Pressure and phase characteristics of orbital angular momentum multiplexed perfect acoustic-vortexes

Profiting from the fixed vortex-radius, perfect acoustic-vortex (PAV) beams can improve the efficiency of orbital angular momentum (OAM) communication. While, characteristics of OAM-multiplexed PAVs with more than two topological charges (TCs) have not been studied. By applying the spatial Fourier transform to Bessel beams, the principle of constructing OAM-multiplexed PAVs is developed. The characteristics of OAM-multiplexed PAVs are investigated by theoretical analyses and numerical simulations, and also demonstrated by the experimental measurement based on quasi-Bessel beams constructed by the saw-tooth phase modulation for the ring-array of sectorial transducers. Numerical and experimental studies demonstrate that the high-pressure annuli are segmented to lobes, with the lobe number equal to the difference of the maximum and minimum TCs. Circular lobe distributions of pressure and phase only rely on TC differences, which may be identical for different TC combinations. The peak-pressure of the primary lobe centered at phi = 0 increases as the increase of the included TC number, with the angular phase slope equal to the arithmetic mean of included TCs. The excellent characteristics of the TC-independent vortex-radius and the remarkable lobe distributions of pressure and phase may guide the choice of TCs for encoding and decoding, and advance a fast, effective and low-cost method for underwater OAM communication.