Design and Analysis of a Nano-photonic Processing Unit for Low-Latency Recurrent Neural Network Applications

Recurrent neural networks (RNNs) have achieved high performance in inference processing that handles time-series data. Among them, hardware acceleration for fast processing RNNs is helpful for tasks where real-time performance is es-sential, such as speech recognition and stock market prediction. The nano-photonic neural network accelerator is an approach that takes advantage of the high speed, high parallelism, and low power consumption of light to achieve high performance in neural network processing. However, existing methods are inefficient for RNNs due to significant overhead caused by the absence of recursive paths and the immaturity of the model to be designed. Therefore, architectural considerations that take advantage of RNN characteristics are essential for low latency. This paper proposes a fast and low-power processing unit for RNNs that introduces activation functions and recursion processing using optical devices. We clarified the impact of noise on the proposed circuit's calculation accuracy and inference accuracy. As a result, the calculation accuracy deteriorated significantly in proportion to the increase in the number of recursions, but the effect on inference accuracy was negligible. We also compared the performance of the proposed circuit to an all-electric design and a hybrid design that processes the vector-matrix product optically and the recursion electrically. As a result, the performance of the proposed circuit improves latency by 467x, reduces power consumption by 93.0% compared with the all-electrical design, improves latency by 7.3x, and reduces power consumption by 58.6% compared with the hybrid design.