Reconfigurable multi bit phase change material silicon photonics directional couplers
The unique optical properties of phase change materials (PCMs) can be exploited to develop efficient reconfigurable photonic devices. Here, we design, model, and compare the performance of programmable 1X2 optical couplers based on: Ge2Sb2Te5, Ge2Sb2Se4Te1, Sb2Se3, and Sb2S3 PCMs. Once programmed, these devices are passive, which can reduce the overall power used in an ONN. In the telecommunication wavelength region (1520 to 1580 nm) the Sb2S3 directional coupler displays a minimum insertion loss of 0.04 dB and 1.1 dB, and the lowest crosstalk of 42 dB and 17 dB for the amorphous and crystalline states respectively. We also design a non volatile 2 bit low loss tuneable Sb2S3 directional coupler that can be used to set the weights of optical neural networks (ONN) Partial crystallization of Sb2S3 strips with dimensions compatible to those in the directional coupler indicate that at least four different crystalline lengths can be reliably programmed. We used this effect to design a 2 bit tuneable Sb2S3 directional coupler with a weight dynamic range close to 17 dB. The bit depth of the coupler appears to be limited by the crystallization stochasticity.