An algorithm to determine cirrus properties from analysis of multiple-scattering influence on lidar signals

A Monte Carlo-based evaluation of the multiple-scattering influence on ground-based Raman lidar measurements is presented. The lidar returns from cirrus clouds are analyzed in order to evaluate vertical profiles of the extinction and backscattering coefficients. Results show that for the typical cirrus cloud, the presence of the multiple scattering can lead to an underestimation of the extinction coefficient by as large as 200% whereas the backscattering coefficient is almost unaffected for the Raman lidar technique. An algorithm to select one or a set of phase functions which fit to the lidar data is also presented. It is an iterative procedure based on Monte Carlo scattering simulation. By comparison of the experimental value of the lidar ratio, corrected for the multiple scattering influence, and the phase function used in the Monte Carlo simulation, one can determine a suitable phase function. The validity and sensitivity of the algorithm have been demonstrated by applying it to simulated cases. The application to some real cases indicates that our procedure allows for the establishing of a practical scattering model for the cirrus clouds.     

浅海小掠射角的海底界面声反向散射模型的简化

在浅海,尤其是负梯度声速剖面和海面较为平静的浅海波导,海底界面反向散射是浅海混响的主要来源.经验散射模型只适用于分析浅海混响平均强度衰减特性,而基于物理机理建立的反向散射模型克服了这一缺陷,但同时也引入了其受地声模型约束的问题.本文结合了海底反射系数的三参数模型,对浅海远场海底反向散射模型进行了简化,以减少地声模型的输入参数.理论分析了海底反射系数的相移参数可以描述海底对声场的散射作用,无需任何海底地声参数的先验知识.通过对海底反向散射模型近似简化,结果表明在临界角附近和甚小掠射角范围内的海底粗糙界面反向散射模型的角度特性和强度特性受海底沉积层的影响不同:在临界角附近,海底反向散射的角度特性受海底反射系数的相移参数加权,而其散射系数则近似与相移参数无关;对于甚小掠射角,海底反向散射的角度特性近似与海底反射系数的相移参数无关,其散射系数则近似与相移参数的4次方成正比.     

舰船尾流气泡层散射相位函数及其后向散射信号特性分析

根据实测舰船的尾流气泡分布模型,利用米氏理论计算得到尾流气泡层的散射相位函数,并将其应用于尾流气泡层后向散射光接收信号的Monte Carlo仿真分析中,以便为实际海况下的舰船尾流气泡层后向散射光探测的实验设置与信号预测、分析提供可靠的依据.通过对不同距离处尾流气泡层的散射相位函数及后向散射光接收信号特性的分析,可以得出:尾流中气泡层散射相位函数的变化小明显,且对接收信号的影响较小;气泡数密度的变化导致的多次散射效应和气泡层衰减系数的变化是引起后向散射光强度变化的主要原因,数密度越大,散射光信号中的多次散射光成分越大;当气泡层厚度达到一定倍数的衰减长度时,继续增加气泡层厚度对后向散射接收信号的影响较小.     

Coherent backscattering and localization in a self-attracting random walk model

Intensity propagation of waves in dilute 2D and 3D disordered systems is well described by a random walk path-model. In strongly scattering media, however, this model is not quite correct because of interference effects like coherent backscattering. In this letter, coherent backscattering is taken into account by a modified, self-attracting random walk. Straightforward simulations of this model essentially reproduce the results of current theories on “non-classical” transport behavior, i.e. Anderson localization in 1D and 2D for any amount of disorder and a phase transition from weak to strong localization in 3D. However, in the strongly scattering regime corrections are necessary to account for the finite number of light modes due to their non-vanishing lateral extention. Within our model this correction leads to the observation that strong localization does not take place. Received 17 September 2001     

Dependence of electron and positron backscattering coefficients on Al film thickness

The backscattering coefficient of 1-4 keV electron and positron beams normally incident impinging on Al thin film targets is stochastically modeled within a Monte Carlo frame work. The aim of the present paper is to study the behavior of the backscattering coefficient as a function of the Al film thickness. To the authors’ knowledge, no theoretical or experimental work on the dependence of the positron backscattering coefficient on film thickness targets has been reported so far. It is found that the backscattering coefficient for both electron and positron beams presents different behaviors when the Al film thickness belongs to the nano-scale. Beyond this scale, the behavior becomes qualitatively similar.     

Examination of Bragg backscattering from crystalline quartz

The backscattering silicon single crystals normally used for hard X-ray inelastic scattering experiments suffer from parasitic reflections and gaps in photon energy where no backscattering reflection exists. Sapphire has been proposed as a possible alternative, but quartz may have advantages over sapphire at low photon energies (5-12.5 keV). Calculations of energy widths of backscattering reflections up to 30 keV for silicon, sapphire, and quartz are compared. The quartz (11 6 0) reflection is examined at 0.03° from backscattering with 0.8 meV bandwidth beam, and its energy width is measured. Finally, the thermal expansions of quartz and silicon are compared.     

Resonator fiber optic gyro using the triangle wave phase modulation technique

Based on the Sagnac effect, resonator fiber optic gyro (R-FOG) has potential as a high accuracy inertial rotation sensor. An R-FOG based on the triangle wave phase modulation technique is proposed in this paper. The slope of the demodulation curve near the resonance dip is found to affect the ultimate sensitivity of the gyro. In order to maximize the slope of the demodulation signal, the parameters of the triangle wave are analyzed. By employing Fourier series and the method of field overlapping, the relationship between the demodulation signal and the resonance frequency deviation is analyzed. The optimum values for the modulation frequency and phase modulation index are obtained. And the relationship between the carrier component and the phase modulation index under analysis in this paper shows that the triangle wave phase modulation technique is more advantageous than the traditional modulation technique in reducing the noise induced by backscattering.     

Weak localization effects in the second-harmonic light scattered by random systems of particles

We report rigorous numerical simulations that show the presence of coherent backscattering effects in the second-harmonic generation and scattering of light by random systems of two-dimensional particles. Since the medium composing the particles is assumed to be homogeneous and isotropic, the second-harmonic field is generated mainly by surface effects. For the fundamental frequency, the results present a clear enhanced backscattering peak. In contrast, the second-harmonic scattering patterns present an intensity dip in the backscattering direction.     

Magnetic field effects on coherent backscattering of light

We have studied the influence of magneto-optical Faraday rotation on coherent backscattering of light experimentally, theoretically and by computer simulations of Monte-Carlo type. The consistency of these three approaches reveals new aspects of the propagation of vector waves in turbid media with and without Faraday rotation. Experimentally, we have demonstrated that the Faraday rotation may almost completely destroy the reciprocity of light paths. However, as shown by the simulations, the cone of coherent backscattering may not only be destroyed but also shifted off the exact backscattering direction, parallel to the magnetic field, as long as the amount of circular polarization is not completely destroyed by the multiple scattering. The relationship between coherent backscattering, depolarization and Faraday rotation are explained by a simple path model of vector waves. This leads to a new characteristic correlation length required to properly describe the influence of Faraday rotation on multiple light scattering. Received 28 January 2000     

Structural investigations of laser-deposited Fe/Al multilayers

Fe/Al multilayers are prepared by crossed-beam pulsed laser deposition and investigated by Rutherford backscattering, conversion electron M?ssbauer spectroscopy, and transmission electron microscopy. The results are compared to purely ballistic simulations of the deposition process using the TRIDYN4.0 code. It is found that the intermixing of adjacent layers must be described in terms of ballistic mixing followed by chemical mixing. The phase formation in the transition layer between adjacent layers follows the non-equilibrium behavior of Fe/Al in analogy to investigations on mechanically alloyed and ion-beam-mixed Fe/Al. In Fe-rich areas a bcc solid solution is formed. In Al-rich environment an amorphous phase is observed. Received: 13 January 1999 / Accepted: 14 January 1999 / Published online: 7 April 1999