Showing papers on "Bessel beam published in 1990"

Ultrasonic nondiffracting transducer for medical imaging

Abstract: The nondiffracting J/sub 0/ Bessel beam is evaluated, and its application to medical imaging is suggested. Computer simulations and experimental results for a ten-ring annular Bessel shaded transducer are described. Both continuous-wave (CW) and pulse-wave (PW) excitations are shown and compared to conventional Gaussian beams. The nondiffracting beam has about 1.27-nm radius main lobe with a 20-cm depth of field compared to the Gaussian transducer of the same size with a 1.27-mm radius main lobe at a focus of 12 cm and 2*4-cm depth of field. The side lobes of the nondiffracting beam are the same as the J/sub 0/ Bessel function. The effects of heterogeneity due to tissue on the nondiffracting beam and on the focused Gaussian beam are also reported. >

Bessel beam radar system using sequential spatial modulation

Abstract: A spatially modulated Bessel beam radar system for enhancing the resolution with which a range and an azimuth of a plurality of closely spaced targets is determined. In a Bessel beam radar system, a Bessel beam is generated by sequential spatial modulation of the radar signal while maintaining a constant spatial polarization, and the return signal from one or more targets is processed to determine its Bessel function content. To spatially modulate the radar beam, the point at which the radar signal is transmitted is moved around a circular orbit. In a first embodiment of the spatially modulated Bessel beam radar system (80), a radar dome (86) mounted on the distal end of a mast (84) is pivoted around an orbit (90). The radar signal is transmitted in a predefined direction, along a Poynting vector that is generally aligned with the plane of the orbit. In a second embodiment (110), a plurality of parabolic antennas (116) are arranged in a spaced-apart circular array around a common center. The radar signal is sequentially spatially modulated as it is transmitted from each of the parabolic antennas in sequence around the circular array, and the Poynting vector of the spatially modulated Bessel beam radar signal is generally transverse to a plane in which the parabolic antennas are disposed. The signal reflected back from plural targets comprises a complex phase history. To determine a range and azimuth for each target, a controller/processor (180), processes this signal to develop closed form Bessel functions from which target azimuth and range are determined. Alternatively, target azimuth is determined from a convolution of the complex phase history using a dot product detector (202).

Ultrasonic Beams with Bessel and Gaussian Profiles

Abstract: A novel technique has been developed for generating ultrasonic beams with spatial profiles of amplitude governed by a truncated Bessel function or Gaussian function [1,2]. Bessel beams have very unique properties; in optics Bessel beams have been shown to be diffractionless (J. Durnin et al, 1987 [3,4]). In a related work, R. W. Ziolkowski et al [5] reported experimental measurements of “acoustic directed energy pulse trains” generated by synthetic line array of ultrasonic transmitters in water. However, a Bessel function ultrasonic transducer has never been reported before. Gaussian beams also have desirable properties; they are very easy to model analytically, and a circular Gaussian function ultrasonic transducer is free of near-field nulls and far-field sidelobes associated with conventional “piston source” transducers [6]. At least three designs of Gaussian transducers have been reported in the literature in the past 30 years [7–9]. We report a method in which piezoelectric ceramic elements are poled with nonuniform electric fields shaped like Bessel or Gaussian functions such that the resulting polarization (and hence the ultrasonic amplitude) follows that of the applied poling field. Like conventional piston source transducers, such Bessel or Gaussian transducers also possess the simple “parallel plate capacitor” configuration and can be packaged likewise. Beam profiles and propagation behavior of these Bessel and Gaussian transducers have been measured experimentally in an immersion tank and the results compared well with model predictions

Critique of nondiffracting beams

Abstract: : A number of researchers have discussed the possibility of generating electromagnetic beams or pulses which can propagate without the usual degree of transverse spreading. Nondiffracting directed radiation beams have been the subject of a number of special sessions at various conferences. Our intention in this note is to discuss i) the Bessel beam which has been called 'remarkably resistant to the diffractive spreading commonly associated with all wave propagation'; and ii) the electromagnetic directed energy pulse train which is claimed to be 'significantly improved over conventional, diffraction-limited beams', and to 'defeat diffraction'. This note shows that diffraction is not eliminated or reduced in any of the proposed schemes and that conventional Gaussian beams will propagate at least as far for a given transmitting antenna dimension.