Focusing Waves Through a Randomly Scattering Medium in the White-Noise Paraxial Regime

TL;DR

This paper investigates wave focusing through complex media using time reversal in the white-noise paraxial regime, analyzing resolution, signal-to-noise ratio, and selectivity with practical constraints.

Contribution

It provides sharp theoretical results on wave focusing, resolution, and imaging capabilities in a realistic setting with limited aperture and finite elements.

Findings

Focusing resolution depends on physical parameters and aperture size.

Signal-to-noise ratio can be optimized for better focusing.

Selective focusing enables imaging near the target point.

Abstract

When waves propagate through a complex or heterogeneous medium the wave field is corrupted by the heterogeneities. Such corruption limits the performance of imaging or communication schemes. One may then ask the question: is there an optimal way of encoding a signal so as to counteract the corruption by the medium? In the ideal situation the answer is given by time reversal: for a given target or focusing point, in a first step let the target emit a signal and then record the signal transmitted to the source antenna, time reverse this and use it as the source trace at the source antenna in a second step. This source will give a sharply focused wave at the target location if the source aperture is large enough. Here we address this scheme in the more practical situation with a limited aperture, time-harmonic signal, and finite-sized elements in the source array. Central questions are then the focusing resolution and signal-to-noise ratio at the target, their dependence on the physical parameters, and the capacity to focus selectively in the neighborhood of the target point and therefore to transmit images. Sharp results are presented for these questions.