Acousto-optic processing for real-time and adaptive memory applications

Two different types of acousto-optic correlators, a real-time large time-bandwidth (∼3000) device and a memory correlator, have been developed. The real-time correlator utilizes successive interactions of a cw laser sheet beam with two “live” surface acoustic waves (SAW's) propagating in the same direction along long (14 cm) lithium niobate (LiNbO3) and bismuth germanium oxide (BGO) crystals that are placed side-by-side. The almost 2:1 SAW velocity ratio for these crystals permits a signal propagating in the LiNbO3 to pass one propagating in the BGO. Focusing the emerging SAW-modulated laser beam onto a nonlinear photo-diode results in an output proportional to the correlation integral. In the memory correlator, a SAW pattern is stored initially (as changes of the index of refraction in LiNbO3) by illuminating the LiNbO3 with high-intensity, short-duration pulses of green laser light during propagation of the (10 MHz) SAW signal to be stored. Subsequently, a live signal may be correlated with the stored signal by modulating a low-intensity laser sheet beam with both the live and stored signals, and focusing the emerging beam onto a photo-diode. Memory storage times of a few months have been demonstrated. Both device types have successfully correlated complex waveforms, such as FM chirps and Barker codes.