Optical processing of pulsed Doppler and FM stepped radar signals

Acousto-optic processors for real-time generation of time-frequency representations

Acousto-optic processors for calculating different two-dimensional (2-D) time-frequency representations for one dimensional temporal signals in real time are described. The various 2-D representations discussed in the literature, such as the Wigner distribution and the ambiguity function, are shown to be obtainable through minor variations in an acousto-optic processor consisting of two Bragg cells in a parallel configuration. Also obtained and discussed are two new time-frequency representations that, for amplitude-modulated signals, correspond to mean-frequency-selective correlation and Doppler-frequency-selective convolution. Experimental results are presented to highlight the special features of the different time-frequency representation.

Optical image plane correlator for ambiguity surface computation

An optical image plane correlator is described which computes the ambiguity function for passive, wideband, complex, and basebanded signals. Quadrature input modulation, single-sideband filtering, and a modified image plane correlator requiring no moving parts are used to achieve the required complex correlations. The scenario, theory, and design are provided together with experimental verification on LFM and Barkercoded waveforms as well as synthetic data.

Multichannel optical correlator for radar signal processing

A multichannel coherent optical processor for radar signals is described in which pulse compression is achieved by matched spatial filtering. The optical system can be used for phased array, linear frequency modulated pulse burst, or other radar systems. However, only its application to linear phased array signal processing is discussed in depth. From the output optical pattern, one can obtain data on the target's fine range and azimuth (for a phased array) or fine range and Doppler (for a pulse burst radar) etc.

Real-time recording medium: photodichroic KF:LiF

Laser recording and readout measurements using the photodichroic crystal KF:LiF indicate that the material has potential as an input transducer and an adaptive spatial filter in an optical spectrum analyzer. These crystals are among the most photosensitive recording materials without gain, can be produced in large sizes at relatively low cost, and can be operated close to room temperature using thermoelectric coolers. Experiments using a high precision laser scanner providing 96.6-MHz subnsec pulses at 514 nm show that adequate recording can be accomplished with exposures less than 10 mJ/cm(2). An MTF of 40% at 72 1/mm was measured with uniform readout illumination of the recorded signal, but this is reduced if the signal is recovered by rescanning due to the convolution of the scanning beam profile with the recorded spot profile. The readout is destructive at the recording wavelength but can be read out with a much reduced decay rate by using 488 nm. The linear dynamic range in the Fourier plane was measured to be 40 dB.

Space-variant processing of 1-D signals

Two general schemes for 1-D space-variant processing are proposed. The direct output display scheme gives the space-variant system output along a line in the processor's output plane. The output spectrum display scheme directly computes the space-variant system's output spectrum. Both of these schemes utilize a 1-D input and a line spread function mask. Example applications and experimental results are also presented.

Ambiguity function display: an improved coherent processor

A coherent optical processor for displaying a signal's ambiguity function is described. The required time delay is realized by 45 degrees rotations of two identical input transparencies and the Doppler shift by a 1-D Fourier transformation. The entire ambiguity function is displayed in the output (Doppler shift-time delay) plane. Examples of the optically computed ambiguity function for single and double pulse signals are shown to be in excellent agreement with theory. Advantages of this approach over other schemes and possible extension to real time processing are also discussed.