Online evolution of a phased array for ultrasonic imaging by a novel adaptive data acquisition method.

Ultrasonic imaging, using ultrasonic phased arrays, has an enormous impact in science, medicine and society and is a widely used modality in many application fields. The maximum amount of information which can be captured by an array is provided by the data acquisition method capturing the complete data set of signals from all possible combinations of ultrasonic generation and detection elements of a dense array. However, capturing this complete data set requires long data acquisition time, large number of array elements and transmit channels and produces a large volume of data. All these reasons make such data acquisition unfeasible due to the existing phased array technology or non-applicable to cases requiring fast measurement time. This paper introduces the concept of an adaptive data acquisition process, the Selective Matrix Capture (SMC), which can adapt, dynamically, to specific imaging requirements for efficient ultrasonic imaging. SMC is realised experimentally using Laser Induced Phased Arrays (LIPAs), that use lasers to generate and detect ultrasound. The flexibility and reconfigurability of LIPAs enable the evolution of the array configuration, on-the-fly. The SMC methodology consists of two stages: a stage for detecting and localising regions of interest, by means of iteratively synthesising a sparse array, and a second stage for array optimisation to the region of interest. The delay-and-sum is used as the imaging algorithm and the experimental results are compared to images produced using the complete generation-detection data set. It is shown that SMC, without a priori knowledge of the test sample, is able to achieve comparable results, while preforming ∼ 10 times faster data acquisition and achieving ∼ 10 times reduction in data size.

Laser induced ultrasonic phased array using full matrix capture data acquisition and total focusing method.

Laser ultrasonics is a technique where lasers are employed to generate and detect ultrasound. A data collection method (full matrix capture) and a post processing imaging algorithm, the total focusing method, both developed for ultrasonic arrays, are modified and used in order to enhance the capabilities of laser ultrasonics for nondestructive testing by improving defect detectability and increasing spatial resolution. In this way, a laser induced ultrasonic phased array is synthesized. A model is developed and compared with experimental results from aluminum samples with side drilled holes and slots at depths of 5 - 20 mm from the surface.

Measurement of material nonlinearity using surface acoustic wave parametric interaction and laser ultrasonics.

A dual frequency mixing technique has been developed for measuring velocity changes caused by material nonlinearity. The technique is based on the parametric interaction between two surface acoustic waves (SAWs): The low frequency pump SAW generated by a transducer and the high frequency probe SAW generated and detected using laser ultrasonics. The pump SAW stresses the material under the probe SAW. The stress (typically <5 MPa) is controlled by varying the timing between the pump and probe waves. The nonlinear interaction is measured as a phase modulation of the probe SAW and equated to a velocity change. The velocity-stress relationship is used as a measure of material nonlinearity. Experiments were conducted to observe the pump-probe interaction by changing the pump frequency and compare the nonlinear response of aluminum and fused silica. Experiments showed these two materials had opposite nonlinear responses, consistent with previously published data. The technique could be applied to life-time predictions of engineered components by measuring changes in nonlinear response caused by fatigue.

A laser-activated MEMS transducer for efficient generation of narrowband longitudinal ultrasonic waves.

In this paper, we demonstrate an optically powered microelectromechanical system (MEMS) transducer. It was designed and fabricated using MEMS techniques, and can generate narrowband ultrasonic bulk waves from a broadband laser excitation pulse with high efficiency. The transducer is a two-mask-level MEMS device with a microdisk seated on a microstem. When a laser pulse is incident on the disk center, a resonant flapping motion of the disk is actuated because of the thermomechanical interaction between the absorbing and non-absorbing parts of the disk, coupling a narrowband longitudinal bulk wave propagating along the axis of the stem into the sample. Finite element (FE) methods were used to simulate the generated ultrasound; the results agree well with experimental measurements. Experiments with the fabricated transducers have shown that narrowband ultrasound with a high SNR/amplitude was generated successfully; compared with normal thermoelastic generation, ultrasound with at least 5 times higher amplitude can be achieved by an optimized MEMS transducer. The transducer is inexpensive, compact, and simple to use.

Design and experimental study of microcantilever ultrasonic detection transducers.

This paper presents the analysis, design, and experimental study of a microcantilever optically-activated ultrasonic detection transducer. An analytical model was derived using 1-D cantilever structural dynamics, leading to the optimization of the transducer design. Finite element modeling enabled dynamic simulation to be performed, with results in good agreement with the analytical model. Transducers were fabricated using MEMS (microelectromechanical systems) techniques. Experimental results are presented on remote noncontact detection of ultrasound using the fabricated transducers; high SNR is achieved for the detected signals, even for relatively low ultrasonic amplitudes. Both analysis and experimental study show that the transducer has a sensitivity approximately 1 to 2 orders of magnitude higher than that of conventional optical detection techniques. Furthermore, we show that the dominant factor in the increased sensitivity of the transducer is the resonant nature of the finger structure.