Abstract

We develop the theory of frequency and temporal compounding of coherent images for the purpose of speckle suppression. Frequency compounding corresponds to the process in which a coherent pulse of a specific bandwidth is passed through a filter bank that divides the pulse into a number of subbands. The image is formed by incoherently summing (compounding) the intensities of the individual subbands. Temporal compounding is a form of nonlinear multirate signal processing in which the final pixel intensity is made up of the intensities of samples that were sampled initially at rates equal to or higher than the pulse bandwidth. The intensities of the samples are then compounded to form the final image. For cases of oversampling by the same factor as the number of combined samples, the final pixel sizes are not altered. In the limit of an infinite sampling rate, temporal compounding is exactly equivalent to analog integrated backscatter in which the sensor continuously integrates the intensity of the incoming signal. The scattering theory that serves as the basis for these calculations is developed from a coherent wave-packet formulation of pulses. The theory is applied specifically to calculations of the effects of frequency and temporal compounding on speckle contrast and axial (range) resolution as a function of coherent pulse bandwidths and signal-sampling frequencies. Trade-offs between speckle reduction and decrease in resolution are discussed. The results presented apply to coherent imaging systems covering the spectral ranges of electromagnetic and acoustic excitations where the signals are amenable to complex detection techniques.

© 1988 Optical Society of America

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