前向散射对激光水下成像的影响

为研究前向散射对水下成像的影响,利用辐射传输理论建立了激光水下成像前向散射理论模型,提出了水下成像前向散射影响系数,用以表征前向散射对水下成像质量的影响程度,并给出了前向散射功率及前向散射影响系数的解析表达式。利用Matlab进行了数值模拟分析,得出了水下成像前向散射光功率和影响系数随探测水深及水衰减系数的变化规律,说明了非对称因子的取值对成像质量具有较大的影响。     

Fractional Laplacian time-space models for linear and nonlinear lossy media exhibiting arbitrary frequency power-law dependency

Frequency-dependent attenuation typically obeys an empirical power law with an exponent ranging from 0 to 2. The standard time-domain partial differential equation models can describe merely two extreme cases of frequency-independent and frequency-squared dependent attenuations. The otherwise nonzero and nonsquare frequency dependency occurring in many cases of practical interest is thus often called the anomalous attenuation. In this study, a linear integro-differential equation wave model was developed for the anomalous attenuation by using the space-fractional Laplacian operation, and the strategy is then extended to the nonlinear Burgers equation. A new definition of the fractional Laplacian is also introduced which naturally includes the boundary conditions and has inherent regularization to ease the hypersingularity in the conventional fractional Laplacian. Under the Szabo's smallness approximation, where attenuation is assumed to be much smaller than the wave number, the linear model is found consistent with arbitrary frequency power-law dependency.     

气泡数密度对尾流光束衰减测量的影响

为准确地测量出尾流气泡群的衰减,提高前向光尾流探测的可靠性,研究了气泡数密度对尾流光束衰减测量的影响.基于辐射传输方程的小角度近似解,得到了探测截面上的约化功率和漫射功率的表达式.根据测量条件,引入了有尾流气泡和无尾流气泡时探测截面上接收到的辐射功率之比作为透射函数,并进一步引进了复散射校正因子,它表征了复散射效应的强弱.针对典型的尾流气泡分布,通过数值计算得到了不同测量条件下气泡数密度与复散射校正因子、透射函数的关系.数值计算结果表明：复散射校正因子都随气泡数密度的增大单调的增大,透射函数都随气泡数密度的增大单调的减小;测量条件不同,气泡数密度对光束衰减的影响也不同.研究结果表明,给定测量条件和透射比时,利用小角度辐射方程可以得到气泡群数密度的量化信息.     

Magnetic field effect on the ultrasonic attenuation in manganese phosphide

The effect of the external magnetic field on the critical attenuation in MnP near the Curie temperature is observed clearly. The critical attenuation is observed on the boundary of the ferro- and para-magnetic phases, and the temperature dependence of the attenuation peak displays a characteristic behavior along this boundary. Furthermore, a new anomalous peak of attenuation is observed in the paramagnetic phase, and this is the first observation of the attenuation arising separately from each of the three components of the spin correlation function by the ultrasonic technique.     

New results and a simple model for the anomalous power dependence of the Kapitza resistance of copper

We report a new anomalous Kapitza Resistance, much stronger than the effect previously described by Cheeke and coworkers.¹ The effect can be described using a localised heat transfer model based on the resonant heat transfer theory.^5,6     

The Influence of Suprathermal Electrons on the Excitation of Short-Wavelength Oscillations in the Ionosphere in the Field of a Powerful Radio Wave

This paper is the continuation of [1]. The expressions obtained in [1] are used to study the influence of suprathermal electrons on the transfer of energy of the upper-hybrid plasma waves excited by the powerful radio emission into the additional region of very short-wavelength oscillations with anomalous dispersion. Using the model examples, we show that this effect can considerably intensify the transfer of plasma-wave energy. According to our calculations, plasma waves in the additional region are excited in fairly broad frequency ranges of a powerful radio wave slightly below the 4th, 5th, and 6th electron gyroharmonics. We discuss the application of the obtained results to the attenuation of the stimulated electromagnetic emission and the auxiliary ionization of the ionosphere by accelerated electrons, which were detected for the pump-wave frequency ranges specified above.     

Low-frequency sound transmission through a gas-liquid interface

Typically, sound speed in gases is smaller and mass density is much smaller than in liquids, resulting in a very strong acoustic impedance contrast at a gas-liquid interface. Sound transmission through a boundary with a strong impedance contrast is normally very weak. This paper studies the power output of localized sound sources and acoustic power fluxes through a plane gas-liquid interface in a layered medium. It is shown that, for low-frequency sound, a phenomenon of anomalous transparency can occur where most of the acoustic power generated by a source in a liquid half-space can be radiated into a gas half-space. The main physical mechanism responsible for anomalous transparency is found to be an acoustic power transfer by inhomogeneous (evanescent) waves in the plane-wave decomposition of the acoustic field in the liquid. The effects of a liquid's stratification and of guided sound propagation in the liquid on the anomalous transparency of the gas-liquid interface are considered. Geophysical and biological implications of anomalous transparency of water-air interface to infrasound are indicated.     

Anomalous normal state acoustic attenuation in niobium

An anomalous temperature dependence in the normal-state electronic attenuation in Nb is investigated. The effect is found to scale with ql. Models for the effect are considered.     

Anomalous attenuation of transverse sound in 3He

We present the first measurements of the attenuation of transverse sound in superfluid 3He-B. We use fixed path length interferometry combined with the magnetoacoustic Faraday effect to vary the effective path length by a factor of 2, resulting in absolute values of the attenuation. We find that attenuation is significantly larger than expected from the theoretical dispersion relation, in contrast with the phase velocity of transverse sound. We suggest that the anomalous attenuation can be explained by surface Andreev bound states.     

Sound propagation in saturated gas-vapor-droplet suspensions with droplet evaporation and nonlinear relaxation

The Sound attenuation and dispersion in saturated gas-vapor-droplet mixture in the presence of evaporation has been investigated theoretically. The theory is based on an extension of the work of Davidson [J. Atmos. Sci. 32(11), 2201-2205 (1975)] to accommodate the effects of nonlinear particle relaxation processes of mass, momentum and energy transfer on sound attenuation and dispersion. The results indicate the existence of a spectral broadening effect in the attenuation coefficient (scaled with respect to the peak value) with a decrease in droplet mass concentration. It is further shown that for large values of the droplet concentration the scaled attenuation coefficient is characterized by a universal spectrum independent of droplet mass concentration.     

Fractional precursors in random media

When a broadband pulse penetrates into a dissipative and dispersive medium, phase dispersion and frequency-dependent attenuation alter the pulse in a way that results in the appearance of a precursor field with an algebraic decay. We derive here the existence of precursors in non-dispersive, non-dissipative, but randomly heterogeneous and multiscale media. The shape of the precursor and its fractional power law decay with propagation distance depend on the random medium class. Three principal scattering precursor classes can be identified: (i) in exponentially decorrelating random media, and more generally in mixing random media, the precursor has a Gaussian shape and a peak amplitude that decays as the square root of the inverse of the propagation distance. (ii) In short-range correlation media, with rough multiscale medium fluctuations, the precursor has a skewed shape with a tail that exhibits an anomalous power law decay in time and a peak amplitude that exhibits an anomalous power law decay with propagation distance, both of which depend on the Hurst exponent that characterizes the roughness of the medium. (iii) In long-range correlation media with long-range memory, the situation mimics that of class (ii), but with modified power laws.     

Experimental demonstration of underwater optical wireless power transfer using a laser diode

We experimentally demonstrate an underwater optical wireless power transfer(OWPT) using a laser diode(LD)as a power transmitter. We investigate the characteristics of a solar cell and a photodiode(PD) as a power receiver. We optimize the LD, the PD, and the solar cell to achieve the maximum transfer efficiency. The maximum transfer efficiency of the back-to-back OWPT is measured as 4.3% with the PD receiver. Subsequently, we demonstrate the OWPT in tap and sea water. Our result shows an attenuation of 3 dB/m in sea water.     

Ultrasonic study of dysprosium in a magnetic field

The ultrasonic attenuation of longitudinal waves propagating along the c axis of single crystal dyprosium is reported, as a function of the applied basal plane field in the paramagnetic region, and as a function of the temperature, at constant applied basal plane field, in the spin-spiral region. In the paramagnetic region, anomalous attenuation behavior is explained on the basis of competing spin-polarization and spin-fluctuation effects. Two anomalous maxima in the temperature dependence of the attenuation were observed: one near T_N is attributed to spin fluctuations associated with short range ferromagnetic ordering; another one at 130 K is attributed to a magnetic phase transition from a fanstructure phase, intermediate between the spin-spiral and ferromagnetically ordered phase     

Non-contact sound velocities and attenuation measurements of several ceramics at elevated temperatures

Laser ultrasonic technique has been employed to carry out the sound velocities and attenuation measurements as a function of temperature in alumina, two kinds of silicon nitride and partially stabilized zirconia (PSZ) samples. Accuracy of the laser technique used has been checked in terms of the diffraction effect and reproducibility of the results. Results of attenuation at room temperature have been compared with quartz transducer technique. In PSZ, velocity behavior has become non-linear and also, a peak in attenuation has been observed around 500 degrees C. In one of the silicon nitride sample, which uses glassy sintering agent, attenuation has shown a sharp peak around 950 degrees C. Interestingly, when the experiment was repeated from 800 to 1000 degrees C, this anomalous attenuation peak has disappeared, leaving a background increasing towards higher temperatures.     

Transmitted power allocation/control for multi-band MC-CDMA

Transmit power allocation over multiple subcarriers is an effective way to improve system error probability performance and to save power in traditional multi-carrier systems. In this paper, we derive the optimum power allocation algorithm for multi-band MC-CDMA systems, where optimum is defined as minimal BER under the constraint of fixed transmitted power, assuming knowledge of power attenuation of different sub-bands. With a two-band MC-CDMA system and Rayleigh channel fading, we show that a BER performance improvement based on the optimum allocation can only be attained over a limited range of transmit power and a limited range of sub-band power attenuation difference values. This performance improvement is also modest compared with a uniform power allocation, which suggests that the uniform power allocation is near optimal for transmit power control under our assumptions. Two simple transmitted power control algorithms are provided, and the controlled transmit power for a two-band system is shown to be a linear function of the power attenuation difference between the two bands for a large range of these attenuation differences. The small non-linear range implies that in using a multi-band channel, any savings in total transmitted power can occur only when the power attenuation difference between the two bands is small.     

The use of power transfer matrices in predicting system loss: Theory and experiment

The phase space diagram for parabolic and step index fibers leads to a graphic representation of the bound, leaky, and refracted rays of ray theory. This concept is used to predict the attenuation of typical components of local area networks. The technique uses power transfer matrices to track the evolution of power distribution in ray packets. In particular, we predict and then measure the power transfer of two ray packets for a step index fiber. The comparison is encouraging.     

The use of power transfer matrices in predicting system loss: Theory and experiment

The phase space diagram for parabolic and step index fibers leads to a graphic representation of the bound, leaky, and refracted rays of ray theory. This concept is used to predict the attenuation of typical components of local area networks. The technique uses power transfer matrices to track the evolution of power distribution in ray packets. In particular, we predict and then measure the power transfer of two ray packets for a step index fiber. The comparison is encouraging.     

Electromagnetic waves propagation in real multimode waveguide structures

A power flow equation describing the power flow of electromagnetic waves in a real multimode waveguide represented by a linear multichannel trasmission system exhibiting an attenuation, dispersion, and an interchannel interaction is solved in three iterative ways. In the case of constant attenuation and constant propagation velocities in all channels and for the convolution interchannel interaction, a closed analytical solution of the problem is presented. Interesting forms of the solution, transfer function, and impulse response of the system which enable us to separate and compare the attenuation, dispersion, and coupling effects independently of each other are derived. The transfer function, the impulse response, and excitation conditions for their measurements are further discussed. Relations which make it possible to compare different experimental data obtained under specific excitation conditions in various laboratories are determined. Finally, a linear system with memory in time is studied and it is shown that the real multimode waveguide as a linear multichannel transmission system can be considered as a general linear system with memory in space.     

Radiation-induced attenuation self-compensating effect in super-fluorescent fiber source

The compact super-fluorescent fiber source(SFS) output spectra variations at different pump currents and under different dose of gamma-ray radiation were measured and compared respectively. The radiation-induced attenuation(RIA)self-compensating effect in SFS based on photo-bleaching was found and the general mathematic model of SFS output spectra variations was made. The radiation-induced background attenuation(RIBA) at the pump wavelength was identified to be the main cause of the total output power and spectra variations and the variations can then be compensated by active control of the pump power to enhance the self-compensating effect. With closed-loop feedback control of pump current,double-pass backward(DPB) configuration and spectrum re-shaping technology, an SFS prototype was made and tested.The mean-wavelength stability of about 87.4 ppm and output power instability of less than 5% were achieved under up to200 krad(Si) gamma-ray irradiation.     

Interaction of surface acoustic waves with moving vortex structures in superconducting films

A method is proposed for describing a moving film vortex structure and its interaction with surface acoustic waves. It is shown that the moving vortex structure can amplify (generate) surface acoustic waves. In contrast to a similar effect in semiconductor films, this effect can appear when the velocity of the vortex structure is much lower than the velocity of the surface acoustic waves. A unidirectional collective mode is shown to exist in the moving vortex structure. This mode gives rise to an acoustic analogue of the diode effect that is resonant in the velocity of the vortex structure. This acoustic effect is manifested as an anomalous attenuation of the surface acoustic waves in the direction of the vortex-structure motion and as the absence of this attenuation for the propagation in the opposite direction.