Analytical solution to LIDAR return signals from clouds with regard to multiple scattering

An analytical approach to LIDAR return signal calculation with regard to multiple scattering is suggested. Two versions of the method are developed. The first one is completely analytical and undispensible for qualitative studies. The second semianalytical approach provides a sufficient accuracy up to a sounding optical depth about 5. Being somewhat more tedious than the proposed analytical solution, the second approach appears to be time saving in comparison with known methods.     

On multiple Compton scattering in plasma clouds

Inverse Compton scattering of low energy photons in a nonrelativistic electron gas is considered. The angle dependence in the Thomson cross section is neglected and spatial and energy transport are separated in a multiple scattering development of the emerging spectrum. The energy transport is determined by a Green function G which may be obtained from the single scattering spectrum for small optical depth or from the Kompaneets equation in the optically thick medium (τ ? 1). The electron cloud is assumed to be spherical and homogeneous with constant temperature $\text{?}{}_{\mathrm{e}}\text{=}\text{kT}{}_{\mathrm{e}}\text{m}{}_{\mathrm{e}}\text{c}{}^2$. Photon escape probabilities P_N from the cloud after N scatterings are calculated from a random flight problem with absorbing walls and alternative methods are mentioned and compared in the limit of large optical depth.     

Optical depth and multiple scattering depolarization in liquid clouds

In this paper, we calculate multiply scattered lidar signals with Monte Carlo method for measuring optical depth (extinction coefficient), effective size of water droplets, and liquid water content of clouds, and present algorithms that implement our method. We calculated multiply scattered lidar signals for various water droplet sizes and liquid water contents using a Monte Carlo method. A simple correspondence between water droplet optical depth and the degree of polarization in a modified gamma size distribution (C₁ cloud) is found. We also calculated the degree of polarization of a lidar signal for a given liquid water content, finding that the degree of polarization is only dependent on optical depth. Since the Raman lidar signal of liquid water depends on the total volume of the water droplet, the effective radius of the water droplet can thus be recovered from the degree of polarization of the lidar signal and the Raman signal of the liquid water.     

Multiple scattering of narrow light beams in aerosols

A multiple scattering propagation model of narrow light beams in aerosol media is described. It is based on a paraxial approximation of the radiative transfer equation in which the flux normal to the incident beam direction is modeled by a diffusion process. The model solutions are the forward- and backscattered intensity profiles for the specified geometry and receiver aperture and field of view. The required inputs are the system parameters, and the aerosol single scattering angular phase function and extinction and scattering coefficients which are allowed to vary along the beam axis. Good agreement is shown with measurements performed in the laboratory over scales ranging from a few tens of mm to a few m, and in the atmosphere over a scale of the order of 1 km. The solutions are valid for optical depths smaller than 10, for phase functions corresponding to average size parameters of order one or greater, and for off-axis positions not exceeding 25% of the reciprocal of the scattering coefficient.     

Simple relation between lidar multiple scattering and depolarization for water clouds

An empirical relationship is derived between the multiple-scattering fraction and the linear depolarization ratio by using Monte Carlo simulations of water clouds measured by backscatter lidar. This relationship is shown to hold for clouds having a wide range of extinction coefficients, mean droplet sizes, and droplet size distribution widths. The relationship is also shown to persist for various instrument fields of view and for measurements made within broken cloud fields. The results obtained from the Monte Carlo simulations are verified by using multiple-field-of-view lidar measurements. For space-based lidars equipped to measure linear depolarization ratios, this new relationship can be used to accurately assess signal perturbations due to multiple scattering within nonprecipitating water clouds.     

Two-dimensional coherent detection imaging in multiple scattering media based on the directional resolution capability of the optical heterodyne method

This paper describes a new application of optical heterodyne detection using a laser beam for two-dimensional imaging of the internal structure of strongly scattering media in which the structure is completely obstructed from normal visual observation. The directional resolution capability for image formation due to the excellent antenna properties of the heterodyne technique is verified experimentally using a ground glass to cause strong scattering of the signal beam. Successful image detection of a test target placed in a highly scattering absorptive medium, with spatial resolution better than 400 m in the case of our experiments, demonstrates that this Coherent Detection Imaging (CDI) method can overcome the diffuse nature of images in media such as those of biomedical interest and others to achieve scanning and tomographic imaging.     

Raman scattering resonances in separated fields

The effects in separated standing-wave fields that are resonant to adjacent Doppler-broadened transitions of many-level atoms are studied. It is shown that due to coherent transfer, at large distances, of polarization on a forbidden transition between the initial and final metastable levels of atoms in a gas, light emission arises at combination frequencies. It is also shown that a resonance with the width reciprocal to the time of flight between separated fields is available in the Raman scattering and absorption line shapes.     

Raman scattering and optical susceptibilities of Nd-doped glasses

The absolute Raman scattering cross-sections of three Neodymium-doped glasses have been measured. These cross-sections have been used to determine the nonlinear optical susceptibility tensor of the three glasses. We have estimated the relative contributions of the “electronic” and “nuclear” nonlinearities to self-focusing index for linearly polarized light in these glasses. We have also performed numerical computations of the Hibert transform of our Raman scattering cross-sections, which are useful in the prediction of any other nonlinear effects. Work supported in part by the National Science Foundation, and in part by the U.S. Atomic Energy Commission under the University of California Lawrence Livermore Laboratory subcontract No. 2713005.     

Optical cavitation probe using light scattering from bubble clouds

To understand the behaviour of systems containing clouds of bubbles (multibubble system) in real sonochemical reactors, a new diagnosis method, i.e., optical cavitation probe (OCP), has been proposed. When a laser beam is introduced into the cavitation bubble cloud, the scattered light intensity changes by the collective oscillation of cavitation bubbles. The frequency domain spectrum of the scattered light contains rich information on the cavitation bubble clouds, comparable with the acoustic emission spectra detected by a hydrophone. The significant merits of OCP, such as capability for spatially resolved, non-invasive measurement of the cavitation bubble clouds, robustness even in a violent cavitation field have been experimentally demonstrated.     

布里渊散射及其在激光雷达中的应用

在简要介绍布里渊散射的基本性质的基础上,重点介绍了布里渊散射在激光雷达中的应用。主要分析了布里渊散射在水中声速测量、粘滞系数的测量和水中目标探测方面的原理和技术实现。     

Radar-radiometric determination of water content of rain clouds with allowance for multiple scattering

We performed combined radiometric and radar measurements of the integral water content of a small-drop fraction of rain cloud systems with allowance for the multiple scattering phenomena. The contribution of rain to emission of the “cloud-rain” system was calculated on the basis of the vector equation of radiation transfer, which enabled us to allow for all orders of multiple scattering by spherical rain drops within the framework of the model of statistically independent particles. It was found that the maximum of the integral water content of overcooled water in clouds does not correspond to the maximum of precipitation intensity. The characteristic values of the integral and specific water contents of a small-drop water fraction in cloud systems with precipitation are measured. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 6, pp. 528–534, June 1999.     

Cavitation clouds created by shock scattering from bubbles during histotripsy

Histotripsy is a therapy that focuses short-duration, high-amplitude pulses of ultrasound to incite a localized cavitation cloud that mechanically breaks down tissue. To investigate the mechanism of cloud formation, high-speed photography was used to observe clouds generated during single histotripsy pulses. Pulses of 5-20 cycles duration were applied to a transparent tissue phantom by a 1-MHz spherically focused transducer. Clouds initiated from single cavitation bubbles that formed during the initial cycles of the pulse, and grew along the acoustic axis opposite the propagation direction. Based on these observations, we hypothesized that clouds form as a result of large negative pressure generated by the backscattering of shockwaves from a single bubble. The positive-pressure phase of the wave inverts upon scattering and superimposes on the incident negative-pressure phase to create this negative pressure and cavitation. The process repeats with each cycle of the incident wave, and the bubble cloud elongates toward the transducer. Finite-amplitude propagation distorts the incident wave such that the peak-positive pressure is much greater than the peak-negative pressure, which exaggerates the effect. The hypothesis was tested with two modified incident waves that maintained negative pressure but reduced the positive pressure amplitude. These waves suppressed cloud formation which supported the hypothesis.     

Laser-frequency stabilization using forward scattering

Frequency stabilization of a single-mode dye laser is demonstrated using a simple magneto-optical forward scattering method. The dye laser was locked to the 2p₄–3s_s2, = 633 nm neon transition. Heterodyne beat-frequency measurements against a¹²⁷I₂-He-Ne meter standard laser showed a frequency stability of a few MHz.     

Gamma holography from multiple scattering

Since the introduction of heterodyne methods for synchrotron radiation (Cousesement et al. in Phys. Rev. B 54:16003, 1996; Callens et al. in Phys. Rev. 67:104423, 2003) one observes interferences between two scattering amplitudes; the scattering amplitude of resonant nuclei in a reference sample and the scattering amplitude of nuclei in the sample under investigation. Theses interferences can easily been observed as resonances in velocity spectra when one uses a time integrated method. They can also been observed as quantum beats, when one would use the time differential method. For both methods it is important that one uses a reference sample and therefore both methods disserved the name “heterodyne methods.” As theses interferences are a product of two scattering amplitudes, the amplitude of a wave scattered form the investigated sample can be known with its phase. But it is assumed that the reference wave is known in advance by a proper choice of the reference sample. At first sight it is very likely that multiple scattering would add more complexity but in this paper it is claimed that on the contrary it provide a bonus, especially for single crystals. It provokes only a line broadening and a line shift of the resonances in the velocity spectra (or a change in the damping and frequency of the quantum beats when the time spectra are registered). Moreover these changes in the line shapes can easily be measured and they provide all the information needed to reconstruct a 3-D picture of the atomic arrangement of resonant nuclei and moreover they distinguish between different hyperfine sites. The method may be more practical for measurements on synchrotron radiation but it does also apply to velocity spectra obtained from resonant scattering with strong sources. The use of radioactive sources suffer from the disadvantage of poorer statistics or much longer accumulation times but they enjoy the advantage to be table-top and at-home experiments. As strong sources are now commercially available this possibility to measure not only the hyperfine fields but also the corresponding crystal structure could give a renewed impetus to the investigations with Moessbauer spectrometry, with “at home and table top” instrumentation.     

Calculation of the bulk electromagnetic properties of thunderclouds using a two-space scattering formalism

Received: 9 July 1997 / Revised version: 30 July 1998 / Published online: 24 February 1999     

A study of various intraband processes inn-InSb by means of stimulated spin-flip raman scattering

The spin-flip Raman (SFR) laser is shown to be an excellent tool to investigate different processes in the conduction band of the SFR laser crystal itself. For the case ofn-InSb, intracavity spectroscopy of inter-Landau transitions is demonstrated to be much more sensitive than conventional transmission spectroscopy. Data on cyclotron harmonic transitions, both free and LO-phonon assisted, are obtained from the output power characteristics of the SFR laser versus magnetic field. The spin-splitting energy and the effectiveg-factor up to 14 T are derived from the SFR laser frequency. The influence of temperature on SFR laser power, threshold and frequency is investigated. The relaxation following the SFR laser pulse is observed in the electrical conductivity. This yields in a direct way the spin-lattice relaxation time at high magnetic fields, which is of the order of 200 ns.     

Correction of imaging errors in spatially resolved laser scattering experiments in flames

Imaging errors in spatially resolved measurements using laser scattering techniques in flames are discussed. An experimental method based on Fourier deconvolution to correct the measurements for the imaging errors is presented. The method is especially important when large gradients are recorded with lowf-number lenses in order to obtain a sufficient signal-to-noise ratio. The technique is demonstrated experimentally on OH profiles measured by laser-induced fluorescence in an atmospheric acetylen-oxygen flame.     

A method based on iterative morphological filtering and multiple scattering for detecting layer boundaries and extinction coefficients with LIDAR

Layer boundaries detection with LIDAR is of great significance for the meteorological and environmental research. Apart from the background noise, multiple scattering can also seriously affect the detection results in LIDAR signal processing. To alleviate these issues, a novel approach was proposed based upon morphological filtering and multiple scattering correction with multiple iterations, which essentially acts as a weighted algorithm with multiple scattering factors in different filtering scales, and applies integral extinction coefficients as media to perform correction. Simulations on artificial signals and real LIDAR signals support this approach.     

气泡线性振动时近海面气泡群的声散射

海洋中的不同成因的气泡群是常见的水下声学目标及声呐混响源,因此对水下气泡群进行声学建模意义重大。利用有效媒质理论描述气泡群内部的相速度及声衰减变化,并考虑到海洋中气泡群往往产生于不同界面附近,进一步利用球面波叠加原理描述海面对气泡群散射声波的再辐射,导出了平海面作用下气泡群声散射截面的一般表达式,建立了其声散射模型,研究了单一尺寸及混合尺寸气泡群的声学特性。数值分析表明,气泡群的谐振频率会随其半径或孔隙率增加而降低;由于海面的存在,气泡群声散射截面会随频率进行周期性变化,且随气泡群远离海面,这一变化逐渐加剧。此外,若气泡的黏滞阻尼项在全部阻尼项中占比较高,气泡群声散射强度会在谐振频率附近存在起伏振荡。该模型可为近海面鱼群、气泡羽流及海底泄漏的甲烷气体的声学建模提供一定的理论基础。