Detection of short pulses by Fresnel preprocessing

Short pulses create unique Fresnel diffraction effects, as they propagate through an acousto-optic signal-processing system, that produce an interference pattern. By following the detection of the interferometric pattern with spectral analysis, we extract information about pulse width, pulse separation, and relative time of arrival. Laboratory models of the Fresnel-transform processing system and an optical spectrum analyzer were built and tested to verify the basic concepts. Pulses as short as the theoretical limit of 20 ns for this system and separated by as little as 60 ns or as much as 17 µs were detected and measured. Short pulses were also detected in the presence of cw signals by use of a frequency-variant matched-filtering operation.