This paper describes a multichannel optical correlator that uses two spatial dimensions to achieve multi-channel capability and operates in real time. Given I signals, ri(t), and J signals, sj(t), it can produce the I × J cross products, ri(t)sj(t), averaged over some interval. Multiplication is based on the fact that the square of the sum of two signals contains their cross product. Light fields modulated by the signals are added and their sum squared by measuring its intensity. An experimental correlator and its performance are described along with applications.
G. J. M. Aitken and L. Wang, "New Multichannel Optical Correlator," Appl. Opt. 10, 2476-2481 (1971)