Application of time reversal in underwater communication

Time reversal is applied to the underwater spreading spectrum coding communication. On the base of analyzing the focusing characteristics of the time reversal in underwater waveguide, the time reversal is studied to overcome the wave distortion of the encoded signal caused by the multi-path effect. The experiment research for underwater coding communication is carried out in a lab water tank and the corresponding theoretical analysis is also conducted by Binary Phase Shift Keying (BPSK) encoding and Barker code with 7 chips for the spreading spectrum signal. The results show that the time reversal can improve the focusing gain and increase the ratio of the principal to the second lobe of the coding signal, and can decrease the bit error rate and increase the communication distance.     

Multi-path propagation of acoustical wave and time reversal field in a solid plate

The multi-path effect of the acoustical wave in a solid plate is studied. The multireflection and wave conversion of the cylindrical compressional and shear waves, which are excited by an infinite strip on a free surface of the solid plate, are analyzed thoroughly by the far-field approximation method. The concise analytical representations of the cylindrical waves are obtained. The time reversal processing is then applied to the propagation of the cylindrical waves and analyzed theoretically and experimentally. It is shown that the waves coming from different array elements and different paths all arrive at the original place after the time reversal operation. It indicates that the time reversal can compensate automatically the wave aberration caused by the multi-path effect. The self-adaptive focusing of the time reversal field is also analyzed quantificationally by the focusing gain and the ratio of the principal to the second lobe. The effects of the focus position and the aperture of the transducer array on the focused field are also investigated. It shows that theoretical and experimental results are consistent to each other very well.     

Study on charge equilibration time of highly charged ions in carbon foils

Charge state distribution of 0.8MeV/u uranium ions after transmission through a thin carbon foil has been studied. It is observed that the charge state distribution is equilibrated after the uranium ions have passed through a 15μg/cm^2 carbon foil. The equilibrated average charge state is 33.72 and the charge equilibration time of uranium ions in carbon foil is less than 5.4fs.     

Application of adaptive filter in time delay estimation——generalized quadratic interpolation method

The time delay estimation(TDE)of two different received sig-nals from the same source has attracted many interests of researchers inthe field of signal processing.A method described for precisely estimatingtime delay in this paper is based on the assumption that little prioriknowledge on statistical characteristics is available for the received signals.The variance of the estimate is derived.The basic architecture of thismethod is to use the adaptive noise canceller,in the steady state,andto interpolate the weight coefficients by using a generalized quadratic inter-polation matrix.The formula of the time delay estimation is presented.The method proposed by F.A.Reed is a special case of this method.The hardware implementation is much easier than that of theconventional time delay estimation method.The results of the systemsimulation and the experimental results at sea show a good agreementwith the theoretical analysis.     

Study of strain and compostion of the self—organized GE dots by grazing incident X—ray diffraction

Grazing incident X-ray diffraction at different grazing angles for self-organized Ge dots grown on Si(001) are carried out and lattice constant expansions of 1.2?parallel to the surface as compared with the Si lattice are found within the Ge dots.A 3.1?lattice expansion of the Ge dots along the growth direction is also fund by ordinary X-ray(004) diffraction.According to the Poisson equation and the Vegard law,our results infer that the Ge dot should be a partially strain relaxed SiGe alloy with Ge content of abuot 55?2001 Elsevier Science B.V.All rights reserved.     

Quantum explanation for time—phase relations in radiation of the plasma induced by laser ablating aluminum

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Slow dynamics of supercooled liquid revealed by Rayleigh scattering of Mössbauer radiation method in time domain

Characteristic relaxation times τ of the microscopic correlation in o-terphenyl and glycerol were obtained by a time-domain interferometry method using nuclear resonant scattering of synchrotron radiation. From the temperature dependence of τ, a new decoupling temperature 278 K, which is below the conventional decoupling temperature 290 K, was obtained for o-terphenyl. Furthermore, we found a liquid-like relation between τ and q (momentum transfer) as $\tau \propto q^{-2}$ at 285 K in o-terphenyl and 260 and 265 K in glycerol. However, an anomalous q dependence of the slow β relaxation time, $\tau \propto q^{-2.9}$ and q ^?3.8, was observed in deeply supercooled (265 K) and glass (240 K) states, respectively, in o-terphenyl, indicating specially restricted dynamics of the slow β process.     

Design of the sample cell in near-field surface-enhanced Raman scattering by finite difference time domain method

The rational design of the sample cell may improve the sensitivity of surface-enhanced Raman scattering (SERS) detection in a high degree. Finite difference time domain (FDTD) simulations of the configuration of Ag film-Ag particles illuminated by plane wave and evanescent wave are performed to provide physical insight for design of the sample cell. Numerical solutions indicate that the sample cell can provide more "hot spots" and the massive field intensity enhancement occurs in these "hot spots". More information on the nanometer character of the sample can be got because of gradient-field Raman (GFR) of evanescent wave.     

Study of the location of testing area in residual stress measurement by Moiré interferometry combined with hole-drilling method

This paper investigates the effect of the location of testing area in residual stress measurement by Moiréinterferometry combined with hole-drilling method.The selection of the location of the testing area is analyzed from theory and experiment.In the theoretical study,the factors which affect the surface released radial strainεr were analyzed on the basis of the formulae of the hole-drilling method,and the relations between those factors andεr were established.By combining Moiréinterferometry with the hole-drilling method,the residual stress of interference-fit specimen was measured to verify the theoretical analysis.According to the analysis results,the testing area for minimizing the error of strain measurement is determined.Moreover,if the orientation of the maximum principal stress is known,the value of strain will be measured with higher precision by the Moiréinterferometry method.     

Preparation of a Waveguide Array in Flame Hydrolysis Deposited GeO2—SiO2 Glasses by Excimer Laser Irradiation

SiO2 galss films doped with GeO2 were prepared by the flame hydrolysis deposition method,then annealed at 1200℃.After exposure to high pressure hydrogen,the as-deposited films were irradiated with excimer laser pulses operated at 248nm,The induced refractive index change(the growth of index change was 0.33%)was measured by a spectorscopic ellipsometer.A waveguide array has been written in the film by irradiation through a phase mask.     

On the modeling of effective constitutive parameters of bianisotropic media by a periodic unfolding method in time and frequency domains

In this paper, we compute the equivalent electromagnetic parameters of a composite periodic material by using the unfolding homogenization method. We are therefore able to solve the Maxwell equations in the unit cell instead of the whole periodic structure. We choose bianisotropic 3D media as they constitute the more general case and we consider dispersive materials to introduce the frequency dependency. The simulations are based on various scenarios studied in the literature. We compare our results with several analytical formalisms and prove the effectiveness of our numerical method in the particular case of the time domain homogenization.     

Study of n-γ discrimination in low energy range(above 40 keVee) by charge comparison method with a BC501A liquid scintillation detector

A VME-based experiment system for n-y discrimination using the charge comparison method was established.A data acquisition program for controlling the programmable modules and processing data online via VME64X bus was developed through the use of Lab VIEW.The two-dimensional(2D) scatter plots of the charge in the slow component vs.the total charge from ~(241)Am-Be and ~(252)Cf neutron sources are presented.The 2D scatter plots of the energy vs.the ratio of the charge in the slow component to the total charge of the pulses are also presented.The quality of n-γ discrimination was checked by the figure-of-merit,and the results showed good performance of n-γ discrimination at the low energy range.Neutrons and γ-rays were separated above 50 keVee(electron-equivalent energy).The quality of n-γ discrimination has been improved compared with others' results at five energies(150,250,350,450,550 keVee).