  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 photonic AI. In this letter, based on MNIST datasets and 4f system,
one- and three-layer optical neural networks are constructed, whose recognition
accuracy can reach 86.06% and 93.66%, respectively. Our network is better than
the existing free-space optical neural network in terms of spatial complexity,
and the three-layer's accuracy.
