Volume holograms for optical correlation

Abstract

Pattern recognition of objects within highly cluttered images is a demanding task, although it has many applications in both the military and civilian sectors. The techniques are extremely computationally intensive, making real-time processing using standard computer equipment difficult if not impossible. One possible solution to overcome the computational limitations employs a coherent optical correlator, i.e., an optical processor that allows objects to be identified and tracked at the speed of light. The coherent optical correlator has had a long and successful history, starting from Vander Lugt’s original design. The basic arrangement is still in use today: an input signal or image is coherently encoded on a beam of light, it is subsequently Fourier transformed, and correlated with a holographically stored Fourier signal or filter. The correlated output signal is again Fourier transformed and a peak is produced in the output plane when the filter matches. One of the nice features of this approach is that it is shift invariant, implying that the peak position corresponds to the previously unknown position of the input object. This means that we can identify the object and track its position in a 2D plane. In modern systems, the filter is implemented on a phase-only spatial-light modulator (SLM) that can be dynamically updated, thus allowing correlation of many templates for real-time target identification. However, the overall bandwidth is limited to the data-bus speed, the frame rate of the SLM, or the frame rate of the camera used for capturing the output. This is a significant weakness. If one wanted to test 36 filter templates for every video frame, each correlation must take less than 926μs or occur at a frequency higher than 1080Hz. If the filter consisted of 1024×1024 pixels, one would need a data bus of over one gigahertz. If the number of templates were increased further, the speed would become even more problematic. Figure 1. Schematic layout of the correlator. MQW SLM: Multiplequantum-well-based spatial-light modulator.