Introduction . Artificial neural networks are the computational models composed of interconnected nodes, and can ‘learn’ to deal with complicated tasks such as image feature recognition, language translation, medical diagnosis, etc., through ‘training’ the parameters1 , 2 , 3 , 4. The optical neural networks (ONNs) can perform the function of the artificial neural networks by using optical elements. They have drawn much attentions in recent years because of the potential to g...