With the exception of the radiometer, all the systems described so far in this book are of the space-integrating type. The name derives from the fact that operations such as spectrum analysis and correlation have their ranges of integration limited by the spatial extent of the input signal. There are, in some applications, additional integration processes implied by bandpass filtering of the electronic signal after detection. However, this postdetection processing integration does not affect the intrinsic frequency resolution or correlation gain produced by the space-integrating nature of the optical system.
In the 1970’s, time-integrating optical signal-processing systems were developed. The early work was due to Montgomery (116), who described a time-integrating system that uses chirp signals to implement operations such as spectrum analysis and correlation. Later, Sprague and Koliopoulos introduced a time-integrating correlator that did not require the use of auxiliary chirp signals (117). From these two major building blocks, new and useful techniques evolved for performing operations such as spectrum analysis, correlation, ambiguity and Wigner-Ville function generation, and range/Doppler processing.
The ability to integrate in time, as well as in space, provides for improved performance. For example, a time aperture of the order of 33 msec is required to provide a 30-Hz frequency resolution in a spectrum analyzer. A one-dimensional ...