A time-reversal mirror with variable range focusing

Recently a time-reversal mirror (or phase-conjugate array) was demonstrated experimentally in the ocean that spatially and temporally refocused an incident acoustic field back to the original position of the probe source [Kuperman et al., J. Acoust. Soc. Am. 103, 25–40 (1998)]. 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 [Brekhovskikh and Lysanov, Fundamentals of Ocean Acoustics, 2nd ed. (Springer-Verlag, Berlin, 1991), pp. 139–145]. Simulations are combined with experimental data to verify this technique.