Long-distance underwater communications via parametric array vortex beams

Previous works have demonstrated the feasibility of utilizing the topological charge (TC) of vortex beams as an additional degree of freedom to convey information and thereby improve the data transmission rate of underwater communications. However, linear vortex beams suffer from wide diffraction angles and rapid attenuation, reducing their practical use for secure long-distance communications. We present the advantages of combining parametric array technology with encoding information within the TC, namely narrow diffraction angles and slower attenuation. Simulations were conducted by solving the Westervelt equation using a split-step approach. Sample letters were encoded into particular TC values of the nonlinearly demodulated difference-frequency wave using standard ASCII codes. Two metrics to evaluate the presence and relative strength of various TCs at various distances from the source were implemented in order to receive the encoded letters. One such metric based on the Fourier series, coupled with weighting the amplitudes of higher-order TC’s at the source, successfully demonstrates the feasibility of transmitting and receiving encoded information using parametric array vortex beams.