键，提出一种新的微循环检测方法及装置。利用基于该原理的正交偏振光谱成像仪器进行了实验，得到对比度清晰的甲襞微循环图像。用线性偏振光聚焦照射被测生物组织时，从组织表面反射回来的光保持了其偏振态，被放置在CCD前的偏振方向垂直的检偏器屏蔽掉；从组织内部返回的光发生复散射后退偏，通过检偏器成像。选择光源波长，使其落在红血球的吸收谱范围内，即可得到清晰的生物活体浅表面微循环图像。     

Mueller matrix-based optimization of reflective type twisted nematic liquid crystal SLM at oblique incidences

Mueller matrix measurements were used to characterize the polarization properties of liquid crystal-based reflective type twisted nematic (TN) special light modulator (SLM) at oblique incidence of the laser beam. The experimentally obtained Mueller matrices were used to obtain the combination of polarization optics required to optimize it for phase only modulation. The results indicate that minimum intensity modulation is obtained with the use of a polarizer followed by a quarter wave plate (QWP) in polarization state generator (PSG) arm and a QWP followed by an analyzer in polarization state analyzer arm (PSA). Polarization parameters such as retardance, rotation and depolarization were calculated from the experimentally obtained Mueller matrices using polar decomposition method at different angle of incidences of the laser beam and the results has been discussed. The similarity between retardance and depolarization curve as a function of address voltage of TNSLM indicated that depolarization is mainly associated with errors in retardance values. Further, spectral Mueller matrix measurements were used to obtain intensity modulation response in the range of wavelengths 450-700 nm for broadband applications.     

Content-addressable Holographic Digital Data Storage Based on Hybrid Ternary Modulation with a Twisted-Nematic Liquid-Crystal Spatial Light Modulator

We propose and demonstrate the use of hybrid ternary modulated digital pages for content-addressable holographic data storage. Display of binary data pages with equal number of ZEROs and ONEs by modulating both amplitude and phase of beams using twisted-nematic liquid crystal spatial light modulator, reduces strong de component and produces a more homogeneous spectral distribution at the recording plane. This technique facilitates better recording of all spatial frequencies, thus improving the discrimination capability of a content-addressable memory. Hence we get better results in associative recall in a holographic memory system, with very low number of false hits. An important advantage of the hybrid ternary modulation over pure phase data pages is that it offers a dark state for coding the undesired portion of the SLM while the search argument is small. The unique orientation of quarter wave plate and the analyzer blocks the light transmitted from OFF pixels leading to near total removal of dark signals. This in turn improves the system performance and reduces the number of false hits when the size of the search argument is small. Our experimental results show good discrimination capability and signal-to-noise ratio for a hybrid ternary modulation based content addressable memory.     

基于TDLAS技术的天然气中痕量硫化氢分析的PLS算法应用

基于可调谐二极管激光吸收光谱技术的光谱分析仪测量天然气中硫化氢浓度的过程中,各种轻烃和二氧化碳等背景成分对光谱产生干扰,导致多个吸收峰叠加,对提取吸收光谱真实特征造成影响.应用偏最小二乘法消除背景成分的干扰,设计了检测天然气中硫化氢气体的可调谐二极管激光吸收光谱实验系统,采用偏最小二乘法和最小二乘法模型,分别检测了天然气中0~50ppm硫化氢成分的直测光谱和差分光谱.偏最小二乘法算法的测量结果均优于最小二乘法算法,且偏最小二乘法算法对于直测谱的测量误差保持在±1ppm范围内,满足分析仪器2%的准确度要求.利用偏最小二乘法算法避免了最小二乘法所必须的大量参考光谱数据的存储,分析仪可省掉复杂的差分光谱系统,从而达到降低成本、提高系统鲁棒性和实时性的效果.