Design of low-energy on-chip electro-optical 1 × M wavelength-selective switches

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A theoretical design is presented for a 1×M wavelength-selective switch (WSS) that routes any one of N incoming wavelength signals to any one of M output ports. This planar on-chip device comprises of a 1×N demultiplexer, a group of N switching “trees” actuated by electro-optical or thermo-optical means, and an M-fold set of N×1 multiplexers. Trees utilize 1×2 switches. The WSS insertion loss is proportional to [log2 (M+N+1)]. Along with cross talk from trees, cross talk is present at each cross-illuminated waveguide intersection within the WSS, and there are at most N−1 such crossings per path. These loss and cross talk properties will likely place a practical limit of N=M=16 upon the WSS size. By constraining the 1×2 switching energy to ∼1  fJ/bit, we find that resonant, narrowband 1×2 switches are required. The 1×2 devices proposed here are nanobeam Mach–Zehnders and asymmetric contra-directional couplers with grating assistance.