Pulse shaper assisted short laser pulse characterization

We demonstrate that a pulse shaper is able to simultaneously act as an optical waveform generator and a short pulse characterization device when combined with an appropriate nonlinear element. We present autocorrelation measurements and their frequency resolved counterparts. We show that control over the carrier envelope phase allows continuous tuning between an intensity-like and an interferometric autocorrelation. By changing the transfer function other measurement techniques, for example STRUT, are easily realized without any modification of the optical setup. PACS 42.65.Re; 42.30.Lr; 42.30.Rx     

Reflector-based phase calibration of ultrasound transducers

Recently, the measurement of phase transfer functions (PTFs) of piezoelectric transducers has received more attention. These PTFs are useful for e.g. coding and interference based imaging methods, and ultrasound contrast microbubble research. Several optical and acoustic methods to measure a transducer’s PTF have been reported in literature. The optical methods require a setup to which not all ultrasound laboratories have access to. The acoustic methods require accurate distance and acoustic wave speed measurements. A small error in these leads to a large error in phase, e.g. an accuracy of 0.1% on an axial distance of 10 cm leads to an uncertainty in the PTF measurement of ±97° at 4 MHz. In this paper we present an acoustic pulse-echo method to measure the PTF of a transducer, which is based on linear wave propagation and only requires an estimate of the wave travel distance and the acoustic wave speed. In our method the transducer is excited by a monofrequency sine burst with a rectangular envelope. The transducer initially vibrates at resonance (transient regime) prior to the forcing frequency response (steady state regime). The PTF value of the system is the difference between the phases deduced from the transient and the steady state regimes. Good agreement, to within 7°, was obtained between KLM simulations and measurements on two transducers in a 1-8 MHz frequency range. The reproducibility of the method was ±10°, with a systematic error of 2° at 1 MHz increasing to 16° at 8 MHz. This work demonstrates that the PTF of a transducer can be measured in a simple laboratory setting.     

Piezoelectrically actuated flextensional micromachined ultrasound transducers

This paper presents novel micromachined two-dimensional array piezoelectrically actuated flextensional transducers that can be used to generate sound in air or water. Micromachining techniques to fabricate these devices are also presented. Individual unimorph array elements consist of a thin piezoelectric annular disk and a thin, fully clamped, circular plate. We manufacture the transducer in two-dimensional arrays using planar silicon micromachining and demonstrate ultrasound transmission in air at 2.85 MHz with 0.15 microm/V peak displacement. The devices have a range of operating resonance frequencies starting from 450 kHz up to 4.5 MHz. Such an array could be combined with on-board driving and addressing circuitry for different applications.     

Piezoelectric multilayer transducers for ultrasonic pulse compression

This paper describes piezoelectric multilayer transducers for application in ultrasonic pulse compression systems. The transducers are constructed in such a way as to produce binary pulse sequences, in particular Barker-coded sequences. This is achieved by the polarization pattern of the active layers. Pulse compression is effected without any electronic circuitry by using a receiver transducer with a pattern corresponding to that of the transmitter.     

Step excitation source for ultrasonic pulse transducers

A circuit is described for exciting ultrasonic transducers by step discharge through avalanching transistors. While a free-running mode is possible, the circuit is designed for synchronous operation at 1 and 10 kHz repetition rates. During the receive mode the transducer is decoupled from the power source. The output is obtained from a buffer amplifier to match a 50 Ω coaxial cable. The circuit described produces a 250 V step discharge into a 24 pf load in 10 ns.     

Conservation laws of the complex short pulse equation and coupled complex short pulse equations

In this paper, the complex short pulse equation and the coupled complex short pulse equations that can describe the ultra-short pulse propagation in optical fibers are investigated. The two complex nonlinear models are turned into multi-component real models by proper transformations. Lie symmetries are obtained via the classical Lie group method, and the results for the coupled complex short pulse equations contain the existing results as particular cases. Based on the linearizing operator and adjoint linearizing operator for the two real systems, adjoint symmetries can be obtained. Explicit conservation laws are constructed using the symmetry/adjoint symmetry pair (SA) method. Relationships between the nonlinear self-adjointness method and the SA method are investigated.     

Acoustic impedance matching of medical ultrasound transducers

Ultrasonic transducers for pulse-echo systems are studied both theoretically and experimentally. For the theoretical calculations the Mason model for thickness-mode disc transducers with and without backing and matching layers is used. By building several of the theoretically investigated transducer configurations it is shown that theory and experiment agree well. Thus the properties of a transducer can be predicted to a good approximation before its experimental realization. To find transducers with good sensitivity and short pulses, the pulse shape and frequency response for the following classes of transducers were studied both theoretically and experimentally: transducers with backing only, transducers with heavy backing and front matching layers, and air-backed transducers with front matching layers.     

超声内镜换能器的应用进展

超声内镜集结了超声检查与内镜检查双重功能,可以获得腹部和胸腔内器官的高质量图像,是一种先进的医疗设备。超声内镜探头的核心部分是超声换能器。超声内镜检查需经过狭窄的消化道或内窥镜的活检通道伸进体内,由于工作环境的限制,超声内镜用换能器与普通超声成像换能器相比,工作频率更高、尺寸更小、制作工艺更精密。本文从超声内镜所用换能器的外形结构、内部组成、工作模式及材料等角度,对目前国内外超声内镜换能器的应用进展情况进行了描述,并根据超声内镜换能器的现状对未来的发展趋势进行了分析。     

Dual-mode transducers for ultrasound imaging and thermal therapy

Medical imaging is a vital component of high intensity focused ultrasound (HIFU) therapy, which is gaining clinical acceptance for tissue ablation and cancer therapy. Imaging is necessary to plan and guide the application of therapeutic ultrasound, and to monitor the effects it induces in tissue. Because they can transmit high intensity continuous wave ultrasound for treatment and pulsed ultrasound for imaging, dual-mode transducers aim to improve the guidance and monitoring stages. Their primary advantage is implicit registration between the imaging and treatment axes, and so they can help ensure before treatment that the therapeutic beam is correctly aligned with the planned treatment volume. During treatment, imaging signals can be processed in real-time to assess acoustic properties of the tissue that are related to thermal ablation. Piezocomposite materials are favorable for dual-mode transducers because of their improved bandwidth, which in turn improves imaging performance while maintaining high efficiency for treatment. Here we present our experiences with three dual-mode transducers for interstitial applications. The first was an 11-MHz monoelement designed for use in the bile duct. It had a aperture that was cylindrically focused to 10 mm. The applicator motion was step-wise rotational for imaging and therapy over a 360°, or smaller, sector. The second transducer had 5-elements, each measuring for a total aperture of . It operated at 5.6 MHz, was cylindrically focused to 14 mm, and was integrated with a servo-controlled oscillating probe designed for sector imaging and directive therapy in the liver. The last transducer was a 5-MHz, 64-element linear array designed for beam-formed imaging and therapy. The aperture was with a pitch of 0.280 mm. Characterization results included conversion efficiencies above 50%, pulse-echo bandwidths above 50%, surface intensities up to , and axial imaging resolutions to 0.2 mm. The second transducer was evaluated in vivo using porcine liver, where coagulation necrosis was induced up to a depth of 20 mm in 120 s. B-mode and M-mode images displayed a hypoechoic region that agreed well with lesion depth observed by gross histology. These feasibility studies demonstrate that the dual-mode transducers had imaging performance that was sufficient to aid the guidance and monitoring of treatment, and could sustain high intensities to induce coagulation necrosis in vivo.     

The bi-Hamiltonian structure of the short pulse equation

We prove the integrability of the short pulse equation derived recently by Schäfer and Wayne from a Hamiltonian point of view. We give its bi-Hamiltonian structure and show how the recursion operator defined by the Hamiltonian operators is connected with the one obtained by Sakovich and Sakovich. An alternative zero-curvature formulation is also given.     

The characterization of capacitive micromachined ultrasonic transducers in air

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. They exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz, when driven from a standard ultrasonic voltage source. Experiments are described using 1 mm square devices in air, operating in both pitch-catch and pulse-echo modes. The dependence on d.c. bias voltage is examined, together with calibration measurements using 1/8 in. microphones. The radiated beam profile, and the farfield directivity pattern, have been measured for both broad bandwidth and one-burst excitation, using a scanned miniature receiver. A 16 element square array is also presented, which has been used to measure the beam cross-sections from a focussed source.     

A programmable pulse generator for piezoelectric multielement transducers

A pulse generator for multielement ultrasonic transducers has been developed with digital control of pulse timing and firing. It can optimize matching to the probe characteristics, and can consequently maximize the amplitude of the received echo.     

Generation and reception of short acoustic pulses by multilayer piezoelectric transducers

By means of the Fourier transformation, a formula is obtained for calculating the pulse response of a transducer when a square pulse of a given duration is supplied to its input and the excited “sound” is reflected from the free flat end of the acoustic line and returns to the transducer. For a frequency of 100 MHz, real lithium niobate transducers that are connected through intermediate layers to the acoustic line made of fused quartz and to a rear load with a given acoustic impedance are considered. The pulse response is calculated for the transducers with different values of the sublayer thickness, rear acoustic load, transmission line impedance, and original pulse duration. The results of the calculations are compared with the experiment.     

小型短电磁脉冲传感器

对一种短偶极子类小型短电磁脉冲传感器进行了研究,分析了传感器电容、有效面积与结构参数之间的解析关系,给出了传感器的工作原理。根据应用需求设计了不同带宽的该类传感器,并对其波形保真性和有效面积进行了数值分析,仿真结果表明：经传感器接收并还原所得场波形与激励电场波形几乎完全重合,传感器有效面积与理论分析结果偏差小于1%。对传感器的焊接、定位和对轴等制作工艺进行了研究,完成了传感器样品的制作。利用单锥TEM室对传感器的波形保真性和有效面积进行了实验测试,测试结果表明：设计制作的传感器样品可以很好地恢复待测脉冲电场的波形和幅度,两支传感器样品的有效面积实测结果与解析计算结果较为一致,偏差分别小于4%和7%。     

Simulation of circular array ultrasound transducers for intravascular applications

The beam shape of a circular array transducer that is commonly used in intravascular ultrasound catheters was investigated in linear mode of operation. For this use, a simulation program which can simulate the radio frequency (rf)-response of a number of scatterers has been developed. The program is based on the impulse response method, which is implemented in the frequency domain. Due to the unusual geometry of the transducer, the far field gets peculiarly shaped for large apertures. Instead of having a far field with its maximum intensity in a single lobe on the acoustical axis, the far field splits up into a dual-lobe far field with maximum intensity in two lobes off the acoustical axis. A formula is derived that predicts the occurrence of these beam shapes.     

Generalizations of the short pulse equation

We classify integrable scalar polynomial partial differential equations of second order generalizing the short pulse equation.     

特形脉冲的设计与计算机模拟

根据LiouvillevonNeumann方程从理论上对特形脉冲做了全面的描述,提出了一种具体的调幅特形脉冲设计方案:首先将待设计的脉冲展成一个有限Fourier级数,然后根据Bloch方程的解析解准确计算出各阶正弦、余弦波的频谱,再将这些频谱组合后与该脉冲的理想频谱进行比较构成误差函数,最后运用鲍威尔-模拟退火组合优化算法计算出全局最优Fourier系数,即可得到所需脉冲的表达式.应用此设计方案,得到了体系处于热平衡态时的特形激励脉冲和反转脉冲的具体表达式.计算机模拟表明,所得脉冲的频谱具有较好的选择性 关键词： 核磁共振 特形脉冲 Bloch方程 鲍威尔模拟退火组合优化算法     

Modeling of nonlinear ultrasound propagation in tissue from array transducers

A computationally efficient model capable of simulating finite-amplitude ultrasound beam propagation in water and in tissue from phased linear arrays and other transducers of arbitrary quasiplanar geometry is described. It is based on a second-order operator splitting approach [Tavakkoli et al., J. Acoust. Soc. Am. 104, 2061-2072 (1998)], with a fractional step-marching scheme, whereby the effects of diffraction, attenuation, and nonlinearity can be computed independently over incremental steps. This approach is an extension to that of Christopher and Parker [J. Acoust. Soc. Am. 90, 507-521; 90, 488-499 (1991)], wherein linear and nonlinear effects are propagated separately over incremental steps, and the computation of the diffractive substeps are based on an angular spectrum technique with a modified sampling scheme for accurate and efficient implementation of diffractive propagation from nonradially symmetric sources. Results of the model are compared with published data. Predicted field profiles for nonlinear propagation in tissue from realistic array transducers using the pulse inversion method are presented.