Investigation of Large Capacity Optical Memories for Correlator Applications

In this investigation of large capacity optical memories the basic memory element, the matched filter, was examined in a number of configurations, including coherent and noncoherent addition, split spectrum, and multiplexed carriers. Three different storage materials were examined: silver halide, dichromated gelatin, and photopolymers. Both thin and thick holographic memories were studied. The matched filters were evaluated using complex terrains such as woodlands, roadsides, and villages in which the principal targets (i.e. tanks, trucks and armored personnel carriers) were located. For each terrain and target, the signal-to-clutter ratio (s/c) was measured. These ranged from 5.5 to 11 dB for the target-terrain combinations studied. The complex matched filters that achieved these s/c's were recorded in dichromated gelatin. It was shown that more than 5 matched filters could be non-coherently added and 15 could be coherently added in the same memory position. In addition, it was demonstrated that carrier multiplexing could be used to achieve 30 matched filters per memory position. It is also shown that the theoretical limit for memory density of matched filters is 2670 MF/sq cm. This is equivalent to 1,000,000 bits of digital data. Various modifications of the basic optical matched filter were studied for the purpose of improving s/c ratio and increasing the storage density.