Electromagnetically induced transparency based six-wave mixing in a Doppler-broadened folded four-level system

We study phase-conjugate six-wave mixing spectroscopy based on electromagnetically-induced-transparency in a Doppler-broadened folded four-level system. It is found that the six-wave mixing spectrum can be either Doppler-free or very broad, depending on whether the interference between the polarisations of atoms with different velocities is constructive or destructive. To obtain the Doppler-free six-wave mixing spectrum in the folded four-level system, the conditions are more stringent in comparison with those in the cascade and N-type four-level systems. This polarisation interference can be controlled in the presence of a strong coupling field.     

太湖水体透明度的分析、变化及相关分析

本文根据1993—2001年太湖站常规监测资料及2001-2002年周年实验资料，分析研究了太湖水体透明度的分布特征、季节变化，重点分析了透明度与光学衰减系数、悬浮物及叶绿素a的相互关系，阐述了影响透明度的主要因子。研究表明：太湖透明度的区域分布为湖心区最低，其次是河口区．东太湖最高；季节变化表现为全湖平均透明度夏秋季大、冬春季小，不同湖区变化不尽相同；光学衰减系数与透明度的关系为：Kd＝0．096 1.852/ST；悬浮质与透明度的关系为：S1/4＝8．103—5．847lnST；对透明度主要影响因子进行分析，发现太湖透明度主要受悬浮物的影响，透明度跟叶绿素的相关性不是很大，只存在微弱的线性相关。     

Study on the simulation of transparency of Lake Taihu under different hydrodynamic conditions

It was indicated in this study that there were negative relations between the concentrations of suspended solid (SS) and transparency according to the analysis of measured data of Lake Taihu. Their relations in pervious studies were reviewed, which showed that the changes of transparency in Lake Taihu could be reflected by simulating suspended solid concentration (SSC). Measured data showed that the changes of SSC with wind speed were similar at different water depths. SSC increased with the increasing of wind speed. Both wave and lake current of Lake Taihu had positive relations with SSC. However, wave was the main factor affecting sediment suspension, while flow took the second place. In this study, a numerical model coupling lake current, wave and SSC of Lake Taihu was developed. In the SS model, the combined effects of wave and current were included. The amounts of suspended and deposited sediments near the lake bed surface layer were treated separately. The stochastic characteristics of turbulent flow pulsation near lake beds were also considered, and the start-up conditions of sediment suspension were introduced to the model. The model elucidated the mutual exchange processes between sediment particles in SS and active sediments within and on the bed surface layer. Simulated results showed that lake current had relatively significant effects on the SSC at littoral areas of Lake Taihu, while SSC at the central area of the lake was mainly influenced by wave. The changes of transparency with SSC were simulated for Lake Taihu using this model. Calculated results were validated by measured data with good fitness, which indicated that the model is basically suitable for the simulation and prediction of transparency of Lake Taihu.     

力觉临场感遥操作机器人（4）：系统的操作性能评价

力觉临场感遥操作机器人系统是一种典型的人-机-环交互系统,临场感是遥操作的核心.如何评价力觉临场感遥操作机器人系统的操作性能即临场感程度,目前大多采用基于人机工效学的主观评价分析方法.通过建立力觉临场感遥操作机器人系统的二端口网络模型,对操作者在遥操作过程的感知和控制行为进行了分析,给出了虚拟阻抗的定义,以此为基础提出了可定量评价系统操作性能即力觉临场感程度的阻抗比较法.利用阻抗比较方法对现有的4种不同反馈控制结构遥操作机器人的操作性能作了分析与评价.     

Biological aspects of pollution in the Heathcote river,Christchurch, New Zealand

The presence and degree of pollution in the Heathcote River was estimated from bacterial and chemical analyses of water sampled at low tide from five stations in summer 1967–68. The river is very badly polluted in the industrial area and downstream of it, but is fairly clean upstream of the industrial area. Results of analyses of micro‐organisms and macrofauna at 20 stations are presented and distributions of these organisms are related to salinity and pollution in the river. Pollution is restricting the macrofauna of the river but is encouraging micro‐organism populations. Three groups of macrofauna are present:

• Those present in clean freshwater only,

• Those tolerating pollution but restricted to freshwater, and

• Those tolerating pollution but restricted to saline water.

The use of these groups of species as biological indicators of pollution in the Heathcote River has been evaluated; pollution appears to be indicated by absence of a group of species rather than by presence of one species.     

太湖梅梁湾冬末春初浮游动物时空变化及其环境意义

根据１９９８年２－３月完全原庆湖梅梁湾地区水环境综合调查,对有关浮游动物时空情况进行了分析,这次调查共设７个点,湾口上的Ｊ２,Ｊ４与湾内的Ｊ６,Ｊ７均是分５层采样,共分无风,小风和大风３次天气过程,每次过程采集２到３次样品与记录。结果显示,太湖梅梁湾地区浮游生物中枝角类和桡足类的生物量与水温变化关系密切,水温越高,生物量越高,温度相同时,其生物量与透明度呈正比。     

云南抚仙湖透明度的时空变化及影响因子分析

根据2005年的综合调查资料和1980年以来的常规监测资料,分析了云南高原深水湖泊抚仙湖透明度的空间分布、年际变化及影响因子.2005年6-7月抚仙湖水体透明度范围为0.5-6.2 m,平均值为4.6 m,透明度分布的总体格局是:南区>北区,湖心区>沿岸区,河口区较低.藻类和悬浮物是影响透明度空间分布的主要因子,随着水体中藻类和悬浮物的增加,透明度下降.近20多年来,抚仙湖透明度呈下降趋势,藻类增加是直接影响因子,水体总氮、总磷的增加促进了藻类的增殖,是重要的间接影响因子.     

水体透明度遥感反演算法研究进展

水体透明度(Secchi Disk depth,SDD)是水环境监测的重要参数,遥感技术对于监测水体透明度具有重要的应用前景。本文旨在分类和比较当前用于监测水体透明度的算法,并提出未来研究的方向,以推动水环境监测技术的进一步发展。文章对目前检索水体透明度的算法进行分类和比较。其中,经验算法、半分析算法和机器学习算法是目前研究的主要方向。通过分析算法特性和优缺点,提出未来研究的重点和方向。经验算法基于透明度与光谱数据、叶绿素a浓度等的相关性,半分析算法基于水下能见度理论,机器学习算法则基于更优的数据特征学习能力。不同算法具有各自的适用范围和限制。未来的研究应该着重于整合多源遥感数据,改进QAA (quasi-analytical-algorithm),深入分析光学参数与水体透明度的关系,将机器学习算法应用到水体透明度模型的建立中,以建立具有高精度、适用性广的反演模型。