Wideband focused film transducer for optoacoustic tomography

The response of a focused film transducer to wideband acoustic signals is studied both theoretically and experimentally. The transducer has the form of a narrow PVDF strip placed on a concave cylindrical surface. A software package is developed for calculating the impulse transient response functions depending on the position of the point source of spherical waves. The experiments are performed using laser thermooptical sources of acoustic spherical wave pulses excited by a pulsed diode-pumped Nd: YAG laser. The theoretical and measured temporal profiles of signals recorded by the transducer are shown to be in good agreement for the source positioned near the transducer’s focus. For this region, a transducer sensitivity map is investigated. For the case of the source positioned at the focus of the transducer, the absolute value of the transducer sensitivity is 8 µV/Pa.     

Modeling the quasicompressional wave field of a rectangular transducer in a transversely isotropic solid

A fast model for simulating the transient quasicompressional wave field of a rectangular ultrasonic transducer directly coupled to a transversely isotropic elastic half-space of general orientation is developed. The so-called two-tier asymptotic approach and the uniform stationary phase method are used to derive the high-frequency asymptotics of time-harmonic displacements. Then, transient fields are modeled by means of harmonic synthesis. In geometrical regions, the formulas involve elementary and inside boundary layers, well-known special functions (Fresnel integral and generalized Fresnel integral), and are applicable in the radiating near field. The asymptotics elucidate the physics in terms of various arrivals and give explicit dependence of the radiated waves upon model parameters. The asymptotic code is tested against a direct numerical solution. It is at least a thousand times faster but describes accurately both arrival times and amplitudes of various pulses radiated by the transducer.     

Signal enhancement in Rayleigh wave interactions using a laser-ultrasound/EMAT imaging system

Enhancement of signal amplitudes from Rayleigh wave interaction at solid surface features has been investigated when signals were detected by an in-plane electromagnetic acoustic transducer (EMAT). A laser-ultrasound system was used to inspect surface-breaking slots, serving as artificial defects. Nd:YAG laser pulses were delivered onto a metal surface via an optical fiber and focused to a line source by a cylindrical lens. An in-plane EMAT receiver detected transient surface acoustic waves. A-scan signals and B-scan images from surface defects revealed increased signal amplitude up to 2.8+/-0.3 depending on the distance of the transducer from a slot. An explanation is based on the interaction of the EMAT sensor with the Rayleigh wave. A supporting computer model was derived to show that experimental signal enhancements were consistent with numerical predictions.     

Comparative study of wide-band ultrasonic transducers

The relative performance of several types of ultrasonic transducer are assessed using reproducible acoustic transients generated from Q-switched Nd:YAG laser pulses incident on an aluminium alloy sample. A laser interferometer, a capacitance transducer, two types of electromagnetic acoustic transducers (EMATs), and a broad-band piezoelectric transducer are examined as detectors. The comparison includes a study of their rise-times, and typical signal-to-noise ratios. In the case of the interferometer and capacitance transducer, displacement measurements are shown to be highly consistent with theory.     

A spatial impulse response based method for determining effective geometrical parameters for spherically focused transducers

This paper proposes a novel method for determining effective geometrical parameters (GPs) of spherically focused transducer. Theoretical basis of the method is the spatial impulse response method (SIRM) that is a powerful tool to calculate transient fields from a piston-like transducer. Experimentally, the method is implemented by measuring arrival times of the pulse signals (either transmission or pulse echo signals) and using them to determine the time intervals between the direct and edge wave pulses. Since the time intervals for a given measurement position depend on the transducer's GPs, then the effective GPs can be obtained. The measurements are carried out in the near-field so that the time intervals can be resolved. The proposed method has been applied to a spherically focused transducer operating in pulse echo mode. The results have shown that the effective GPs used in the SIRM give a closer agreement between the theory and the measurements than the nominal GPs. The method is best suited for characterizing broadband transducers with short pulse excitations. With certain modification it can be also applied to narrowband transducers. The method can be also applied to transducers of other shapes, e.g., planar circle, and planar or curved rectangle provided the piston approximation is appropriate.     

Explosive boiling in water exposed to q-switched erbium laser pulses

Pressure signals generated in water under the action of erbium laser pulses (100–200 ns, 2.94 μm) are investigated with lithium niobate piezoelectric transducer. For the first time multiply short (subnanosecond) pressure pulses standing against smooth pressure background are observed when the laser intensity exceeds explosive boiling threshold.     

Nonstationary radiation mode of a thin-walled piezoceramic sphere with a correcting electric circuit

A transducer in the form of a water-loaded thin-walled piezoceramic sphere is considered. On the electric side, the transducer is connected to an RL circuit. The pulsed radiation mode of the transducer is studied using the theory of equivalent schemes of piezoelectric transducers and the spectral Fourier transform. For the system under study, the parameters that provide the minimal emitted signal duration are determined for different values of the relative wall thickness of the sphere. The durations and amplitudes of acoustic pulses emitted by the transducer in the presence and absence of the electric circuit are compared.     

On driving a transducer to produce pulses shorter than the natural period of the transducer

When an acoustical transducer is given a short electrical impulse, the pressure wave produced is usually some sort of damped sine wave whose exact shape is determined by the physical properties of the transducer and the medium to which it is coupled. In order to produce pressure pulses shorter than this, it is necessary to drive the transducer with a more complex waveform. This paper describes how to calculate and produce the voltage waveform for any desired pressure waveform and shows some examples of the voltage waveforms necessary to drive a loudspeaker to produce pulses shorter than the natural resonant period of the loudspeaker.     

Ultrafast band-gap shift induced by a strain pulse in semiconductor heterostructures

The conventional piezospectroscopic effect is extended to picosecond time scales by using ultrashort strain pulses injected into semiconductor heterostructures. The strain pulses with durations of approximately 10 ps are generated in a metal transducer film by intense femtosecond laser pulses. They propagate coherently in the GaAs/(Al,Ga)As heterostructure over a distance of 100 microm and shift the band gaps by several meV as detected optically for quantum well exciton resonances by pump-probe techniques and time-resolved photoluminescence.     

Excitation of oscillations of an immiscible liquid interface by a pulsed ultrasound beam parallel to the interface

For several pairs of immiscible liquids, a new opportunity to excite oscillations of their interface by ultrasound pulses propagating parallel to the interface has been discovered experimentally. A plane ultrasound transducer is placed so that the interface between liquids halves its aperture. The evolution of the shape of the interface oscillations under the variation of the amplitude and duration of excitation pulses, as well as of the distance from the transducer, has been analyzed. The possibility of the excitation of various modes of the interface oscillations in a bounded volume has been revealed.     

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.     

The optimum bandwidth of chirp signals in ultrasonic applications

A new formula for the pulse-compression ratio achievable with linear FM pulses (chirp signals) in ultrasonic applications is derived. The calculations take into account the frequently omitted effects of the transfer function of an ultrasonic transducer. For a squared-cosine type transducer spectrum, the optimum chirp signal bandwidth is 1.14 times the 6 dB bandwidth of the transducer. The pulse-compression ratio is 2.24 dB lower than in the case of an ideal broadband transducer. Measurements show a close agreement with the presented theory.     

The transducer vibratory profile effects on the detection of the transient ultrasonic field scattered by a rigid point reflector

In this study a generalized approach, based on the impulse response technique, is developed to evaluate the transient ultrasonic field scattered by a rigid point reflector and detected by a planar circular transducer characterized by a non-uniform vibratory profile. For this purpose, several analytical functions are used to represent the non-uniform vibration amplitude of the transducer. All these functions have maximum amplitude at the transducer’s center and decrease monotonically away from axis. The amplitude variation of these functions has a direct effect on the shape of the average pressure detected by the transducer. Some numerical results are presented to illustrate the effects of a non-uniformly vibrating source on the detected transient pressure. The results show the relative importance of the edge wave modifications in the case of source having non-uniform vibration amplitude distribution.     

Transducer resolution enhancement by combining different excitation pulses

This work presents a low-cost solution to improve the axial resolution of an arbitrarily loaded and backed piezoelectric transducer. The proposed solution is based on the use of different excitation pulses and, in particular, the combination of two excitation pulses. The characteristic parameters of the second pulse, i.e. amplitude and time delay with respect to the main pulse, are obtained by studying the waveform of a reference pulse emitted or received by the transducer. This reference pulse is the transducer's response to a generic excitation pulse which has the same shape and duration as the main pulse. The work also presents a procedure to estimate the characteristic parameters of the second excitation pulse by means of the electrical simulator SPICE. Then, to illustrate the effectiveness of the simulation, the simulated and experimental values are compared through two experimental cases.     

超声检测用压电换能器瞬态特性可控可调

超声检测用压电换能器瞬态特性主要包括瞬态空间响应和瞬态时间响应。本文通过调整控制换能器和激励源,实现瞬态空间响应和瞬态时间响应的某些特性的可控可调。其中包括在空间响应方面消除边缘波以获得平行声束和消除平面波以获得聚焦绳声束;在时间响应方面,调整换能器的背衬阻抗以获得可调首次波幅比和调整换能器结构和激励电信号以获得任意检测信号等。     

Detection of nanosecond optoacoustic pulses in steel

The optoacoustic transformation of 5-ns laser pulses was used to excite broadband longitudinal acoustic pulses in a 0.4-mm-thick steel plate. An electromagnetic acoustic transducer that incorporated a flat wire coil with a diameter of 5 mm and an amplifier was proposed as a device for contactless ultrasound detection. A permanent cylindrical magnet and a coil close to the printed-circuit board of an operational amplifier were placed on one side of the sample, thus allowing the detection of the alternating electromagnetic field produced by pressure pulses excited on the opposite side. The frequency spectrum of detected pulses ranged from 5 to 200 MHz.     

Miniature piezoelectric conical transducer: fabrication, evaluation and application

This paper reports a new type miniature-conical transducer for acoustic emission measurements. The transducer follows the basic idea and structure of a conventional NBS conical transducer, but is much compact in size and easier to use. The improvements are made possible by introducing an excimer laser micromachining method for fabricating smaller PZT conical elements, which play a key role in the transducer. Conical PZT elements with contact size less then 300 microm are laser-machined and the miniature-conical transducers are constructed. Standard quantitative acoustic emission testing is performed on a plate using the fabricated transducers and good results are observed. The transducers can be very useful in many applications involving quantitative measurements of transient elastic waves.     

Photoacoustic Generation: Optical Fiber Ultrasonic Sources for Non-Destructive Evaluation and Clinical Diagnosis

An innovative photoacoustic source for ultrasonic non destructive evaluation (NDE) and clinical diagnosis is proposed. The working principle is based on thermal conversion of laser pulses into a metallic film evaporated directly onto the tip of an optical fiber. Attractive advantages of the proposed transducer are high frequency, large bandwidth and good efficiency.     

强流电子束时间分辨系统瞬态脉冲干扰的抑制

强流电子束时间分辨测量系统在直线感应加速器(LIA)环境中会受到一些短暂的高能脉冲干扰,这些瞬态脉冲干扰既针对电路又针对测量系统,这对测量系统电子设备危害很大。介绍了时间分辨测量系统的原理,分析了瞬态脉冲干扰的成因和抑制方法,给出了束参数测量系统的实验布局和特点,分析瞬态脉冲干扰对LIA中测量系统电子器件电性能的影响和变化规律,并进一步探讨电子器件电性能受瞬态脉冲干扰后的抑制措施。通过采用光纤传输控制信号的措施,能很好地传输窄脉冲,信号延时抖动小,达到了高速信号的可靠传输要求,利用紧凑嵌入式方法,提高了抗电磁干扰的能力,这样可以更好地保护束参数测量电子器件,提高了整个系统的抗干扰能力及可靠性。     

Focused, phased-array plane piston and spherically-shaped concave piston transducers: comparison for the same aperture and focal point

It has sometimes been assumed that the phased-array plane piston transducer and the spherically-shaped concave piston transducer are equivalent structures when both have the same aperture and focal point. This assumption has not been previously examined, nor has an expression for the on-axis impulse response of the focused, phased-array plane piston transducer been derived. It is shown in this paper how such an expression can be obtained. Comparisons of the impulse response for both structures show similarities, as well as some differences that could be significant as the observation point approaches the focal point. Comparisons are also performed for wide-band pulses close to the focus as well as for sinusoidal excitation. A physical explanation for the cause of the impulse response discrepancy is shown to be due to the nature of the piston focusing delay and its effect on the Rayleigh integral.