  With its unique parallel processing capability, optical neural network has
shown low-power consumption in image recognition and speech processing. At
present, the manufacturing technology of programmable photonic chip is not
mature, and the realization of optical neural network in free-space is still a
hot spot of intelligent optical computing. In this article, based on MNIST
datasets and 4f system, three-layer optical neural networks are constructed,
whose recognition accuracy can reach 93.66%. Our network is programmable, high
speed, reconfigurable and is better than the existing free-space optical neural
network in terms of spatial complexity.
