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Fresnel Zone Plates are planar lenses that can be used to focus ultrasound beams. This kind of acoustic lenses can play a key role in the resolution of ultrasonic NDT systems. In this type of pulse-echo applications, the pulse duration is an important parameter that specifies the axial resolution, and thus, shorter ultrasound pulses provide higher resolutions. However, acoustic lenses exhibit a transient response that should be considered when setting the pulse duration, as pulses shorter than the transient state duration result in degradation in the response of acoustic lenses in terms of focal intensity, focal displacement, and lateral and axial resolutions. In this work, a thorough analysis of the transient response of Fresnel Zone Plates is discussed, demonstrating that the transient state should be considered in order to achieve optimal focusing performance. Theoretical and numerical results are presented, showing very good agreement.

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Abstract. Non-destructive ultrasonic testing of composite materials requires imaging algorithms which could be applied for anisotropic structures. At present, time-domain techniques are used for this purpose. However, it is evident that this method has lower computational speed compared with frequency-domain algorithms. This paper discusses advanced approach off frequency-domain techniques for multilayer composite materials inspection. The proposed method is based on multi-layer omega-k (MULOK) ultrasound frequency-domain imaging algorithms. It is shown that MULOK algorithms enable highly accurate reconstruction within a higher computational speed.