Variable range focusing in a time-reversal mirror

Abstract

Recently, a time-reversal mirror (or phase conjugate array) was demonstrated experimentally in the ocean, which spatially and temporally re-focused an incident acoustic field back to the original position of the probe source. [Kuperman et al., accepted for publication in J. Acoust. Soc. Am.]. Here, this waveguide time-reversal mirror technique is extended to refocus at ranges other than that of the probe source. This procedure is based on the acoustic-field invariant property in the coordinates of frequency and range in an oceanic waveguide [G. A. Grachev, Acoust. Phys. 39(1), 33–35 (1993)]. This technique has been verified in a second phase conjugate experiment conducted in the Mediterranean Sea in May 1997. The theory behind variable range focusing will be presented along with experimental results demonstrating the ability to shift the focal range ±1 km for a probe source in the vicinity of 15 km from the source–receive array.