Generation of a nondiffracting beam with frequency- independent beamwidth

Abstract

A method is presented for generating a beam with a beam shape that is independent of distance and frequency over large axial and frequency ranges. The half-width of the beam can be less than a wavelength at the low end of the frequency range and can hold this narrow waist for hundreds of wavelengths, giving a large depth of field. The phase fronts of this beam are planar and perpendicular to the beam axis over the depth of field. By virtue of the flat phase fronts and the frequency independence of the beam, a pulsed beam with flat pulse fronts and an unchanging time response can be created. This beam is shown to share some of the properties of an axicon. A numerical simulation is done to demonstrate the feasibility of generating such a beam with an annular array.

A new method for estimation of velocity vectors

“…In practice the beams of interest are only diffraction-free for a limited range because of the finite aperture used [17]. It has been shown [18], that the fields generated by axicon transducers are similar to the nondiffracting beams proposed by Durnin [16]. The axicon focusing, originally presented by McLeod [19], uses a linearly increasing time delay function from the edge to the center of the aperture.…”

Effect of decentering ratio of elements in a variable radiation direction sound source on beam profile

“…A nondiffraction optical beam [1,2] may be useful for the above purposes. Therefore, the creation of such a beam in acoustics has been actively pursued using various methods [3][4][5][6][7][8].…”

Ultrasonic transcutaneous energy transfer using a continuous wave 650kHz Gaussian shaded transmitter

“…From the perspective of power transfer, while meeting the safety requirements it is better not to generate an intense main pressure lobe which is mandatory for imaging purposes or focused ultrasound applications, instead it is better to spread the power flow over the cross sectional area as much as possible and by doing this it is possible to keep the intensity under the safe limit of 94 mW/cm 2 [12,13]. Although Gaussian beam diverges after the Rayleigh distance compared to a Bessel non-diffracting beam [9,19], the measured pressure which decreased along the acoustic axis, as illustrated in Fig. 15, shows that the axial pressure distribution is almost uniform (to within ±1.9 dB).…”

“…Durnin and Miceli [10] showed (for an optical signal) that excitation in the shape of zero order Bessel function of the 1st kind, J 0 of an infinite aperture transducer, results in an Axicon which is a non-diffracting propagating wave. A practical implementation of the Bessel beam consists of a transducer having a finite aperture size that proves also to be effective [9,11]. Unfortunately, for continuous power transfer through a living tissue, the Bessel beam has two disadvantages.…”

“…Unfortunately, for continuous power transfer through a living tissue, the Bessel beam has two disadvantages. It generates a pressure field that consists of a main lobe concentrated along the acoustic axis, and several (<10 depends on the accuracy of the Bessel stepwise approximation) lower amplitude (À0.4, +0.3, À0.25, +Á Á Á, normalized to the amplitude of the main lobe) side lobes [7] that travel in parallel with the main lobe [9,19]. Side lobes tend to decrease the amount of energy harvested by the receiver as it has an alternating pressure polarity.…”