Evaluation of a Binary Spatial Light Modulator Correlator Using Time-Domain Polarity Coincidence Correlation Theory

Time-domain Polarity Coincidence Correlators (PCC) have been studied for the possible extension of these theories to the spatial-domain in the context of an optical PCC. The existing body of knowledge has been found to be inadequate for direct application to the spatial-domain. However, by remodeling the type of clipper or hardlimiter used in the development of the existing theory from a signum (bipolar) clipper to a binary (unipolar) clipper, a new expression for the output signal-to-noise ratio of an optical PCC is developed. This modification is necessary to account for the unipolar nature of light intensity. Both the existing time-domain and new spatial-domain PCC expressions are valid for the case of additive narrowband (bandwidth much less than the signal frequency) Gaussian-distributed noise at the inputs of the correlator. An optical experiment was constructed to evaluate a new type of binary spatial light modulator, the Litton Data Systems 128 x 128 Magneto-Optic Array. To accomplish this, a noiseless single frequency (fc) sinusoidal one-dimensional reference was correlated with a sinusoidal signal of equal frequency and phase to which gaussian-distributed noise was added. Both signals were hardlimited prior to correlation. Noise sources having bandwidths of 0.1 fc and0.5 fc were investigated to test the robustness of the narrowband constraints. The input data sets had signal-to-noise ratios of -10.0dB and -20.0dB. For both of these conditions the experimental data was found to be in excellent agreement with the new expression for the narrowband case. In the wideband case, the results were significantly different from the new expression, thereby corroborating the necessity of meeting the narrowband constraint. Both the time-domain and spatial-domain expressions are highly-dependent on the output filtering (after hardlimiting and multiplication) characteristics. The effect of filter insertion loss, at the signal frequency (fc), for the unipolar or optical case was examined as part of this research. It was found that the output signal-to-noise ratio increases as the carrier frequency components are attenuated. This affect, which is a direct result of the multiplication of the reference with the input signal within the PCC, is greater for input signal-to-noise ratios above -10dB. For input signal-to-noise ratios below -10dB the output signal-to-noise ratio asymptotically approaches a constant. This output signal-to-noise independence on input signal-to-noise is due to the combined effect limiting signal (which is mostly noise) amplitude information by the hardlimiter and the bounding of the spatial correlation by the low pass filter