An ultrasonic transducer with an electrode pattern in the shape of a Fresnel zone plate (FZP) is useful in acoustic imaging. However a major problem in focusing with an FZP is that the final image will have a large zero-order component and this will restrict the dynamic range of the system. One way to solve this problem is to use a Fresnel phase plate pattern (FPP).

Fresnel Zone Plate and Fresnel Phase Plate Patterns for Acoustic Transducers

Piezoelectric transducers with an electrode pattern in the form of a Fresnel zone plate (FZP) on the air side and a full, planar electrode on the water side have been used in many acoustical imaging systems to focus the acoustic beam. The goal is to get an acoustic pressure pattern in the shape of an FZP. The acoustic pressure pattern, however, is never an exact replica of the FZP electrode pattern because of mechanical aberrations introduced by the piezo-electric plate. In a previous paper we derived the focal-plane acoustic field distribution of such a transducer. The distribution just in front of the transducer, however, was not given in that paper. In this paper we use a linear spatial piezo-electric model to construct a spatial transfer function for the piezoelectric plate. We show that elastic-wave generation and propagation inside the piezoelectric plate cause the acoustic field pattern just in front of the transducer to differ substantially from the electrode pattern. A plot of amplitude versus position on the piezoelectric plate shows that the sharp edges associated with the electrode pattern are severely rounded off by these effects and the amplitude is greatly reduced in the region of the outer zones. The aberrations also cause a vari-ation with the position of the phase of the out-put field.© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Near Field Acoustic Pattern Of Fresnel Zone Plate Transducers

A prototype of an ultrasonic transducer has been developed that uses an optical input to control the ultrasonic output. This transducer is called an optoacoustic transducer (OAT) and provides an ultrasonic pattern that is spatially similar to the optical pattern used to illuminate it. When a focus-inducing optical pattern, such as a zone plate, is used, an acoustic focus is achieved. The success of this procedure depends on the use of amplitude-modulated light at the input and on filtering the received signal to eliminate the primary frequency. This provides an increase, from 30 dB to 70 dB, in the ratio of acoustic pressure in the illuminated regions to that in the dark regions. The prototype operates at 2.8 MHz and has been used to provide a good acoustic focus in water. A 3-dB beamwidth of 3.5 mm was measured at a range of 92 mm. The construction techniques and materials used are discussed. >

Focusing with an optoacoustic transducer

Ultrasonic imaging transducers have been proposed in which a piezoelectric plate is used in conjunction with a Fresnel Zone Plate (FZP) to produce a focused beam of ultrasound. Focusing is achieved with these devices by activating the piezoelectric plate with an electrode pattern in the shape of an FZP. The focal plane distribution of these transducers cannot, in general, be predicted by diffraction theory alone. This is because the acoustic field pattern in front of the transducer is not an exact replica of the FZP electrode pattern. In this paper we show that elastic wave generation and propagation inside the piezoelectric plate cause the acoustic field pattern directly in front of the plate to differ from the electrode pattern. These effects are called mechanical aberrations. We show that mechanical aberrations cause the focal plane distribution to be broader than that predicted by diffraction theory. The effects on resolution of material properties and other device parameters are discussed. Experimental results are also presented.

M. Mortezaie

Papers

Effect of Mechanical Aberrations on the Resolution of Fresnel Zone Plate Transducers