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Nonlinear echoes, phase conjugation, time reversal, and electronic holography

In this paper we review the experiments on nonlinear echo phenomena during the last three decades, from spin echoes to echoes in piezoelectric powders. We show how the common principle is one of a physical Fourier transform space in which time reversal is brought about through phase conjugation. It will be seen how this leads to intriguing applications in signal processing, signal storage, and electronic holography.