The applicability of a material-treatment laser pulse in non-destructive evaluations

A practical optodynamic study was performed to determine the usability of different lengths of laser pulses for the generation of ultrasonic transients in a solid material. The aim of the study was to evaluate the possibility of a dual use for a laser pulse-for laser material processing, on the one hand, and for the ultrasonic wave generation on the other-with both processes being combined on the same production line. The propagation of the laser-generated ultrasonic waves is evaluated by detecting and measuring with a PID-controlled stabilized interferometer. Thus, both systems provided the basic tools, the generation and detection of ultrasonic waves, for an ultrasonic, laser-based, non-destructive material evaluation. The ultrasonic transients generated by 'classical' nanosecond laser pulses were compared with the transients generated by industrial laser pulses with a duration of a few tenths of a microsecond. The experimental results are compared with the results of a time-of-flight analysis that also involved part of a mode-conversion analysis for both regimes in a layered material structure. The differences between the two waveforms were assessed in terms of their visibility, wavelength and resolution. The limit values were calculated and estimated for the laser-pulse parameters, when such pulses are intended for use in an ultrasonic, laser-based, non-destructive evaluation. The possibility of using an industrial marking laser for laser ultrasound generation is thus demonstrated.     

The spatio-temporal structure of wideband acoustic pulses in shallow sea

We study the influence of a bottom-sediment layer on the spatio-temporal structure of wideband acoustic pulses in shallow sea. The shallow sea is modeled as a uniform liquid layer above a layer of liquid sediments located on an elastic half-space. The influence of various acoustic parameters of the problem, in particular the thickness of the sedimentary layer, damping parameters, etc. on the structure of wideband pulses is considered. We analyze the features of constructive mode interference due to which the spatial structure of the pulses takes the form of beams.     

High-Resolution Ultrasound Technologies for Studying Biological Objects

Russian experience in the development of high-resolution ultrasound technologies for bioimaging is considered. Two types of ultrasound biomicroscopy (UBM) systems for the in vivo imaging of skin are described: a UBM system based on a resonant transducer with the electrical excitation of probing pulses and a UBM system based on a wideband polyvinylidene difluoride detector (PVDF) with laser thermoelastic excitation of the probing pulses.     

Materials testing by ultrasonic spectroscopy

Ultrasonic spectroscopy is a development of the pulse-echo technique which uses broadband (0.5–10 MHz) ultrasound and analyses the spectra of the echo pulses. It has already proved its value in cases where the geometry is essentially two-dimensional. Examples include the determination of grain size in metals and quality control in carbon-fibre composites and glued joints, while a new development has opened up the possibility of measuring the depth and width of surface cracks down to sub-millimetre size. When the problem is essentially three-dimensional, as in characterization of discrete defects in metals, full interpretation of the echo signal cannot as yet be achieved except in a few special cases.This paper is the first of two reviews on ultrasonic spectroscopy. The second will cover the generation and reception of wideband ultrasound.     

Developments in direct ultrasonic visualization of defects

Direct ultrasonic visualization of defects (duvd) is a method of instantaneous ultrasonic imaging in which reflections of repetitive ultrasonic pulses are focused into a transparent medium and made visible by a stroboscopic Schlieren apparatus. An improved system using two polystyrene lenses in water and short pulses of 1.5 MHz ultrasound is described. Resolution of 1 mm both in the direction of the ultrasonic beam and transversely is demonstrated over a 25 x 25 mm² field. The potential performance is discussed; good resolution can be expected but the sensitivity is about 50 dB short of that of conventional pulse-echo techniques, so that general applications in non-destructuve testing and medicine are ruled out. Uses may exist in short-range underwater work for rapid, low sensitivity inspection of materials.     

Backward Raman amplification in plasmas with chirped wideband pump and seed pulses

Chirped wideband pump and seed pulses are usually considered for backward Raman amplification(BRA)in plasmas to achieve an extremely high-power laser pulse.However,current theoretical models only contain either a chirped pump or a chirped seed.In this paper,modified three-wave coupling equations are proposed for the BRA in the plasmas with both chirped wideband pump and seed.The simulation results can more precisely describe the experiments,such as the Princeton University experiment.The optimized chirp and bandwidth are determined based on the simulation to enhance the output intensity and efficiency.     

Controllable optical delay of wideband laser pulses by means of coherent effects

We report the first experimental observation of controllable temporal delay of wideband laser pulses (input spectral bandwidth of 3.3 GHz) using electromagnetically induced transparency. We obtain this result with limited temporal distortion of our pulses and excellent values of the delay-bandwidth product. Our experimental results are in agreement with a theoretical analysis.     

Remote cure monitoring of epoxy materials using optical techniques

A remote all-optical technique for the generation and detection of ultrasonics in materials is described. This technique uses a robust wideband optical fiber interferometer for detection. Using this interferometer, non-contact measurements of ultrasonic propagation times in epoxy materials are reported throughout the complete cure cycle.     

Electro-optic detection of femtosecond electromagnetic pulses by use of poled polymers

We report the generation and coherent detection of freely propagating ultrashort baseband electromagnetic pulses. Using optical rectification in ?110? GaAs for wideband emission and electro-optic sampling in a poled polymer for wideband detection, we demonstrate spectral sensitivity that extends from the far infrared (lambda~100 mum) to ~33 THz(lambda = 9 mum) . Over a band of nearly 20 THz, a relatively flat frequency response is observed. We discuss issues that limit the response bandwidth.     

Testing a thermoacoustic sensor of high-power microwave pulses

We present the results of testing a new thermoacoustic sensor designed to detect microwave pulses having durations from 3 to 120 ns at wavelengths of 0.8 and 3 cm. Operation of the sensor is based on the effect of generation of acoustic signals during absorption of microwave pulses in a radiotransparent substrate–absorber–liquid layered structure . A thin nanometer-thick film deposited on a substrate is used as an absorber. Microwaves are converted to an acoustic pulse in the film and the adjacent liquid. The pulse is received by a wideband acoustic receiver and then recorded by a digital oscilloscope. It is shown that for a pulse duration of 120 ns, the shape of the signal recorded by the thermoacoustic sensor completely corresponds to the signal of a tube-diode detector of microwave pulses. The response of the thermoacoustic sensor to shorter pulses (3 and 5 ns long) is a pulse with a duration of 18 ns which is determined by a limited frequency band of the acoustic receiver.     

Alfvén Pulses in the Ionospheric Waveguide

Exact explicit solutions, describing the propagation of wideband non-sinusoidal waveforms in the ionospheric Alfvén waveguide by means of non-separable electromagnetic fields, are presented. The dispersive modulation of these fields is shown to result in the formation of quasimonochromatic oscillations at the wings of the Alfvén pulses. The frequencies of these oscillations are close to the cut off frequencies of the waveguide.     

螺纹边界法对圆柱超声缓冲杆尾随脉冲干扰的抑制*

对于高温管道壁厚的超声波在线监测,需要超声波探头与缓冲杆搭配使用,以降低探头的接触温度。超声波在圆柱形缓冲杆的边界会发生波型转换,在缓冲杆的第一次和第二次底面回波之间产生等间隔的尾随脉冲干扰,影响管道内壁回波的识别与提取。该文创新性地采用螺纹边界法改变缓冲杆的边界特征以抑制尾随脉冲干扰,经实验验证螺纹边界法对尾随脉冲干扰有较好的抑制效果;不同螺纹螺距的实验对比表明,对于直径为20 mm的钢材料圆柱缓冲杆,在1 mm、2 mm和3 mm三种螺距中,螺距为2 mm的螺纹边界对5 MHz超声波的尾随脉冲干扰的抑制效果最强。     

Probing of ultrasonic pulses by multidirectional light

An acousto-optical reconstruction method for acoustic signals using multidirectional light diffraction by finite amplitude ultrasonic pulses is presented. When crossing the ultrasonic field, the far field diffracted laser light intensity of an incident convergent lightbeam becomes modulated in time. It is found that for special conditions, concerning direction of observation, ultrasonic frequency, power level and interaction length, the modulated light intensity is almost an exact copy of the diffracting acoustic pulse. Reconstruction can be completed by applying a fast Fourier transform (FFT) routine. Examples are provided and applications of this optical probing technique are suggested.     

Probing of laser-induced crack modulation by laser-monitored surface waves and surface skimming bulk waves

All-optical monitoring of the nonlinear motion of a surface-breaking crack is reported. Crack closing is induced by quasi-continuous laser heating, while Rayleigh surface acoustic pulses and bulk longitudinal surface skimming acoustic pulses are also generated and detected by lasers. By exploiting the strong dependence of the acoustic pulses reflection and transmission efficiency on the state-open or closed-of the contacts between the crack faces, the parametric modulation of ultrasonic pulses is achieved. It is observed that bulk acoustic waves skimming along the surface can be more sensitive to crack motion than Rayleigh surface waves.     

Physical and technological aspects of ultrasonic imaging of brain structures through thick skull bones: 1. Theoretical and model studies

The basic effects that restrict the potentialities of the ultrasonic imaging of brain structures by echo sounding through thick skull bones are considered. It is demonstrated that the main interfering factors are the echo signals from multiple reflections in the bone, its unknown inhomogeneity in depth, and the strong attenuation. To eliminate these effects, the use of a matched spatial processing and complex wideband signals with their subsequent correlation compression is proposed. A simulation of the proposed signal processing techniques confirmed the possibility of brain structure imaging with a spatial resolution of about 1 mm by ultrasonic echo sounding with acceptable intensities through bones up to 20 mm thick at frequencies of 1–2 MHz.     

光学参量啁啾脉冲放大中时间抖动对飞秒脉冲对比度的影响

 基于宽带脉冲的计算模型,采用4阶龙格-库塔法研究了光学参量放大所满足的三光波耦合方程。数值结果表明:时间不同步导致大量本底能量不能被压缩,使本底形成,导致飞秒脉冲展宽,如抖动从0变化到150 ps,则其预脉冲宽度由0.2 ps展宽到0.7 ps,降低了输出飞秒激光对比度,并导致输出能量稳定性降低。采用长而平滑泵浦光脉冲、选择适当长度的非线性晶体、并使系统工作于增益饱和区,能实现高对比度和高稳定的飞秒激光输出。     

Wideband light diffraction on bulk acoustic waves in uniaxial crystals

We study diffraction of light on ultrasonic waves, excited by a system of interdigitial transducers, in uniaxial crystals. We show that wideband acoustooptical diffraction in trigonal crystals in the presence of additional anisotropy differs significantly from the isotropy of diffraction on a shear ultrasonic wave in the absence of it. We establish that an exact Bragg angle correction in a wide frequency retuning range is achieved owing to electrically induced anisotropy in the external electric field.