Modulated helicity for acoustic communications and helicity-selective acoustic receivers.

Helicoidal or vortex beams have an azimuthal phase gradient and an axial null in amplitude. The sign and magnitude of the azimuthal phase gradient determine the helicity and topological charge of the beam and the associated handedness. Beams with a unit-magnitude charge have been generated with a four-sector array, adjacent quadrants being driven with a 90 deg phase offset [B. T. Hefner and P. L. Marston, J. Acoust. Soc. Am. 106, 3313–3316 (1999)]. The present work shows that the helicity can be rapidly modulated with an appropriately timed reversal of the excitation to a single pair of diagonal sectors. This reversal was demonstrated with an electronic commutator that inverts (at a zero crossing of the sine-wave excitation) two of the channels prior to amplification. While the wave helicity may be inferred with processed signals from a detection array, it may also detected with simple superposition of outputs from a four-element detector using appropriately offset elements. Digital communications was demonstrated where signals of the wrong helicity were suppressed. Scattering by symmetric objects on the axis of the beam preserves the helicity of the radiation [P. L. Marston, J. Acoust. Soc. Am. 124, 2905–2910 (2008)]. [Work partially supported by ONR.]