Stability of time reversed waves in changing media

TL;DR

This paper investigates the stability and quality of time-reversed wave refocusing in media with changing properties, demonstrating statistical stability and identifying effects that cause de-focusing when media decorrelate.

Contribution

It provides a rigorous analysis of the high frequency limit of time-reversed waves in different media, revealing conditions for stability and effects leading to de-focusing.

Findings

Refocused signals are statistically stable and independent of media realizations.

Media decorrelation causes de-focusing through absorption and phase modulation effects.

The cross correlation of media influences the refocusing quality.

Abstract

We analyze the refocusing properties of time reversed waves that propagate in two different media during the forward and backward stages of a time-reversal experiment. We consider two regimes of wave propagation modeled by the paraxial wave equation with a smooth random refraction coefficient and the It\^o-Schr\"odinger equation, respectively. In both regimes, we rigorously characterize the refocused signal in the high frequency limit and show that it is statistically stable, that is, independent of the realizations of the two media. The analysis is based on a characterization of the high frequency limit of the Wigner transform of two fields propagating in different media. The refocusing quality of the back-propagated signal is determined by the cross correlation of the two media. When the two media decorrelate, two distinct de-focusing effects are observed. The first one is a purely absorbing effect due to the loss of coherence at a fixed frequency. The second one is a phase modulation effect of the refocused signal at each frequency. This causes de-focusing of the back-propagated signal in the time domain.