Perfect Acoustic-Vortexes Constructed by the Fourier Transform of Quasi-Bessel Beams Based on the Simplified Ring Array of Sectorial Planar Transducers.

Benefiting from the independence of the vortex radius on the topological charge (TC), the perfect acoustic-vortex (PAV) with an angular phase gradient exhibits important perspectives in acoustic applications. However, the practical implementation is still restricted by the limited accuracy and flexibility of the phase control for large-scaled source arrays. An applicable scheme of constructing PAVs by the spatial Fourier transform of quasi-Bessel AV (QB-AV) beams is developed using the simplified ring array of sectorial transducers. The principle of PAV construction is derived based on the phase modulation of the Fourier and saw-tooth lenses. Numerical simulations and experimental measurements are carried out for the ring array with the continuous and discrete phase spirals. The construction of PAVs is demonstrated by the annuli at an almost identical peak pressure with the vortex radius independent of the TC. The vortex radius is proved to increase linearly with the increase of the rear focal length and the radial wavenumber, which are determined by the curvature radii and the acoustic refractive index of the Fourier lens and the bottom angle of the saw-tooth lens, respectively. The improved PAV with a more continuous high-pressure annulus and lower concentric disturbances can be constructed by the ring array of more sectorial sources and the Fourier lens of a bigger radius. The favorable results demonstrate the feasibility of constructing PAVs by the Fourier transform of QB-AV beams and provide an implementable technology in the fields of acoustic manipulation and communication.

Quasi-Bessel Acoustic-Vortex Beams Constructed by the Line-Focused Phase Modulation for a Ring Array of Sectorial Planar Transducers.

Acoustic Bessel beams are commonly used as ideal sources to study the characteristics for acoustic-vortex (AV) beams, exhibiting prosperous perspectives in contactless object manipulations and acoustic communications. However, accurate Bessel beams are difficult to construct using 2-D arrays in practical applications. By integrating active phase control and passive phase modulation to a ring array of sectorial planar transducers, quasi-Bessel AV (QB-AV) beams of arbitrary order are built by the line focus of AV fields in the current study. Based on Snell's refraction law, a circular sawtooth lens of phase modulation is designed to converge incident waves toward the beam axis at the same deflection angle. QB-AV beams constructed by the main lobes of the sectorial sources are demonstrated by theoretical derivations, numerical simulations, and quality evaluations, while those created by the sidelobes are neglected to avoid the pressure fluctuations in the near field. Experimental measurements for AV beams of different orders coincide basically with the simulations, demonstrating that line-focused QB-AV beams can be generated along the beam axis across the pressure peak. With the increase of the topological charge, the peak pressure of the beam decreases accordingly with a reduced effective axial range. The favorable results prove that, as a special kind of diffraction sources, the adjustable QB-AV beams may enable more important biomedical applications where Bessel beams are necessary, especially for the line-focused manipulation of biological and drug particles.