Variable sensitivity photodetector for optical neural networks

A novel type of photodetector called a variable sensitivity photodetector (VSPD) has been developed for an optical implementation of a neural network. The function of the VSPD is a photodetector with functions of both a spatial light modulator and a photodetector. The VSPD with a metal-semiconductor-metal structure is shown to be suitable for the implementation of the analog synaptic weight with a bipolar value. A two-dimensional 8×8 array of VSPDs was fabricated on a GaAs substrate and integrated with a light-emitting-diode array with eight lines in hybrid form. This chip can simulate an eight-neuron system with 64 variable synaptic connections. The application to pattern classification was demonstrated by using the chip. The chip was used to classify 12 patterns into three categories by the backpropagation learning algorithm in a three-layered network. The authors studied how the nonlinear and nonuniform characteristics of the VSPD array device affect the learning performance