共查询到19条相似文献，搜索用时 234 毫秒
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研究了以InAs量子点为有源区的二维GaAs基光子晶体微腔的设计与制作,测试并分析了室温下微腔的光谱特性.观察到了波长约为1137 nm,谱线半高宽度约为1 nm的尖锐低阶谐振模式发光峰.我们比较了不同刻蚀条件下光子晶体微腔的发光谱线,结果表明空气孔洞截面的垂直度是影响光子晶体微腔发光特性的重要因素之一.通过调节干法刻蚀工艺,改变空气孔半径与晶格常数的比率,可以在较大范围内调节谐振模式发光峰位置,达到谐振模式与量子点发光峰调谐的目的. 相似文献
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非稳腔主动式直接获取纳秒近似无衍射贝塞尔绿光 总被引：1，自引：0，他引：1
通过对基于轴棱锥的贝塞尔谐振腔和贝塞尔-高斯谐振腔的研究,设计了一台腔内倍频Nd:YAG纳秒近似无衍射贝塞尔绿光激光器. 非稳激光器谐振腔由轴棱锥和凸面镜组成. 实验采用单灯脉冲抽运激光增益介质Nd:YAG晶体,倍频晶体选用KTP. 当抽运电压为350 V时,由主动式直接产生纳秒近似零阶贝塞尔绿光,其脉冲宽度为55.1 ns,波长为532 nm,线宽为0.8 nm,近似无衍射零阶贝塞尔绿光的中心光斑直径为192 μm. 利用Fresnel-Kirchhoff 衍射积分和Fox-Li迭代法,通过数值计算得出 相似文献
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悬浮泥沙质量浓度（SSC）是我国河口水体研究的重要水质参数之一,遥感测定SSC是一种重要的方法。为了研究遥感光谱数据和表层悬浮泥沙质量浓度（SSSC）之间的定量梯度关系,本文采集了长江口南汇嘴附近的表层沉积物,定量设计了SSSC的梯度变化值,利用ASD高光谱仪对不同质量浓度含沙水体进行了光学特性测量。结果表明,随着SSSC的增加,水体的光谱反射率曲线具有相似的形态,但也存在一定的差异。当SSSC<36.59 mg/L时,光谱曲线仅在560～580 nm之间有1个反射峰;当SSSC>52.69 mg/L时,光谱曲线第一反射峰位于570～710 nm,并在近红外波段780～820 nm处出现第二反射峰。光谱曲线第一峰值区间存在明显的“红移”现象,且波长的“红移”变化与SSSC的线性拟合R2达到0.91,说明两者呈较强的线性相关性。在400～900 nm光谱范围内,600～860 nm波段对0～200 mg/L范围内的SSSC变化较敏感且比较稳定,740～900 nm波段对>200 mg/L的SSSC的变化具有更强的响应能力。 相似文献
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利用等离子体增强化学气相沉积技术,在高氢稀释条件下,研究不同激发频率对纳米晶硅薄膜生长特性的影响.剖面透射电子显微镜(TEM)分析结果显示,不同激发频率下制备的纳米晶硅薄膜晶化区均呈锥状结构生长,但13.56 MHz激发频率下制备的纳米晶硅薄膜最初生长阶段存在非晶态孵化层,即纳米晶硅薄膜的形成经历了由非晶态孵化层到晶态结构层的转变.而高激发频率(40.68 MHz)下硅纳米晶则能直接在非晶态衬底上生长形成.Raman谱和红外吸收谱测量结果表明高激发频率(40.68 MHz)下制备的纳米晶硅薄膜不但具有较高 相似文献
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光作为一个复杂的环境因子,对鱼类的存活、摄食、生长及视觉均产重要影响。为了探究LED光谱对许氏平鲉(Sebastods schlegelii)视网膜结构和视网膜各层细胞数目与厚度的影响,本文选取1 000尾许氏平鲉幼鱼(体质量为38.80±0.43 g、体长为10.20±0.17 cm)分别养殖在5种光谱下(蓝光(λ450 nm)、绿光(λ525 nm)、黄光(λ590 nm)、白光(λ460 nm)、红光(λ630 nm)),其中白光组为对照组,其余4组为处理组。经过为期60 d的处理后,分别对各组的视网膜进行取样观察。结果表明:不同的LED光谱对许氏平鲉的视网膜结构未产生影响,但各个光谱下视网膜细胞数目略有差别,在5种不同光谱条件下,在单位面积(100μm×100μm)内,蓝光组内网层细胞数量(24.33±2.08)显著低于白光组(P<0.05);红光组外核层细胞数量(33.33±3.51)显著低于除白光组外的其他各组(P<0.05),通过对比各个光谱下视网膜各层细胞数量可得,蓝光组与红光组数量最少,其他各层差异不大。综上所述,饲养于不同光谱处理组的许氏平鲉幼鱼的视网膜... 相似文献
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极低太阳高度条件下穿透海冰的太阳辐射研究 总被引：2，自引：0，他引：2
基于2007年冬季北极的透射辐射光学数据,研究了极低太阳高度条件下的太阳辐射特征。低太阳高度时较短波长的可见光受到削弱,而较长波长的光得以保留,呈现明显的双峰结构。由于考察时的海冰都是新冻结的当年冰,厚度小,积雪层很薄,海冰的反射率较低,有较高比例的太阳辐射进入海冰或海洋。没有积雪的冰面由于反射率随波长增加而减少,更高比例的长波进入海冰。但在透射辐射光谱中,490 nm的光重新占优势,较长波长的光在穿过海冰时受到很大的削弱。因此,在低太阳高度的条件下,较短波长的光在大气中受到显著削弱,而较长波长的光在海冰中受到显著削弱,大气和海冰共同作用的结果使进入冰下海水的太阳辐射能更加微弱。但是,海冰对较长波长光的吸收使海冰获取较多的热量,减缓海冰的冻结过程。 相似文献
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针对影响拖曳线列阵声纳系统目标检测性能的两种典型近场强干扰源,由宽带近场阵列模型,提出了基于功率谱相关的干扰抵消方法,通过比较基元域频域信号与干扰波束信号功率谱之间的相似关系,找到与每路基元信号相匹配的干扰分量信号,最后通过频域块自适应滤波算法实现每路基元域信号中的干扰抵消。宽带仿真结果与海试表明,这种方法在强干噪比和低信噪比条件下,比最小方差无失真响应和传统基元域干扰抵消方法在阵增益方面提高约10dB。相比传统基元域干扰抵消方法,这种方法能够实现抵消拖船干扰的同时抵消邻近目标强干扰。 相似文献
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本文以野生型和突变型龙须菜为材料,测定了完整藻体的室温吸收光谱、77K低温荧光发射光谱和叶绿素诱导荧光动力学曲线.野生型(Wt)和黄色突变体(Ye100)的室温吸收光谱有435、498、540、566、618、640和680nm7个吸收峰;绿色突变体Gr180和Gr184则只有除540nm以外的6个吸收峰.3种突变体的藻红蛋白含量均比野生型少,两种绿色突变体的藻蓝蛋白含量相对有所增加,别藻蓝蛋白含量变化不明显.77K低温荧光发射发谱(激发光分别为436、540和580nm)的结果表明:在发射峰的波长上,4种材料没有明显的区别,但Wt中PE吸收的光能传递效率比较低.叶绿素诱导荧光动力学曲线显示Wt的光系统Ⅱ活性、光系统Ⅱ原初光能转化效率及光合碳代谢活性都高于突变体. 相似文献
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An echo sounder has been developed with features ideally suited to oceanographic and fisheries research. Instruments commonly used for such research are inaccurate, limited in dynamic range, unstable, and generally inflexible. An effort has been made to overcome these deficiencies with the sonar system discussed here. The echo sounder to be described has a time-varied-gain receiver (20 log_{10} R or40 log_{10} R + 2alphaR ) accurate to withinpm0.5 dB over a 100-dB range. The equivalent dynamic range is 140 dB (the ratio of the maximum signal at minimum gain to the equivalent input noise at maximum gain in a 4-kHz bandwidth). The temperature stability ispm0.5 dB from10deg to35deg C at any range. Operating parameters, including frequency, can be easily altered to accommodate a variety of needs. 相似文献
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This paper investigates the properties of thickness extension mode excited by lateral electric field on LiNbO3 by using the extended Christoffel--Bechmann method. It finds that the lateral field excitation coupling factor for a-mode (quasi-extensional mode) reaches its maximum value of 28% on X-cut LiNbO3. The characteristics of a lateral field excitation device made of X-cut LiNbO3 have been investigated and the lateral field excitation device is used for the design of a high frequency ultrasonic transducer. The time and frequency domain pulse/echo response of the LiNbO3 lateral field excitation ultrasonic transducer is analysed with the modified Krimholtz--Leedom--Matthae model and tested using traditional pulse/echo method. A LiNbO3 lateral field excitation ultrasonic transducer with the centre frequency of 33.44 MHz and the --6 dB bandwidth of 33.8% is acquired, which is in good agreement with the results of the Krimholtz--Leedom--Matthae model. Further analysis suggests that the LiNbO3 lateral field excitation device has great potential in the design of broadband high frequency ultrasonic transducers. 相似文献
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本文提出了一种新型的多波长全光再生方案,利用相位时钟光纤光参量放大,并采用相邻信道偏振正交的方法,实现对由异步信源产生的双波长信号全光再生.理论分析了参量放大中的增益饱和现象用于幅度噪声抑制,以及利用相位时钟及后续色散实现对信号定时的机理.在这个基础上,对两个独立信源产生的异步双波长10 Gbit/s信号进行再生实验,实验表明该方案有效的抑制了基于多波长3R再生系统中信道间的四波混频与交叉相位调制等非线性干扰.系统在单波长和双波长情况下分别将两路信号信噪比改善了至少6.5 dB与4.5 dB.误码率测试结 相似文献
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Biot反演在夏威夷钙质沉积物原位测量声速和衰减中的应用 总被引：9，自引：1，他引：9
对夏威夷檀香山岛的两个站(H3和H4)钙质沉积物进行了20～100kHz的原位纵波声速和声衰减测量.它们均有轻微的频散.随频率的增加H3站位声速从1691m/s增加到1708m/s,H4站位的声速从1579m/s增加到1585m/s.随频率的增加H3站位的有效衰减从15dB/m增加到75dB/m,H4站位的有效衰减从22dB/m增加到62dB/m.运用Biot-Stoll模型对所测得的纵波速度和声衰减数据进行了Biot模型未知参数反演,发现粒径较大的H3站的沉积物(孔隙率为45%)比粒径较小的H4站的沉积物(孔隙率为56%)具有曲率小和渗透率及孔隙半径都大的性质. 相似文献
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Multibeam echosounders (MBES) have become a widely used acoustic remote sensing tool to map and study the seafloor, providing co-located bathymetry and seafloor backscatter. Although the uncertainty associated with MBES-derived bathymetric data has been studied extensively, the question of backscatter uncertainty has been addressed only minimally and hinders the quantitative use of MBES seafloor backscatter. This paper explores approaches to identifying uncertainty sources associated with MBES-derived backscatter measurements. The major sources of uncertainty are catalogued and the magnitudes of their relative contributions to the backscatter uncertainty budget are evaluated. These major uncertainty sources include seafloor insonified area (1–3 dB), absorption coefficient (up to >?6 dB), random fluctuations in echo level (5.5 dB for a Rayleigh distribution), and sonar calibration (device dependent). The magnitudes of these uncertainty sources vary based on how these effects are compensated for during data acquisition and processing. Various cases (no compensation, partial compensation and full compensation) for seafloor insonified area, transmission losses and random fluctuations were modeled to estimate their uncertainties in different scenarios. Uncertainty related to the seafloor insonified area can be reduced significantly by accounting for seafloor slope during backscatter processing while transmission losses can be constrained by collecting full water column absorption coefficient profiles (temperature and salinity profiles). To reduce random fluctuations to below 1 dB, at least 20 samples are recommended to be used while computing mean values. The estimation of uncertainty in backscatter measurements is constrained by the fact that not all instrumental components are characterized and documented sufficiently for commercially available MBES. Further involvement from manufacturers in providing this essential information is critically required. 相似文献
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In this paper, we consider the use of multiple antennas and space-time coding for high data rate underwater acoustic (UWA) communications. Recent advances in information theory have shown that significant capacity gains can be achieved by using multiple-input-multiple-output (MIMO) systems and space-time coding techniques for rich scattering environments. This is especially significant for the UWA channel where the usable bandwidth is severely limited due to frequency-dependent attenuation. In this paper, we propose to use space-time coding and iterative decoding techniques to obtain high data rates and reliability over shallow-water, medium-range UWA channels. In particular, we propose to use space-time trellis codes (STTCs), layered space-time codes (LSTCs) and their combinations along with three low-complexity adaptive equalizer structures at the receiver. We consider multiband transmissions where the available bandwidth is divided into several subbands with guard bands in between them. We describe the theoretical basis of the proposed receivers along with a comprehensive set of experimental results obtained by processing data collected from real UWA communications experiments carried out in the Pacific Ocean. We demonstrate that by using space-time coding at the transmitter and sophisticated iterative processing at the receiver, we can obtain data rates and spectral efficiencies that are not possible with single transmitter systems at similar ranges and depths. In particular, we have demonstrated reliable transmission at a data rate of 48 kb/s in 23 kHz of bandwidth, and 12 kb/s in 3 kHz of bandwidth (a spectral efficiency of 4 bs-1Hz-1) at a 2-km range. 相似文献
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利用济南、青岛和烟台S波段双偏振多普勒天气雷达资料和常规观测资料以及天气实况,对2020年5月17日和6月1日两次强对流天气过程的关键环境物理量和风暴低层强冰雹区偏振量、三体散射、强冰雹衰减等偏振特征等进行了分析.结果 表明:1)两次强对流天气都具有大的温差和强垂直风切变,中层较干.5月17日山东半岛0～6 km垂直风... 相似文献
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珠江口水体叶绿素荧光特性研究 总被引：3，自引：0，他引：3
利用2003年1月和2004年1月珠江口实测的遥感反射率和叶绿素浓度,建立了基于实测1nm带宽及其模拟MERIS遥感反射率数据的叶绿素荧光特征与叶绿素浓度之间的拟合关系。基于实测1nm带宽遥感反射率光谱,叶绿素荧光峰的位置和高度分别与叶绿素浓度之间存在良好的线性关系和指数关系;基于模拟MERIS遥感反射率数据,叶绿素荧光峰的高度与叶绿素浓度之间存在良好的线性关系,叶绿素荧光峰位置与叶绿素浓度之间不存在明显的相关关系,但可作为叶绿素浓度数量级的有效探针。利用2006年5月和8月的珠江口实测遥感反射率和叶绿素浓度对拟合结果进行了检验,结果表明其精度较高,说明叶绿素荧光特征在探测叶绿素浓度时效果良好且MERIS数据在近岸二类水体叶绿素浓度反演、水质监测等方面有着良好的应用前景。 相似文献
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Development of a high target strength passive acoustic reflectorfor low-frequency sonar applications
The objective of this study was development of broadband, high target strength, passive acoustic reflectors. These reflectors would provide convenient, low-cost targets for low-frequency sonar trials and fleet exercises. The primary development goals included controlled, stable monostatic and bistatic reflectivity, and adjustable deployment depth down to 90 m (300 ft). The development process used both computer models and scaled physical models to find the most effective configuration capable of meeting the project goals. Review of acoustic scattering theory showed that acoustically soft reflectors (bubbly liquids or air-filled spheres and cylinders) provide higher target strength values than similarly sized hard reflectors (metal spheres or corner reflectors). Air-filled cylinders were found to provide the highest target strength values and widest useful bandwidth for a given reflector volume. This is a result of the tube resonance that occurs at ka=0.02 at the depth range of interest where k is the acoustic wavenumber and a is the tube radius. Air-filled cylinder target strength at resonance is about equal to its target strength at ka=1.0 with a deviation less than ±4 dB between these values. Guided by these study results, a prototype reflector was assembled using a 16-m length of air-filled gum rubber tubing, 5.7 cm in diameter (3.2-cm ID). A SCUBA regulator attached to a small air tank maintained tubing inflation. Test results at a depth of 90 m showed an effective target strength of 12 dB at 250 Hz with a deviation less than ±2 dB from 200 to 400 Hz. This is equivalent to the target strength of a 16-m-diameter perfectly reflecting sphere-an object with a volume about 52000 times that of the prototype cylinder reflector 相似文献