Nonlinear Activation Function Generation Based on Silicon Microring Resonators for Integrated Photonic Neural Networks

To overcome the interconnect problem of CMOS Neural Network (NN) implementations (increased power consumption while inhibiting speed), small-scale linear optics-based solutions have been proposed to replace the electronic NN layer in multiple works - e.g. [1-3]. Nevertheless, an all-optical NN is difficult to achieve as it would imply substituting the existing electro-optic signal conversion and digital-driven activation function necessary between NN layers. In this work, we demonstrate how feedback controlled microring resonators (MRR) can be used as activation functions in NNs. The design we focus on is shown in Fig. 1-a. Pulses of light at different frequencies carry signals while weights are applied using PIN ring modulators with proper free spectral range. Pulses are used to ensure that the detuning due to heating of the device is mostly avoided. The light is then coupled in the main ring resonator responsible for the non-linear transfer function. The power dependent response is governed by an interplay between free carrier dispersion and free carrier absorption [4]. An electronic feedback loop will ensure carrier lifetime control, crucial for output stability and reproducibility. The output of this resonator is then filtered again to extract the necessary signal, before passing it to the next NN layer.