GENERATION OF A NONDIFFRACTING BEAM WITH FREQUENCY-INDEPENDENT BEAMWIDTH

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. © 1990, Acoustical Society of America. All rights reserved.