Physical model of acoustic forward scattering by cylindrical shell and its experimental validation

Research on the underwater target scattering can provide important theoretical support for target detection. The scattering model of cylindrical shell is established in this paper. It is found that the forward target strength is much stronger and varies with angles of incident wave less significantly than backward target strength. The received forward signal strength fluctuates with the target moving due to the interference between direct signal and scattering signal, which is most significant when the target approaches the baseline. An experiment is carried out in an anechoic tank to validate the scattering model. The method of acquisiting forward scattering in the tank is proposed. The forward and the backward target strengths are achieved by using the pulse compression technology, and they axe about 3dB less than the modeling results. The forward scattering phenomena of quiescent and moving targets are measured, which are similar to modeling results with different target types.

Forward scattering bistatic radar imaging method and practice data processing

A new forward scattering bistatic radar motion compensation method based on spectral analysis and minimum waveform entropy is proposed. In order to demonstrate effectiveness of the presented method and ground vehicles imaging function of forward scattering radar, a simple bistatic forward scattering radar experiment system is set up on both sides of a road to collection ground vehicles experimental data. Finally, experimental ground vehicles imaging results confirm validity of the proposed motion compensation method and the experimental imaging results are identical with computer simulation results in the same parameter and condition.

Ground moving target signal model and power calculation in forward scattering micro radar

Forward scattering micro radar is used for situation awareness; its operational range is relatively short because of the battery power and local horizon, the free space propagation model is not appropriate. The ground moving targets, such as humans, cars and tanks, have only comparable size with the transmitted signal wavelength; the point target model and the linear change of observation angle are not applicable. In this paper, the signal model of ground moving target is developed based on the case of forward scattering micro radar, considering the two-ray propagation model and area target model, and nonlinear change of observation angle as well as high order phase error. Furthermore, the analytical form of the received power from moving target has been obtained. Using the simulated forward scattering radar cross section, the received power of theoretical calculation is near to that of measured data. In addition, the simulated signal model of ground moving target is perfectly matched with the experimented data. All these results show the correctness of analytical calculation completely.

Simultaneous detection of the acoustic-field aberration and Doppler shift in forward acoustic scattering

The aberration in the received acoustic field and the Doppler shift in the forward scattered field are simultaneously induced when a submerged target crosses the source–receiver line. Formulations for the two variations are developed upon an ideal forward scattering configuration. Both the field aberration and the Doppler shift are expressed as functions of the same argument — the target motion time. An experimental validation was carried out in a tank, in which the continuous wave was transmitted. The field aberration and the Doppler shift were extracted from the collected data by the simple Hilbert transform and a hybrid technique, respectively. The measured aberration and Doppler shift agree with the theoretical results.Simultaneous detection outputs are beneficial to enhance the reliability on target detection by providing both the aberrations in the received acoustic field and the Doppler shift in the forward scattered field.

Forward Scattering Series and Pade Approximants for Acoustic Wavefield Propagation in a Vertically Varying Medium

We present the application of the theory of Pade approximants to extending the perturbative solutions of acoustic wave equation for a three dimensional vertically varying medium with one interface.These type of solutions have limited convergence properties depending on either the degree of contrast between the actual and the reference medium or the angle of incidence of a plane wave component.We show that the sequence of Pade approximants to the partial sums in the forward scattering series for the 3D wave equation is convergent for any contrast and any incidence angle.This allows the construction of any reflected waves including phase-shifted post-critical plane waves and,for a point-source problem,refracted events or headwaves,and it also provides interesting interpretations of these solutions in the scattering theory formalism.

Distributed analysis of forward stimulated Brillouin scattering for acoustic impedance sensing by extraction of a 2nd-order local spectrum

Distributed fiber sensors based on forward stimulated Brillouin scattering(F-SBS)have attracted special attention because of their capability to detect the acoustic impedance of liquid material outside fiber.However,the reported results were based on the extraction of a 1st-order local spectrum,causing the sensing distance to be restricted by pump depletion.Here,a novel post-processing technique was proposed for distributed acoustic impedance sensing by extracting the 2nd-order local spectrum,which is beneficial for improving the sensing signal-to-noise ratio(SNR)significantly,since its pulse energy penetrates into the fiber more deeply.As a proof-of-concept,distributed acoustic impedance sensing along~1630 m fiber under moderate spatial resolution of~20 m was demonstrated.

Scattering by Large Bubbles: Correction for Debye Coefficient at a Small Number of Sub-Waves

I found that when the Debye theory calculates the far-field scattered light intensity of bubbles, the forward scattered light intensity is quite different from the result calculated by the Mie theory due to the convergence problem, so the expression of the Debye coefficient has been revised. I derived the Debye reflectance and transmittance according to the physical meaning of Debye theory and compared them with Fresnel’s formula. I modified the Debye coefficient expressions for bubbles based on the differences between the Debye reflectance and transmittance from the Fresnel formula. Finally, compared with the far-field scattered light intensity calculated by the original Debye theory, the far-field scattered light intensity calculated based on the modified Debye coefficient can obtain more accurate forward scattered light intensity with fewer sub-waves.

Detection method of forward-scatter signal based on Rényi entropy

The application scope of the forward scatter radar(FSR)based on the Global Navigation Satellite System(GNSS)can be expanded by improving the detection capability.Firstly,the forward-scatter signal model when the target crosses the baseline is constructed.Then,the detection method of the for-ward-scatter signal based on the Rényi entropy of time-fre-quency distribution is proposed and the detection performance with different time-frequency distributions is compared.Simula-tion results show that the method based on the smooth pseudo Wigner-Ville distribution(SPWVD)can achieve the best perfor-mance.Next,combined with the geometry of FSR,the influence on detection performance of the relative distance between the target and the baseline is analyzed.Finally,the proposed method is validated by the anechoic chamber measurements and the results show that the detection ability has a 10 dB improvement compared with the common constant false alarm rate(CFAR)detection.

A NEW ALGORITHM OF RELAXATION METHOD FOR PARTICLE ANALYSIS FROM FORWARD SCATTERED LIGHT

A new algorithm of the relaxation method is developed for the inversion of forward scattered light to obtain the size distribution of spherical particles. Numerical tests are performed for a laser particle analyzer using the Mie theory and the diffraction approximation. The algorithm efficiency, in the presence of experimental noises, is studied. The results show that the technique is fast in convergence, stable against random noise and insensitive to the distribution of particles and the initial trial distribution.

Numerical calculation of multiple scattering with the layer model

The optical measurement technique based on Mie scattering has been applied to various areas, in which single scattering at low particle concentration is assumed. Nevertheless, since multiple scattering is usually unavoidable in online measurements, we present in this work a multiple scattering calculation method, in which a layer model is employed. The three-dimensional particle system is divided into a pile of layers the number of which is automatically determined, depending on the obscuration of the particle system. The calculation is found to be fast, reasonable and reliable.

Application of the direct blast adaptive suppression method to the forward scattered broadband signals

The basic principles of target detection by forward acoustic scattering are presented.A direct blast suppression approach based on adaptive filtering(DBS-AF) is proposed to suppress the direct blast.The DBS-AF technique is extended to the linear frequency modulation(LFM) signal,where the envelope of the signal is regarded as a 'general waveform' and imported into the adaptive filter.Application of the DBS-AF method to the data collected from a lake trial yields an output detection curve,in which the direct blast is mapped to the background while the acoustic field aberration is represented by the peak value fluctuation.The inhibitory effect in single hydrophone is approximately- 5 dB,and is then enhanced by exploiting the mean value removal approach as a preprocessing technique.The direct blast is further suppressed to a level of-10 dB by making full use of multichannel receptions.The main factors affecting the algorithm performance are as follows:the fluctuation degree of the receptions during the weighting vector training period and the power ratio of the forward scattered wave to the direct blast when the target is present.

Optical properties of conductive silver-nanowire films with different nanowire lengths

Transparent electrodes made of silver nanowires （Ag NWs） exhibit a higher flexibility than conventional indium tin oxide electrodes.For this reason,Ag NWs may find applications in future flexible electronic and optoelectronic devices.However,different optoelectronic devices have different specific requirements for Ag NWs.For example,the optical transmittance haze is an important but rarely studied aspect of Ag NW films.In this study,the optical transmittance and optical scattering of long （5-50 μm,L-NWs） and short （1-20 μm,S-NWs） Ag NW films were investigated.The L-NWs exhibited better optical transmission than the S-NWs,whereas the S-NWs exhibited better light-scattering properties than the L-NWs.Our results indicate that the L-NWs are suitable for touch-screen displays,whereas the S-NWs are better suited as transparent conductive films for solar cells.We analyzed the scattering ratio of forward-scattered light to backscattered light for both the L-NWs and S-NWs and discovered that the mesh size affected the scattering ratio.For longer wavelengths,a larger mesh yielded a higher backscattering ratio,whereas a smaller mesh yielded a lower backscattering ratio.We formulated an equation for calculating the reflection haze using the total reflection （Ag NWs/glass）,R and the reflection of glass,R0.The reflection haze of the S-NWs and L-NWs exhibited different trends in the visible-near-infrared region.An omnidirectional scattering model for the Ag NWs was used to evaluate the Ag NW scattering properties.The results of this study have great significance for the evaluation of the performance of Ag NWs in optoelectronic devices.

Higher-Order Rytov Approximation for Large-Scale and Strong Perturbation Media

In the field of geophysics,although the first-order Rytov approximation is widely used,the higher-order approximation is seldom discussed.From both theo-retical analysis and numerical tests,the accumulated phase error introduced in the first-order Rytov approximation cannot be neglected in the presence of strong velocity perturbation.In this paper,we are focused on improving the phase accuracy of forward scattered wavefield,especially for the large-scale and strong velocity pertur-bation case.We develop an equivalent source method which can update the imaginary part of the complex phase iteratively,and the higher-order scattered wavefield can be approximated by multiplying the incident wavefield by the exponent of the imaginary part of the complex phase.Although the convergence of the proposed method has not been proved mathematically,numerical examples demonstrate that our method can produce an improved accuracy for traveltime(phase)prediction,even for strong perturbation media.However,due to the neglect of the real part of the complex phase,the amplitude change of the scattered wavefield cannot be recovered.Furthermore,in the presence of multi-arrivals phenomenon,the equivalent scattering source should be handled carefully due to the multi-directions of the wavefield.Further investigations should be done to improve the applicability of the proposed method.

Enhancement of light trapping in thin-film solar cells through Ag

Forward-scattering efficiency （FSE） is first proposed when an Ag nanoparticle serves as the light-trapping structure for thin-film （TF） solar cells because the Ag nanoparticle’s light-trapping efficiency lies on the light-scattering direction of metal nanoparticles.Based on FSE analysis of Ag nanoparticles with radii of 53 and 88 nm,the forward-scattering spectra and light-trapping efficiencies are calculated.The contributions of dipole and quadrupole modes to light-trapping effect are also analyzed quantitatively.When the surface coverage of Ag nanoparticles is 5%,light-trapping efficiencies are 15.5% and 32.3%,respectively,for 53and 88-nm Ag nanoparticles.Results indicate that the plasmon quadrupole mode resonance of Ag nanoparticles could further enhance the light-trapping effect for TF solar cells.