(a) Base 4−3−2 network, light circles indicate matrix–vector products, dark circles indicate merging/averaging (see (b) and (c)), boxes indicate activation functions. The light box indicates a single instance of a photonic layer as in figure 1. (b) Example for the tree-like design with 2 layers as input copies to each subsequent layer. The light circles indicate the linear operations/matrix-vector products. The results of the linear operation is averaged (single solid-blue circle) and fed through the activation function, producing the multiple version of the layers output (boxes). (c) Example of accordion-like design.