Broadband wave packet dynamics of minimally diffractive ultrasonic fields from axicon and stepped fraxicon lenses.

A fraxicon is a stepped phase plate lens designed to be a discrete approximation of the axicon, a refractive cone-shaped lens. Both lenses generate minimally diffractive Bessel beams with long depths of focus. Here the characteristics of broadband pulses modified by these lenses to compare and contrast the dynamics of the propagating wave packets were examined. Pulses from a spherically focused Fresnel lens are also examined to provide the context of a conventional design. The wave packets generated through the fraxicon exhibit many of the same characteristics of those from the axicon such as lateral compactness and axial integrity, although the fraxicon packets do exhibit noticeable dispersion in comparison. Both the fraxicon and axicon wave packets have a much tighter lateral extent than those of the Fresnel throughout the propagation region. The most significant difference between the fraxicon and axicon is with the group speeds of the packets with the fraxicon group subsonic and the axicon supersonic across a 50 mm path. Supplemental movies are provided for direct visualization of the propagation (for movies of the waveforms and energy profiles of the wave packet propagation that was experimentally obtained). To assess the conformity of the low profile lenses to their design parameters, frequency domain comparisons of measurements with simulations are also presented and are in good agreement.