This paper reviews the optical hardware required for photonic switching and optical computing systems that are based on free-space digital optics. It includes a comparison of the devices, interconnects, and systems that have been proposed for both types of systems.
Harvard Scott Hinton,
"Photonic switching and optical computing based on free-space digital optics", Proc. SPIE 1215, Digital Optical Computing II, (1 July 1990); doi: 10.1117/12.18057; https://doi.org/10.1117/12.18057