基于平面反射式光栅的微型紫外-可见光谱仪设计

光谱仪具有快速、无损、多元数据分析等显著特点,已成为环境监测、食品安全检测、生物医学、航空航天和国家安全等众多涉及国民经济和安全领域的必备检测装备。但传统的光谱仪体积大、功耗高、成本高,已严重制约现代光谱分析技术的快速发展。微型化成为重要发展方向。针对上述诸多领域对微小型光谱仪的广泛需求,提出一种基于平面反射式衍射光栅的宽光谱、高分辨率微型紫外-可见光谱仪光学系统设计方法。考虑微型光谱仪宽光谱范围与高分辨率的矛盾,提出基于光线追踪的设计并结合系统像差消除方法。通过ZEMAX软件完成光学系统的优化设计,实现了微型紫外-可见光谱仪的宽光谱、高分辨率设计。设计的仪器光谱范围为200～800 nm,在50μm入射狭缝下整个波段的光谱分辨率优于0. 5 nm。该方法可广泛应用于基于平面反射式衍射光栅的宽光谱、高分辨率微型光谱仪光学系统设计。     

计算成像前沿进展

计算成像是融合光学硬件、图像传感器和算法软件于一体的新一代成像技术,突破了传统成像技术信息获取深度(高动态范围、低照度)、广度(光谱、光场、3维)的瓶颈。本文以计算成像的新设计方法、新算法和应用场景为主线,通过综合国内外文献和相关报道来梳理该领域的主要进展。从端到端光学算法联合设计、高动态范围成像、光场成像、光谱成像、无透镜成像、低照度成像、3维成像和计算摄影等研究方向,重点论述计算成像领域的发展现状、前沿动态、热点问题和趋势。端到端光学算法联合设计包括了可微的衍射光学模型、折射光学模型以及基于可微光线追踪的复杂透镜的模型。高动态范围光学成像从原理到光学调制、多次曝光、多传感器融合以及算法等层面阐述不同方法的优点与缺点以及产业应用。光场成像阐述了基于光场的3维重建技术在超分辨、深度估计和3维尺寸测量等方面国内外的研究进展和产业应用,以及光场在粒子测速及3维火焰重构领域的研究进展。光谱成像阐述了当前多通道滤光片,基于深度学习和波长响应曲线求逆问题,以及衍射光栅、多路复用和超表面等优化实现高光谱的获取。无透镜成像包括平面光学元件的设计和优化,以及图像的高质量重建算法。低照度成像包括低照度情...     

激光衍射光栅测试仪的设计与实现

激光二极管衍射光栅光斑亮度比和透过率的测定是影响VCD、DVD等设备中激光读写头以及光学镜头质量的关键因素.本设计采用高性能单片机AVR Atmegal6L设计并实现了650nm激光衍射光栅测试仪,对衍射光栅0级和1级光斑亮度比和光栅透过率测试误差小于2%,方差小于2.2×10-3,满足了光栅测试的精度和稳定性要求.     

Offner成像光谱仪的设计与航拍实验

介绍一种Offner结构高分辨率推扫式成像光谱仪的设计,包括光学结构设计、数据获取与存储系统配置、基于数据库的光谱图像拼接软件设计以及成像光谱仪波长标定和辐射标定方法.其次介绍成像光谱仪在青藏高原陆面敏感因子航空遥感实验中的使用情况,包括实验概况、成像光谱仪及其采集系统在机舱内的安装调试、光谱仪的航拍参数以及航带光谱图像的拼接处理,最终获取到高分辨率的高光谱图像立方体.仪器的首次航拍飞行实验取得预期成果.     

微型可编程光栅光谱模拟特性及其应用研究

研究了基于MOEMS技术的微型可编程光栅的光学特性.对平行复色光正入射时微型可编程光栅的衍射场进行了理论分析,分析表明可以通过对可编程光栅编程状态的预先设定,使其衍射输出对应于不同目标物质的光谱,即模拟光谱.最后介绍了微型可编程光栅的光谱模拟特性在相关光谱检测法中的应用,分析了其相对于传统方法的优点.     

褐潮土的光谱特性及用土壤反射率估算有机质含量的研究

本文通过ASDFR便携式光谱仪对132个风干土壤样品的光谱反射率进行了实验室测定。根据土样光谱反射率变化,获得了褐潮土土壤剖面的不同诊断层反射光谱特征。结果表明,在400～1200nm范围之间,土壤有机质含量与土壤光谱反射率有较好的相关性。利用导数光谱方法建立了预测土壤有机质含量的方程,提出了预测北京地区褐潮土有机质光谱的最佳波段。在波长447nm处采用反射率和A值(反射率倒数的对数)所建立的预测方程的预测精度较高。采用反射率的一阶微分建立的预测方程的最佳波段在516nm处。而A值一阶微分光谱在615nm处相关性最好。作为一项参考指标用光谱分析法评价土壤中有机质含量,以期对精准农业中土壤养分或肥力的预测具有一定的指导作用。     

DWDM系统啁啾光栅色散补偿器的设计

首先对D W D M通信系统中的啁啾光栅色散补偿器进行了研究,介绍了啁啾光栅的耦合模算法,并针对其色散补偿的功能详述了用逆散射法设计啁啾光栅补偿器的过程;最后通过matlab仿真,给出了一个啁啾光栅滤波器8信道色散补偿反射率仿真图,从图可以看出,采用逆散射法设计的补偿器可以较好地达到D W D M系统中色散补偿的目的。     

基于移动最小二乘法的微型光谱仪标定方法

针对现有微型光谱仪缺少一种统一、可靠的标定方法,提出了一种基于移动最小二乘法（Moving Least Squares, MLS）的微型光谱仪标定方法。首先,微型光谱仪分别获取汞氩灯和氖灯的标准光谱图;然后对含有高频噪声的原始光谱图进行小波去噪,之后通过峰值定位算法找到特征峰并筛选出需要参与拟合的特征峰所对应的像元序号,最后筛选出一定数量的标定点使用MLS进行拟合。选取一定数量的未参与标定的特征峰代入到拟合函数中进行精度验证。实验表明：基于MLS拟合标定后,标定集的误差标准差为0.136 nm,测试集的误差标准差为0.192 nm,高于传统的最小二乘法曲线拟合,该方法实现了快速、准确地对微型光谱仪进行标定,在实际工程应用中具有重要的指导意义。     

光纤Bragg 光栅的温度传感研究􀀁

本文分析了光纤 Bragg光栅在受到温度调制时的滤波特性。与温度对光纤光栅反射谱波长的影响相比 ,对其带宽的影响可忽略。实验中 ,光纤光栅受温度的调制 ,并且其反射谱由光谱仪记录。结果表明该光纤 Bragg光栅对温度产生的波长偏移是 0 .0 0 991nm/℃。值得注意的是实验中最大标准误差为 0 .0 3nm,能满足目前波分复用技术所需的信道精度。     

基于FPGA便携式光谱分析仪设计

为了大幅提高光谱检测的抗干扰能力和实时数据处理能力,设计了基于现场可编程门阵列(FPGA)的静态光谱分析系统.选用静态干涉具取代机械扫描,从而提高系统的抗震动能力,使其适用于便携式光谱仪的设计要求.采用FPGA芯片完成干涉数据的高速处理,从而达到实时响应的设计要求.设计了系统的结构、硬件电路以及光谱分析算法,给出了光谱还原的理论推导与计算过程.实验采用Virtex2-pro型FPGA开发板,对660 nm激光光谱进行测试,频谱计算效果与Matlab仿真数据作对比.实验结果显示:本系统可以有效地将被测激光光谱分布函数解出,速度快,准确度高,并具有较好的抗干扰能力,适合于便携式光谱分析仪的应用特点.     

High-resolution electrostatic analog tunable grating with a single-mask fabrication process

We present the design, modeling, fabrication, and characterization of the microelectromechanical systems (MEMS) analog tunable diffraction grating with the concept of transverse actuation. In contrast to the vertically actuated "digital" tunable grating, our prototype design trades angular tunable range for tuning resolution. The prototype shows an angular tunable range of 250 /spl mu/rad with 1-/spl mu/rad resolution at 10 V. Grating pitch changes corresponding to the full range and resolution are 57 nm and 2.28 /spl Aring/, respectively confirmed by experimental measurement and theoretical calculation. Simulation shows that subradian tunable range is feasible with better lithographic design rules or higher actuation voltage. The single-mask fabrication process offers several advantages: 1) Excellent optical flatness; 2) ease of fabrication; and 3) great flexibility of device integration with existing on-chip circuitry. Tunable gratings such as the one presented here can be used for controlling dispersion in optical telecommunications, sensing, etc., applications.     

The role of spectral resolution and classifier complexity in the analysis of hyperspectral images of forest areas

Remote sensing hyperspectral sensors are important and powerful instruments for addressing classification problems in complex forest scenarios, as they allow one a detailed characterization of the spectral behavior of the considered information classes. However, the processing of hyperspectral data is particularly complex both from a theoretical viewpoint [e.g. problems related to the Hughes phenomenon (Hughes, 1968) and from a computational perspective. Despite many previous investigations that have been presented in the literature on feature reduction and feature extraction in hyperspectral data, only a few studies have analyzed the role of spectral resolution on the classification accuracy in different application domains. In this paper, we present an empirical study aimed at understanding the relationship among spectral resolution, classifier complexity, and classification accuracy obtained with hyperspectral sensors for the classification of forest areas. We considered two different test sets characterized by images acquired by an AISA Eagle sensor over 126 bands with a spectral resolution of 4.6 nm, and we subsequently degraded its spectral resolution to 9.2, 13.8, 18.4, 23, 27.6, 32.2 and 36.8 nm. A series of classification experiments were carried out with bands at each of the degraded spectral resolutions, and bands selected with a feature selection algorithm at the highest spectral resolution (4.6 nm). The classification experiments were carried out with three different classifiers: Support Vector Machine, Gaussian Maximum Likelihood with Leave-One-Out-Covariance estimator, and Linear Discriminant Analysis. From the experimental results, important conclusions can be made about the choice of the spectral resolution of hyperspectral sensors as applied to forest areas, also in relation to the complexity of the adopted classification methodology. The outcome of these experiments are also applicable in terms of directing the user towards a more efficient use of the current instruments (e.g. programming of the spectral channels to be acquired) and classification techniques in forest applications, as well as in the design of future hyperspectral sensors.     

MOMS-02 sensor simulation and spectral band selection

Abstract

MOMS-02 is a push-broom scanner with four spectral bands in the 450–810 nm region (each with a 15 m ground resolution element at a 310 km orbit) and a panchromatic (520–760 nm) stereo mode with on-track stereoscopic capability. The stereo mode employs three look angles: nadir (with a 5 m ground resolution element), 24° forward and backward (each with a 10 m ground resolution element). The sensor which is funded by the German Ministry for Research and Technology (BMFT) is scheduled for launch on a Space Shuttle mission at the end of 1991. The selection and radiometric performance of the panchromatic and the multispectral bands are discussed.     

MERIS atmospheric correction over coastal waters: validation of the MERIS aerosol models using AERONET

Standard aerosol models (SAMs) are used for the Medium-Resolution Imaging Spectrometer (MERIS) level-2 processing over water, first to remotely sense the aerosols in the near-infrared and secondly to perform the atmospheric correction for ocean colour analysis. However, are these SAMs still suitable over coastal areas? The present work was intended to answer that question through the use of the Aerosol Robotic Network (AERONET) by selecting CIMEL radiometers operating over the sea surface or near the coastline. The current official MERIS algorithm overestimates aerosol optical thickness (AOT) over coastal waters at 865 nm. This can be related either to incorrect assumptions of the underlying surface assumption or to the assumptions of the aerosol properties (e.g. phase function). This study looks at the importance of aerosol modelling and confirms that the improved aerosol models must be used in the retrieval chain. Extinction measurements were first used to derive the aerosol optical thicknesses (AOTs). The spectral dependency of the AOTs between 670 nm and 865 nm allowed the selection of a standard aerosol model. The ability of the standard aerosol models to retrieve the AOTs at 440 nm was then analysed as a key element in the extrapolation of the aerosol path radiance from the near-infrared to the blue spectral range. The two outputs of this analysis are systematic biases in this retrieval process and accordingly they are an estimation of the dispersion. The first output can be defined as a corrective factor in the aerosol path radiance at 440 nm and the second output can be used for error analysis. A radiative transfer code was used to simulate the sky radiance in the principal plane of acquisition. Comparisons at 870 nm illustrated the ability of the standard aerosol models to retrieve the aerosol path radiances with a direct impact on the AOT retrieval from satellite observations at 865 nm.     

Experiment and design of a high-resolution optical encoder

An optical mechanism proposed in this paper is composed of a movable part and a fixed part for the increment high-resolution optical displacement encoders. A parallel light source is set on the moving part and a double-concave lens, a specially designed optical grating and a phototransistor array receiver are set on the fixed part. The parallel light emitted from the movable part passes through the double-concave lens and the specially designed optical grating; it is then projected onto the phototransistor array receiver to indicate the displacement of the movable part. The relationship equation of the lens is developed to design an optical mechanism, which can enlarge the displacement so that it becomes observable. Based on the simulation results, three different radii in the curvature of the double-concave lens were proposed. The simulation results indicated that the optical mechanism with 50 times magnification could make the 10 nm movement intervals of a light source be about 500 nm movement intervals in the detecting surface. Furthermore, considering the limitation of the precision of existing tools, an experimental system with a 200 nm resolution was established to verify the possibility of the proposed structure. The experimental result indicates the possibility of using the proposed theorem to achieve the desired results.     

Microfluidic electrodischarge devices with integrated dispersion optics for spectral analysis of water impurities

This paper reports a microfluidic device that integrates electrical and optical features required for field-portable water-chemistry testing by discharge spectroscopy. The device utilizes a dc-powered spark between a metal anode and a liquid cathode as the spectral source. Impurities are sputtered from the water sample into the microdischarge and characteristic atomic transitions due to them are detected optically. A blazed grating is used as the dispersion element. The device is fabricated from stacked glass layers, and is assembled and used with a charge-coupled device (CCD) sensing element to distinguish atomic spectra. Two structural variations and optical arrangements are reported. Detection of Cr and other chemicals in water samples has been successfully demonstrated with both devices. The angular resolution in terms of angular change per unit variation in wavelength (/spl part//spl theta///spl part//spl lambda/) is experimentally determined to be approximately 0.10 rad//spl mu/m, as opposed to the idealized theoretical estimate of 0.22 rad//spl mu/m. This is because the microdischarge is uncollimated and not a point source. However, this is sufficient angular resolution to allow critical spectra of metal impurities to be distinguished.     

Optical fiber sensing element based on luminescence quenching of silica nanowires modified with cryptophane-A for the detection of methane

An optical fiber sensing element based on luminescence quenching of cryptophane-A/silica nanowires was successfully constructed and used to dynamically monitor methane gas at low concentration below 3.5% (v/v). The optical fiber device was designed to operate via luminescence reflection. The sensing properties of optical sensing element to methane at room temperature were characterized. The sensing element shows an intensive and stable blue luminescence when excited by UV light source at wavelength of 380 nm, and it is efficiently quenched by molecular methane. The response of the sensing element demonstrates excellent linear Stern-Volmer behavior at the fixed wavelength 439 nm within the methane concentration range between 0.1% and 3.5% (v/v). A detection limit of below 0.1% (v/v) is estimated for the methane sensing element. This newly developed methane sensing element has significant advantages over the currently available methane sensors such as fast response and recovery (within seconds), good repeatability, selectivity, and long-term stability.     

Spectral calibration and atmospheric correction of ultra-fine spectral and spatial resolution remote sensing data. Application to CASI-1500 data

Imaging spectrometers operating in the solar spectrum measure the upwelling reflected solar radiation, and are an important tool in the bio/geochemical characterization of the Earth system. Surface reflectance is usually the starting point for the retrieval of biophysical parameters from remote measurements. Reliable radiometric and spectral calibrations and accurate atmospheric correction are mandatory in the interpretation of the surface reflectance. A complete surface reflectance retrieval scheme specifically designed for ultra-fine spectral resolution (bandwidth from 10 to 2 nm) and spatial resolution (pixel size less than 10 m) imaging spectrometers is presented in this work. The assessment of the spectral calibration is coupled to the removal of the atmospheric distortion so that maps of surface reflectance are derived, as well as columnar water vapor (CWV) maps, estimations of aerosol optical thickness (AOT) and updated sensor gain coefficients and spectral calibration. Radiative transfer calculations are performed by an optimized version of the MODTRAN4 code, which is run before processing each image. The method is tested against Compact Airborne Spectrographic Imager (CASI) 1500 images acquired during the ESA SENtinel-2 and FLuorescence EXperiment (SEN2FLEX) campaigns held in the Barrax (La Mancha, Spain) study site during June and July 2005. A peak-to-peak spectral shift variation of up to 2.3 nm is detected in CASI. Concerning atmospheric products and surface reflectance retrievals, an extensive validation is performed using ground-based measurements. A good correlation between ground measurements and CASI-derived AOT and CWV is found, with a Pearson correlation coefficient r² up to 0.71 and 0.74, respectively. The subsequent surface reflectance retrievals also hold a good correspondence with ground-based measurements.     

A multichannel surface plasmon resonance sensor using a new spectral readout system without moving optics

Surface plasmon resonance (SPR) sensors with spectral interrogation provide a high refractive index resolution, a large dynamic range and a fixed optical detection module. In this work, we propose a new multichannel spectral detection unit that uses only one spectrometer to measure the reflection spectrum from multiple sensing spots serially without any mechanical movement. This spectral detection unit is designed based on a spatial light modulator (SLM) configured as a programmable optical aperture for the spectrometer. To demonstrate this concept, a five-channel laboratory SPR prototype was built based on the proposed multichannel detection unit, and we evaluated the device's sensitivity and resolution using a refractive index test. Refractive index resolution of 1.4 × 10⁻⁶ refractive index units (RIU) can be reached using the five-channel prototype. This sensor is suitable for low-cost multichannel biosensing applications that do not contain fast kinetics.     

Remote detection of Trichodesmium blooms in optically complex coastal waters: Examples with MODIS full-spectral data

Remote detection of the Trichodesmium spp. cyanobacteria blooms on the west Florida shelf (WFS) has been problematic due to optical complexity caused by sediment resuspension, coastal runoff, and bottom interference. By combining MODIS data measured by the ocean bands and land bands, an approach was developed to identify surface mats of Trichodesmium on the WFS. The approach first identifies possible bloom patches in MODIS FAI (floating algae index) 250 m resolution imagery derived from the Rayleigh-corrected reflectance at 667, 859, and 1240 nm. Then, spectral analysis examines the unique reflectance characteristics of Trichodesmium at 469, 488, 531, 551, and 555 nm due to specific optical properties (absorption, backscattering, and fluorescence) of the unusual pigments in Trichodesmium. These spectral characteristics (i.e., high-low-high-low-high reflectance at 469-488-531-551-555 nm, respectively) differentiate Trichodesmium mats unambiguously from other features observed in the FAI imagery, such as Sargassum spp. Tests in other coastal locations show that the approach is robust and applicable to other optically complex waters. Results shown here can help study Trichodesmium bloom dynamics (e.g., initiation and bloom formation) and may also help design future sensors to better detect and quantify Trichodesmium, an important N₂ fixer in the global oceans.