Self-focusing in the ocean using out-of-band phase conjugation

Abstract Phase conjugation methods have been used in underwater acoustics because of their remarkable spatial self-focusing performance in inhomogeneous, dynamically changing environments. However, phase conjugation devices have stringent constraints such as acoustic transmitters and receivers working simultaneously in the same frequency band, which complicates the implementation of these devices and prevents their wide deployment in the ocean. Here, we introduce the out-of-band phase conjugation (OBPC) for ocean acoustics through which phase conjugation waves are generated at integer multiples of the incident wave frequency. The different bands translate into remarkably simple implementations while maintaining excellent spatial focusing in challenging shallow ocean environments. Furthermore, the resolution of OBPC is significantly higher than that of conventional phase conjugation. An interesting phenomenon typical to out-of-band phase conjugation is the generation of phantom paths along which some of the energy in the phase conjugated wave is focused.