Multiplexed parametric vortex beams for underwater communications.

Underwater acoustic communication is constrained by limited bandwidth and large aperture requirements at low frequencies. Parametric acoustic arrays offer a means to generate highly directive low-frequency fields from compact sources, while acoustic vortex beams carrying orbital angular momentum can encode digital information in their phase structure, quantified as their topological charge (TC). In this work, a three-dimensional mixed-domain split-step solver of the Westervelt equation is developed to model the simultaneous generation and propagation of multiple TCs via self-demodulation of vortex-bearing primary waves. Digital information is encoded using 8-bit ASCII mappings to TC channels and recovered using two metrics. Simulated results demonstrate successful recovery of six representative ASCII characters over meter-scale propagation distances from a ∼40 cm circular aperture in homogeneous water at 4 kHz with a carrier frequency of 40 kHz. A demultiplexing metric based on the Fourier series is introduced that captures the relative strength of constituent TCs as a function of distance from the central axis, informing the choice of amplitude weighting to improve detectability of higher-order TCs.