OptiLookUp: a microring-based photonic read-only memory lookup table engine for photonic accelerators

Read-only memory (ROM) provides deterministic access to predefined data mappings. Extending ROM concepts to the optical domain enables high-bandwidth, low-latency, and parallel memory access, but realising compact, function-selectable optical ROM remains challenging due to loss, wavelength control, and integration constraints. This work presents a high-speed, function-selectable photonic ROM architecture implemented using integrated microring resonators (MRRs). The ROM encodes predefined input-output mappings directly in the spectral response of the photonic devices, enabling deterministic lookup-based operation without dynamic computation during readout. To improve scalability and reduce cumulative insertion loss, the architecture employs compact banked sub-arrays that are selectively addressed through an optical decoding mechanism. Run-time function selection is achieved using transistor-based optical selectors, allowing different ROM banks to be activated without physical light rerouting or interferometric structures. The proposed photonic ROM is designed and evaluated using device-level simulations based on the GlobalFoundries 45SPCLO silicon photonics platform. Simulation results support an access rate of 12.5 Gaccess/s with a maximum settling time of 31.3 ps, with stable light-to-current transfer characteristics obtained through integrated photodiode readout. The optical ROM can be used to implement nonlinear activation functions utilised in photonic accelerator architectures, including sigmoid, tanh, ReLU, and exponential mappings. In this use case, the ROM operates as a lookup table that stores predefined nonlinear transfer characteristics, demonstrating that complex nonlinear behaviours can be realised through static optical responses rather than active photonic computation. The use of activation functions in this work serves to illustrate the flexibility of the proposed ROM framework and does not constrain its general applicability to other lookup-based functions.