Bispectral magnitude and phase recovery using a wide bandwidth acousto-optic processor

A hybrid optical-digital processor has been developed that computes both the magnitude and phase of the bispectrum for wide bandwidth (10 MHz to 1 GHz) rf signals. The overall optical architecture is that of a modified Mach-Zehnder interferometer that contains three acousto-optic modulators and appropriate transforming lenses. The intensity distribution in the output plane of the interferometer contains an interference term that represents the real part of the bispectrum multiplied by a spatial carrier (the interference fringes). To isolate the bispectrum information, the output image is digitized and digitally filtered. The imaginary part of the bispectrum is obtained by Hilbert transforming the real part, and then computing the bispectrum magnitude and phase. The processor is tested with four different combinations of rf test signals. Each signal has a bandwidth of either 6 or 12 MHz. Test results that illustrate the performance of the processor in the recovery of magnitude and phase information for the bispectrum of quadratically related signals are presented.