Free-space coherent optical dot-product multiplier with lensless fan-in

Since a significant power burden in modern digital AI processors lies in linear operations, optics' inherent linearity and parallelism offer an attractive route to analogue processors for multiply-and-accumulate (MAC) operations. We present a free-space dot-product multiplier that uses coherent detection to enable signed vector representation. The key innovation lies in a strictly common-path architecture where a single spatial light modulator (SLM) performs both phase modulation and optical fan-in, while the reference beam is generated within the active aperture itself. Such fan-in embedding allows for a leaner and phase-drift-tolerant architecture, as well as active misalignment compensation and enhanced parallelism. With commercially available device formats, our system is capable of performing dot products of 8-bit vectors of size N = 2025 or, alternatively, 4-bit vectors of size N = 32400 with RMS error as low as 0.06% (relative to full scale), with an equivalent SNR of 42 dB. Fan-in embedding allows us to seamlessly partition the SLM aperture to perform independent dot products of reduced-size vectors simultaneously with little precision penalty. Although simulation-based, our work evaluates key practical device limitations and identifies mitigation strategies for issues likely to arise during experimental deployment. Beyond the results themselves, we believe this to be a novel design that represents a step towards robust coherent optical processors, with the potential to contribute to future platforms that reduce the energy burden of linear computation.