Shock-wave generation during dry laser cleaning of particles

Generation of shock waves during a dry laser-cleaning process was examined with the probe-beam-deflection technique. Our experimental set-up allows measurement of a density gradient and a time of flight of the generated optoacoustic waves. With analysis of the measured signals we observed supersonic propagation velocities, density profiles matching the shock-wave profile, and reasonable agreement between the shock-wave theory and measured data. Furthermore, we inspected the mechanisms for the generation of these optoacoustic waves. For the clean surface a rapid thermoelastic surface expansion is the main generation mechanism, creating only weak shock waves; however, during the dry laser cleaning the ejected contaminants generate relatively strong shock waves. This difference is easily observed with our experimental set-up, thus enabling the on-line monitoring of the laser-cleaning process. PACS 79.20.Ds; 81.65.Cf; 43.25.+y     

Real-time optodynamic monitoring of pulsed laser decoating rate

This paper deals with a laser-cleaning process for removing a layer of paint. The effects of each individual laser pulse were monitored by a three-probe system. Ultrasonic signals from the rear surface of the substrate were obtained using an arm compensated Michelson interferometer, the optoacoustic waves in the surrounding air were detected using a laser beam deflection probe and the shape of the growing crater was measured using a laser anamorph profilometer. Two different cleaning lasers were used: an Excimer and a Nd:YAG laser. Typical parameters of the optodynamic signals versus the laser-pulse number are presented. A linear correlation between the quantity of ablated material and the integral of the ultrasonic signals was found together with an exponential relationship between the time-of-flight of the optoacoustic signals and the decoating rate.     

Intermediate regime of light diffraction by surface ultrasonic waves in planar gyrotropic optical waveguides

The intermediate regime of optoacoustic interaction, similar to the Bragg one, with Rayleigh surface ultrasonic waves in gyrotropic planar waveguides is studied. A system of equations of bound waves is presented that describes planar optoacoustic interaction in the intermediate, Raman-Natoh, and Bragg regimes of light diffraction by surface ultrasonic waves. It is shown that the optical activity of a wavequide film favors the incident light energy pumping to diffracted light for a TE-polarization wave and decelerates this process for an incident TM-polarization wave. Mozyr' State Pedagogical Institute, 28, Studencheskaya St., Mozyr', 247760, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 412–416, May–June, 1997.     

复Ginzburg-Landau方程中模螺旋波的稳定性研究

研究了复Ginzburg-Landau方程系统中模螺旋波与其他斑图在同一平面内的竞争行为,发现演化结果在系统参数平面内可分为四个主要区域:在I区和III区中,模螺旋波与相螺旋波相比稳定性较差,模螺旋波的空间被相螺旋波所入侵.在II区中,模螺旋波具有较强的稳定性,相螺旋波的空间被模螺旋波所入侵.在IV区内,由于时空混沌所导致的频率不稳定性,演化的结果较为复杂.我们通过对模螺旋波、相螺旋波以及时空混沌的频率分析,发现当模螺旋波的系统参数为α1=-1.34,β1=0.35时,较高频率的模螺旋波具有较好的稳定性,高频模螺旋波可以入侵低频斑图空间.竞争结果主要受系统变量实部的频率影响,频率分析所得到的理论结果与数值实验结果符合得非常好.     

手持半导体泵浦固体激光清洗机

针对轮胎模具和金属锈迹去除,介绍了自主研发生产的一种新型手持激光清洗机。采用半导体泵浦固体激光器（DPSS）手持方案,使光-光转换效率和清洁速率得到提升。激光平均功率超过160W。对橡胶轮胎模具和严重锈蚀钢板进行清洗,效果明显,清洗速度分别达到1.08m^2/h、1.44m^2/h。参数调整后可适应多种其他清洗应用。     

Nonlinear Evolution of a Gaussian Wave Perturbation in the Presence of a High Power Electromagnetic Plane Wave in the F Region Ionosphere

The interaction of a high power electromagnetic plane wave of uniform intensity with a weak electromagnetic Gaussian wave has been studies analytically. The two waves propagate vertically with almost the same frequency in the F region ionosphre. It is seen that the Gaussian wave acquires an oscillatory structure due to the nonlinear coupling of the strong plane wave and the weak Gaussian wave and the amplitude of these fluctuations grows with height. The growth, however, occurs only above a certain altitude z₀, which is determined by the plasma and wave parameters.     

Surface preparation of Ti–3Al–2.5V alloy tubes for welding using a fiber laser

Ti–3Al–2.5V tubes are widely used in aircraft hydraulic systems. Meticulous surface preparation before welding is necessary to obtain a sound weld involving these alloy tubes. Conventionally this is done by cleaning with environmentally malign toxic chemicals, such as, hydrofluoric acid and nitric acid. This paper describes the laser-cleaning process of the surface of these tubes with a fiber laser as a preparation for pulsed gas tungsten arc welding and results obtained. A simple one-dimensional heat equation has been solved to evaluate the temperature profile of the irradiated surface. It is shown that surface preparation by laser cleaning can be an environmentally friendly alternative process by producing acceptable welds with laser-processed tubes.     

Reconstruction of Optical Energy Deposition for Backward Optoacoustic Imaging

Visualizing optical properties, such as the optical absorption coefficient, helps us to obtain structural information of biological tissues. In this paper, we present an efficient reconstruction algorithm for optical energy deposition in backward optoacoustic imaging. Note that econstruction of optical energy deposition is the first step to imaging the optical absorption coefficient distribution. This algorithm is derived from the optoacoustic wave equations with line focusing, in which the focusing techniques were utilized to reduce the reconstruction problem from three dimensions (3-D) to one dimension (1-D). Simulations and experiments were conducted to verify efficacy of this algorithm. In the simulations, optoacoustic signals were generated based on the solution of the optoacoustic wave equations. In the experiments, a 3-D backward mode optoacoustic imaging system was built. The system consisted of a Nd YAG laser for optical irradiation and an acoustic detection system with a broadband hydrophone. A phantom was used to illustrate validity of the proposed algorithm. The results show that optical energy deposition can be efficiently reconstructed in both simulations and experiments.     

Diffraction efficiency of sequentially stored gratings in reflection volume holograms

On the basis of the theory of coupled waves the amplitudes of the first-order diffracted waves and the diffraction efficiency is calculated for sequentially superposed phase gratings stored in dielectric reflection holograms. The investigations are restricted to the case of a fixed reference wave during recording and Bragg-angle incidence.By analogy to the transmission volume grating the coupling effects which appear between incident beams and diffracted waves give the possibility of building up beam-couplers and -splitters with certain coupling parameters determined by the recording process. Moreover, reflection gratings allow a favourable combination between coupling and reflecting properties and avoid the diffraction efficiency oscillations that are characteristic of transmission gratings. The performance of the reconstruction process with several waves simultaneously allows the tuning of the intensities of diffracted waves with dependence on amplitude or phase differences between incident beams.     

Image reconstruction in optoacoustic tomography for dispersive acoustic media

Conventional image reconstruction methods for optoacoustic tomography (OAT) assume an idealized, non-dispersive acoustic medium. However, the linear attenuation coefficient and the phase velocity of acoustic waves propagating in soft tissue depend on temporal frequency and satisfy a known dispersion law. These frequency-dependent effects are incorporated into an optoacoustic wave equation, and a corresponding reconstruction method for OAT is developed. The improvement in image fidelity that can be achieved over conventional reconstruction methods is demonstrated by use of computer-simulation studies.     

Influence of static and dynamic internal actions on elastic nonlinear properties of a granulated unconsolidated medium

We have studied the influence of external static (pressure) and dynamic (caused by an elastic wave with a finite amplitude) actions on the linear and nonlinear elastic properties of a granulated unconsolidated medium, which was simulated by steel spheres with diameters of 2 and 4 mm. We have analyzed the equation of state for such a medium taking into account the presence of weakly and strongly deformed contacts between individual spheres. We have obtained expressions for the elasticity coefficient and second- and third-order nonlinear elastic parameters, and we have experimentally studied the influence of external static pressure on their values. We have measured the dependence of the velocity of elastic waves on external static pressure and the probing signal amplitude. In the studied medium, a number of structural phase transitions were observed, related to rearrangement of the packing of spheres, which were caused by both static and dynamic actions. The structural phase transition was accompanied by an anomalous change in the nonlinear elastic parameters of the medium and the velocity of elastic waves. We have analyzed the results based on the Hertz theory of contact interaction.     

Lateral spatial resolution of widened focused transducer array for optoacoustic tomography

Numerical investigation of a point spread function for a optoacoustic transducer array is described. Analysis of the minimal reconstructed dimension for an optoacoustic image of a point source of spherical waves is performed within the proposed approach. The influence of the array geometrical parameters, number of array transducers, and the frequency band of a single transducer on the lateral resolution in the image plane is investigated in detail. It is demonstrated that the lateral resolution obtained with the help of a transducer array in the image plane is determined unambiguously by the frequency band of a transducer, the flare angle of the array, and the transducer width and does not depend on the number of transducers.     

Modelling of Internal and Surface Waves of the Real Ocean in the Large Thermostratified Experimental Tank at the Institute of Applied Physics of the Russian Academy of Sciences

We discuss the results of studies of surface-wave transformation by nonuniform flows, performed in the tank of the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS), and the results of modelling of the influence of iceberg motion on regular background internal waves in the subsurface pycnocline.Transformation of surface waves in the flow field past an immersed sphere is studied both experimentally and theoretically. It is shown that even fairly weak nonuniform flows can cause noticeable changes in the surface-wave field. The sizes of the spatial region in which the characteristics of the surface waves are changed exceed considerably the sizes of the nonuniform-flow region. It is found that the nonlinearity of surface waves leads to an increase in the variability of the surface-wave amplitude in a broad frequency range. The proposed theoretical model describes well the main experimentally observed features of the transformation of nonlinear surface waves in the nonuniform-flow field.It is proved experimentally that background internal waves with frequencies close to those of internal waves in an iceberg wake lead to a considerable transformation of the field of lee waves. The parameters of the resulting wave system are independent of characteristic horizontal sizes of the iceberg model and the length of the internal wave. The total wave system is stationary in the entire velocity range of the model in the case of counterpropagation of background waves. In the case of copropagation of background waves, the nature of the wave system depends on the ratio between the towing velocity and the phase velocity of background waves. In particular, the wave system in the wake can have both a pronounced nonstationary nature and a typical stationary phase pattern.     

Oblique Interaction of Ion-Acoustic Solitary Waves in e-p-i Plasmas

In this study, we investigate the oblique collision of two ion-acoustic waves (IAWs) in a three-species plasma composed of electrons, positrons, and ions. We use the extended Poincare-Lighthill-Kuo (PLK) method to derive the two-sided Korteweg-de-Vries (KdV) equations and Hirota’s method for soliton solutions. The effects of the ratio (δ) of electron temperature to positron temperature and the ratio (p) of the number density of positrons to that of electrons on the phase shift are studied. It is observed that the phase shift is significantly influenced by the parameters mentioned above. It is also observed that for some time interval during oblique collision, one practically motionless composite structure is formed, i.e., when two ion-acoustic waves with the same amplitude interact obliquely, a new non-linear wave is formed during their collision, which means that ahead of the colliding ion-acoustic solitary waves, both the amplitude and width are greater that those of the colliding solitary waves. As a result, the nonlinear wave formed after collision is a new one and is delayed. The oblique collision of solitary waves in a two-dimensional geometry is more realistic in high-energy astrophysical pair plasmas such as the magnetosphere of neutron stars and black holes.     

Localization of nonlinear waves in elastic bodies with inclusions

By the example of the problem of the motion of a semi-infinite string lying on an elastic base, a method for describing wave localization near inclusions is proposed for the case of a cubic nonlinearity of the base. The method applies the perturbation technique to the amplitude of a localized mode. The nature of the divergences is revealed, and the secular terms are found to belong to one of two types: inphase or antiphase with the localized wave. It is shown that a combination of the renormalization method and multiscale method provides a convergence of the solutions, which are sought for in the form of power series in the amplitude of the localized mode. It is found that the localization process is determined by the type of the discrete spectrum, type of the nonlinearity, and type of dispersion. The nonlinearity of the elastic base produces two characteristic effects. First, the frequency of the localized wave becomes dependent on the wave amplitude. Second, the system can generate traveling waves at multiple frequencies, which withdraw energy from the localized wave and cause it to decay. The decay behavior is determined by the minimum frequency of these traveling waves (because it must be higher than the cutoff frequency). The lifetime of the localized wave as a function of the mass of a dynamic inclusion exhibits a number of maxima. In particular, the first maximum corresponds to the minimum amplitude of the traveling wave at the triple frequency.     

Solitary waves on a curved space-time

A nonlinear partial differential equation is derived which admits plane solitary waves on a conformally flat Riemannian space-time. The metric is determined by the amplitude of these waves. By interpreting these solitary waves as particles we arrive at the following picture: these particles are confined to regions exhibiting singular (very large) amplitudes in an otherwise continuous wavetrain. There is, thus, no distinction between the notion of a particle and that of a wave.     

考虑频散效应的一维非线性地震波数值模拟

非线性理论是解决地学问题的重要手段, 充分认识非线性波动特征有助于解释实际观测资料中的一些特殊地震现象. 本文基于Hokstad改造的非线性本构方程, 利用交错网格有限差分法实现了固体介质中一维非线性地震波数值模拟; 采用通量校正传输方法消除非线性数值模拟中波形振荡, 提高模拟精度. 通过与解析解的对比验证了模拟结果的正确性. 研究结果显示了非线性系数对地震波的传播有重要影响, 并且当取适当的非线性和频散系数时, 地震波表现出孤立波的传播特性. 最后分析了不同的非线性地震波在固体介质中的传播演化特征.     

Saturation effects at backward-stimulated scattering in the single-mode regime of interaction

The effects of amplitude modulation of interacting waves at stimulated Mandelstam-Brillouin scattering (SMBS) in a monomode fibre caused by the noise origin of the initial Stokes wave are investigated both theoretically and experimentally. Depending on the determined conditions, there can taken place either a smoothing of the noise amplitude modulation or its preservation in the Stokes radiation in the deep saturation regime of SMBS. Amplitude modulation with period corresponding to the double time of radiation propagation through the fibre is observed along with noise-amplitude modulation of the Stokes and pump waves. The effect of this period prolongation is discussed. The influence of phase fluctuations on amplitude modulation of interacting waves is investigated. The role of phase correlations in the feedback regime of SMBS is considered. Experimental results are in good agreement with numerical calculations.     

Analysis of the rogue waves in the blood based on the high-order NLS equations with variable coefficients

The research of rogue waves is an advanced field which has important practical and theoretical significances in mathematics, physics, biological fluid mechanics, oceanography, etc. Using the reductive perturbation theory and long wave approximation, the equations governing the movement of blood vessel walls and the flow of blood are transformed into high-order nonlinear Schrödinger (NLS) equations with variable coefficients. The third-order nonlinear Schrödinger equation is degenerated into a completely integrable Sasa-Satsuma equation (SSE) whose solutions can be used to approximately simulate the real rogue waves in the vessels. For the first time, we discuss the conditions for generating rogue waves in the blood vessels and effects of some physiological parameters on the rogue waves. Based on the traveling wave solutions of the fourth-order nonlinear Schrödinger equation, we analyze the effects of the higher order terms and the initial deformations of the blood vessel on the wave propagation and the displacement of the tube wall. Our results reveal that the amplitude of the rogue waves are proportional to the initial stretching ratio of the tube. The high-order nonlinear and dispersion terms lead to the distortion of the wave, while the initial deformation of the tube wall will influence the wave amplitude and wave steepness.     

An optodynamic method for the real-time determination of the depth of a laser-drilled hole

An experimental method for determining the real-time depth of laser-drilled holes is presented. The proposed method involves detecting the laser-induced optoacoustic waves generated during the interaction of the laser beam with the material. Our optodynamic study involved measuring the propagation times of these waves as they traveled through the material and analyzing their temporal behavior during the drilling process. The experimental observations revealed an exponential relationship between the propagation time of the longitudinal stress wave and the number of consecutive laser pulses. Received: 25 October 2001 / Accepted: 27 October 2001 / Published online: 20 December 2001