Compression of the Wave Power Density by Means of a Time-Varying Medium

Abstract

Propagation of waves in a medium homogeneous in space but with time-varying properties is considered. It is shown that this can result in compression of a signal and a gain in power. The signal becomes also frequency modulated. For nondispersive waves, such as acoustic waves, there is an increase in frequency, whereas for transverse electromagnetic waves the frequency is lowered for the particular scheme of propagation considered.