土壤表层湿度影响下冬小麦晚霜冻害及冠层光谱检测

为探究土壤表层湿度影响下冬小麦冠层光谱反射率响应晚霜冻害的特征,并检验敏感光谱波段预测小麦产量变化的能力,于2013和2014年小麦拔节期,分别设置了表层土壤含水量为10%(干)、10%~20%(中)和20%(湿)的三个湿度处理的冻前试验,并在低温室内进行降温处理。分析了不同土壤表层湿度下受冻冬小麦的穗数、穗粒数、千粒重、单株产量、冠层光谱反射率及其一阶微分值的差异,对冬小麦冻害产量变化率和高光谱特征参量进行了相关分析和一元线性拟合。结果表明:冬小麦穗粒数和单株产量总体上均随土壤表层湿度的降低而呈减少态势,在土壤表层干处理条件下冻害对冬小麦产量造成的影响最为显著(p0.05);在绿峰(523nm附近)、黄边(571nm附近)、红边(732nm附近)和近红外台的两个水分吸收带(952和1 145nm附近),干+冻害处理冬小麦冠层反射率的一阶微分值与中+冻害、湿+冻害处理的差值明显;剥离了土壤表层湿度对光谱的影响后,冬小麦冠层反射率一阶微分差值在以570nm为中心的黄边区域和以710nm为中心的红边区域对干、中和湿梯度处理下晚霜冻害响应的差异明显;两年试验中的黄边面积(SDy)和570nm处一阶微分值(d570)均与冻害产量变化率达到显著正相关(p0.05),说明黄边区域的高光谱特征参量可用于检测土壤表层湿度影响下的冬小麦晚霜冻害程度。本研究可为土壤表层湿度和晚霜冻害叠加影响下冬小麦产量变化预测方法的探讨提供参考。     

不同碱化程度土壤对其上覆植被冠层光谱特征的影响

通过实地光谱测量,研究了不同碱化程度土壤上向日葵在不同生育期冠层光谱反射率、红边参数及与对应的土壤碱化度和pH之间的关系。结果表明,不同碱化程度土壤上向日葵生育期冠层反射光谱具有绿色植物的光谱反射特征,随着向日葵生育期的推进,向日葵冠层光谱反射率逐渐增大,到开花期在809nm处形成一个高的反射峰。在近红外短波范围,向日葵光谱反射率随碱化程度的减轻而增大。用红边一阶微分光谱特征参数分析,向日葵冠层反射光谱红边位置在整个生育期处于702~720nm之间,并随着碱化程度的加重,红边位置和红边斜率呈"蓝移"现象;而在同一碱化程度水平上,随向日葵生育期的推进,红边位置和红边斜率均出现先"红移"后"蓝移"的现象。相关分析和回归分析显示,土壤碱化度、pH与红边位置均呈极显著正相关和二次多项式关系,R2值分别为0.745 6和0.615 3。     

水分胁迫对小麦光谱红边参数和产量变化的影响

在不同的水分胁迫梯度下,利用实验区小麦不同生长期光谱反射率观测数据,研究水分胁迫对小麦光谱反射率、红边参数及小麦产量的影响。首先分析水分胁迫对小麦光谱反射率的影响,然后利用小麦光谱反射率的一阶微分得到小麦光谱反射率的红边位置和红边幅度参数,分析了水分胁迫对小麦光谱红边参数的影响,最后利用水分胁迫下的红边幅度和小麦产量的关系,阐述了小麦水分胁迫下的光谱反射率特征与小麦产量的关系。研究结果表明,水分胁迫下小麦的红边位置在生长期前期出现红边位置红移现象,生长期后期出现红边位置蓝移现象。水分胁迫下的小麦的红边幅度在不同的生长期表现出不同的变化特征：生长期初期随着水分胁迫的增加而红边幅度增大,生长期后期随着水分胁迫的增加而红边幅度减小。小麦的红边幅度在拔节生长期前与小麦产量呈负相关而拔节生长期之后呈正相关,且不同生长期小麦的红边幅度与小麦的产量的相关系数不同。     

无人机高光谱的玉米冠层大斑病监测

大斑病是一种对玉米危害严重的病害,迫切的需要一种可以快速了解玉米大斑病病情的方法。以无人机遥感作为新的技术平台,探究玉米冠层受到大斑病胁迫时的光谱响应情况,并利用无人机高光谱成像技术对大斑病病情进行监测和可视化研究。采集玉米多生育期(抽雄期、灌浆期、完熟期)冠层500～900 nm的高光谱影像,根据采集影像的原始光谱和一阶微分光谱特征,提取出12个大斑病敏感波段位置, 12个波段位置分别为:514, 532, 553, 680, 714, 728, 756和818 nm,近红外、红、绿波段及红边位置。根据前人提出的植物病害监测参数结合提取的敏感波段位置,构建13组针对玉米冠层大斑病的监测光谱参数,研究不同波段对大斑病病情指数(DI)值的敏感性,并构建玉米冠层大斑病的监测模型,验证利用无人机遥感监测大斑病DI值的精度及稳定性。结果表明:随病情指数增加,一阶微分光谱图出现典型的"蓝移"现象,病害冠层DI值与红光(680～714 nm)和近红外(770～818 nm)的反射率及一阶微分光谱图的红边位置(680～756 nm)相关性更显著,与绿光波段相关性较低。在13组监测光谱参数中, 8组与建模样点冠层大斑病实测DI值达到极显著相关水平,决定系数(R~2)均达到0.8以上,选取各生育期R~2达到0.8以上的光谱参数用于玉米冠层大斑病监测模型的构建,将检验样本的实测值与监测模型的预测值进行相关性分析。检验表明,在抽雄期,模型DI-NDVI(SD_(λ_i), SD_(λ_j))的回归斜率(0.829 3)和决定系数(R~2=0.842 7)都最接近1,均方根误差(RMSE=4.59)和相对误差(RE=12.3)更小,说明模型DI-NDVI(SD_(λ_i), SD_(λ_j))的预测能力和精度更高。各生育期对应模型均取得较好监测效果,说明本研究利用无人机遥感对植物病害监测具有指导意义,对精准农业的发展具有一定的借鉴价值。     

冬小麦生长期光谱变化特征与叶绿素含量监测研究

在分析冬小麦生长期冠层反射光谱和叶绿素含量变化特征的基础上,对二者之间的相关性进行了研究。表明从小麦拔节期开始,冠层反射光谱在可见光区(400～750 nm)的反射率先降低而后升高,以孕穗期反射率最低;在近红外区(750～1 000 nm)冠层反射率由拔节期至孕穗期反射率降低,然后开始上升。扬花期上升至最高点后又开始下降,直至乳熟期降至最低。冬小麦冠层反射率与叶绿素含量相关分析结果表明,冬小麦拔节期和孕穗期二者呈正相关,扬花期二者呈负相关;整个生长期中,孕穗期可见光区552 nm处反射率与叶绿素含量相关系数最大达0.89。依据冬小麦生长期冠层反射光谱红边拐点位置,分别建立了拔节期叶绿素含量线性检测模型(R2=0.92)和孕穗期二项式模型(R2=0.91),用于冬小麦叶绿素含量的无损检测是可行的。     

倒伏胁迫对水稻可视茎叶穗比率的影响及光谱响应解析

倒伏胁迫下作物的冠层光谱响应机理解析,是大范围作物倒伏灾情遥感监测的重要基础。倒伏胁迫直接改变了遥感光谱探测视场内的可视茎叶穗比率,通过解析冠层光谱与可视茎叶穗比率间的关系,探索不同强度的倒伏胁迫下水稻可视茎叶穗组分变化规律及其与冠层光谱响应规律,为大范围作物倒伏灾情遥感监测提供理论支持。以2017年江苏省兴化市、大丰区的实发倒伏水稻为研究对象,在野外观测实验的支持下,分析不同倒伏强度的倒伏水稻冠层光谱变化规律,并对不同倒伏强度下的冠层可视茎叶穗比率与倒伏角度进行相关性分析,筛选能有效表征倒伏强度的敏感农学参数,采用灰色关联分析法构建倒伏水稻冠层光谱指标与敏感农学参数之间的响应模型,实现水稻倒伏灾情的光谱诊断,并利用野外实测样本评价诊断精度。研究结果表明,随着倒伏强度的加大,冠层光谱表现出规律性变化,红光波段与近红外波段响应较为明显,“红边”位置明显“蓝移”,且“红边”振幅与“红边”面积增大,说明红光波段和近红外波段对水稻倒伏胁迫强度较为敏感;冠层可视叶茎比存在随倒伏强度增加而减少的规律,其相关性可达0.715,说明倒伏后的水稻冠层可视叶茎比对于倒伏强度有着较好的表征能力;通过对可视叶茎比与冠层高光谱反射率进行相关性分析,分别于红光波段和近红外波段内筛选出698与1 132 nm作为敏感波段,进而计算特征植被指数;利用灰色关联分析构建了基于特征植被指数的水稻可视叶茎比光谱响应模型,检验样本的决定系数为0.635,以可视叶茎比预测结果进行倒伏灾情等级划分的精度达到82%。因此,倒伏发生后水稻冠层的茎、叶、穗等组分在光谱探测器视场中的贡献比例发生了规律性改变,茎、叶、穗本身光谱反射率差异和视场内比率差异直接反映于倒伏水稻冠层光谱差异,其中可视叶茎比能有效表征受倒伏胁迫的水稻群体结构变化,与倒伏强度具有较好的响应关系,不同倒伏强度的可视叶茎比与水稻冠层光谱之间的响应规律可以有效区分倒伏灾情等级,有助于为区域尺度的水稻倒伏灾情遥感监测提供先验知识。     

洪涝胁迫的水稻叶面积指数变化及其光谱响应研究

通过水稻淹水实验模拟洪涝胁迫状态,分析不同生育期、不同洪涝胁迫强度下的水稻叶面积指数变化及其冠层高光谱响应规律,建立洪涝胁迫下水稻叶面积指数(LAI)的估测模型。结果表明,分蘖期、拔节期、抽穗期水稻LAI均随淹水深度的增加而降低;水稻冠层680~760 nm光谱的一阶微分具有“双峰”现象,主峰位于724~737 nm,701和718 nm有次峰出现,并随淹水深度的增加出现“三峰”;冠层红边位置在各个生育期出现“蓝移”,红边幅值、红边面积在分蘖期、拔节期、灌浆期均呈现“蓝移”,在抽穗期出现“红移”;生育前期水稻LAI与光谱特征参量存在显著相关,生育后期相关性不显著;最后以D_λ737/D_λ718光谱参量建立了水稻生育前期的LAI估测模型。表明在洪涝胁迫下,水稻LAI的变化幅度直接反映胁迫强度的大小,估测模型LAI=3.138(D_λ737/D_λ718)-0.806可用于估测洪涝胁迫不同强度下的水稻叶面积指数。     

基于可见-近红外光谱技术的炭疽病侵染后油茶叶片叶绿素含量预测研究

分析炭疽病侵染后油茶冠层的可见-近红外光谱特征,探索建立病害胁迫下油茶冠层叶片叶绿素含量的预测模型。通过实地调查病情指数,获取不同病害程度的油茶冠层叶片光谱数据及其叶绿素含量,并对光谱数据进行了一阶微分与滑动平均滤波相结合的预处理,再通过光谱数据重采样,提取敏感波段建立了叶绿素含量的BP神经网络预测模型。结果表明:(1)随着病情的加重,油茶冠层光谱可见光区域的反射峰和吸收谷逐渐消失;红光到近红外陡峭的红边被逐渐拉平;在近红外区域,健康油茶的光谱反射率明显大于感病油茶的光谱反射率。(2)微分光谱484～512,533～565,586～606和672～724nm四个波段是叶绿素吸收和反射的敏感波段。(3)以敏感波段为输入变量建立的BP神经网络模型,其计算出的预测值与观测值之间的相关系数r和均方根误差分别为0.992 1和0.045 8。因此,利用可见-近红外光谱技术预测炭疽病侵染后油茶叶片叶绿素含量是可行的。     

Sentinel-2宽波段光谱指数预测拔节后受冻冬小麦减产率

冬小麦拔节期后的晚霜冻害在区域尺度上表现出一定的空间差异,决定了应对冻害采取分区措施。基于Sentinel-2卫星数据的宽波段光谱指数预测区域冬小麦减产率,对灾情评估和生产管理决策具有重要意义。采用人工模拟霜冻试验,以光谱重采样将ASD FieldSpec® 3光谱辐射计获取的冠层反射率模拟为Sentinel-2传感器对应的波段反射率。采用19个已有光谱指数和以3种形式（简单比值、简单差值、归一化）组合构建的光谱指数构建冬小麦减产率线性回归模型。在每种形式中,筛选出决定系数排名前三的宽波段光谱指数作为候选指数。以商丘地区自然霜冻事件为契机,以Sentinel-2卫星数据计算候选指数,利用地面采样点的实测减产率验证候选指数精度。结果表明：（1）随着处理温度的降低,冬小麦冠层反射率在近红外区呈降低趋势,在可见光区和短波段红外区呈升高趋势;（2）重采样前后反射率数据计算的19个已有光谱指数中,大部分指数与减产率呈显著相关(p<0.001)。筛选出的12个候选光谱指数预测冬小麦减产率的线性回归精度较好,在校正集和验证集中的决定系数均高于0.631;（3）Sentinel-2卫星数据计算的候选光谱指数精度验证结果表明,包含波段B9的3个光谱指数未能通过显著性检验,其他9个光谱指数通过极显著性检验（p<0.001）。基于波段B8,B8a和B12组合的2个光谱指数(B8a-B12和B8-B12)精度较好,决定系数R2分别为0.543和0.492,均方根误差RMSE分别为8.510%和8.971%。该研究构建光谱指数B8a-B12和B8-B12符合简单差值形式,是预测冬小麦减产率宽波段光谱指数的最佳组合。研究成果揭示了冬小麦幼穗发育期冠层反射率在不同低温胁迫下的响应机制,表明Sentinel-2宽波段光谱指数预测冬小麦减产率有良好的精度,在霜冻后区域尺度上的冬小麦减产率预测具有可行性,对于不同地区霜冻灾害的措施制定具有指导作用。     

玉米苗期干旱胁迫对关键生育期的光谱特征影响分析

植被的光谱特征是监测植被生长发育状态、健康状况的基础,探索玉米苗期干旱胁迫后的关键发育期光谱特征变化,不仅为植被光谱数据库的建设提供理论数据,也为植被水分胁迫高光谱识别提供理论依据。以辽宁西部锦州生态与农业气象站大型农田土壤水分控制场为研究区,对玉米苗期、拔节期、抽雄期和乳熟期4个发育期进行苗期干旱胁迫小区和水分适宜小区光谱观测,通过分析原始光谱、一阶导数光谱、光谱参数和水分敏感植被指数,对比苗期干旱胁迫光谱与同期对照的光谱特征差异性。结果表明:(1)对于原始光谱来说,玉米在苗期受到干旱胁迫后,与同期水分适宜的玉米光谱特征有较为明显区别,在可见光波段和短波红外波段反射率均高于同期对照反射率,近红外波段明显低于同期对照反射率,拔节期差异性最为显著,达到了5%左右,随着作物的生长,差异性逐渐减弱。(2)4个发育期玉米一阶导数光谱在可见光波段均出现双峰,红光位置峰值在抽雄期达到最大;苗期干旱胁迫一阶导数光谱的红光位置峰值均低于同期对照,拔节期差异明显,在0.003左右,乳熟期差别明显减小,可区分性减弱。(3)苗期胁迫玉米光谱参数与同期对照相比,从苗期到乳熟期,红边位置存在"蓝移"—"红移"—"蓝移"的现象,绿峰位置均存在向长波方向移动的现象;在抽雄期和乳熟期,蓝边位置、蓝边幅值、黄边位置和黄边幅值两者差异不明显;在三边面积中,红边面积均低于同期对照,黄边面积均高于对照,两者的蓝边面积无明显差异。(4)在8个水分敏感植被指数中, NDWI和NDW_2的差异性指数在玉米四个关键发育期均达到50%以上,可区分性明显。该研究旨在为植物水分胁迫光谱库提供基础数据,为作物干旱识别的谱段选择及高光谱波段设置提供基础依据。     

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.