Experimental realization of arbitrary activation functions for optical neural networks.

We experimentally demonstrate an on-chip electro-optic circuit for realizing arbitrary nonlinear activation functions for optical neural networks (ONNs). The circuit operates by converting a small portion of the input optical signal into an electrical signal and modulating the intensity of the remaining optical signal. Electrical signal processing allows the activation function circuit to realize any optical-to-optical nonlinearity that does not require amplification. Such line shapes are not constrained to those of conventional optical nonlinearities. Through numerical simulations, we demonstrate that the activation function improves the performance of an ONN on the MNIST image classification task. Moreover, the activation circuit allows for the realization of nonlinearities with far lower optical signal attenuation, paving the way for much deeper ONNs.

High-speed grating-assisted all-silicon photodetectors for 850 nm applications.

This paper presents the design, fabrication, and measurement results of a novel lateral p-i-n silicon photodetector (Si-PD) for 850 nm in a silicon-on-insulator (SOI) platform. In the proposed photodetector, the incident light is directed horizontally using a grating coupler, significantly increasing optical absorption in the depletion area thereby increasing the PD's responsivity. The measurement results show that the grating coupler increases the responsivity by 40 times compared with the Si-PD without a grating coupler. The grating-assisted Si-PD with 5µm intrinsic width has a responsivity of 0.32 A/W and a dark current of 1 nA at 20 V reverse-bias voltage. Further, it shows an open eye diagram for 10 Gb/s PRBS­31 non-return-to-zero on-off keying (NRZ-OOK) data and has a 3-dB bandwidth of 4.7 GHz at this bias voltage. In addition, the design parameters of three variations of the grating-assisted Si-PD for high-speed applications (>25 Gb/s) are presented. The optimized grating-assisted Si-PD uses a focusing grating coupler and its p-i-n diode has a 0.3 µm intrinsic width. It has a responsivity of 0.3 A/W, an avalanche gain of 6, a dark current of 2 µA, and a 3-dB bandwidth of 16.4 GHz at 14 V reverse-bias voltage that, to the best of our knowledge, is the largest 3-dB bandwidth reported for a Si-PD. As a result, it has a figure of merit (FoM) of 4920 GHz×mA/W. Further, it shows an open eye diagram of 35 Gb/s PRBS­31 NRZ-OOK data.