Single-layer metasurface enables a compact rotational speed detection system

The detection of rotational speed based on the rotational Doppler effect (RDE) has gained notable attention due to its quick response, wide measurement range, and non-contact operation. However, previous methods for generating conjugate topological vortex beams in the rotational speed detection process rely on a series of bulky optical elements, such as the spatial light modulator (SLM) or digital micromirror device (DMD), making the system complex and difficult to integrate. In this Letter, we propose a method to generate conjugate topological vortex beams via a single-layer metasurface, thereby realizing, to the best of our knowledge, the first compact version of an RDE-based rotational speed detection system. By analyzing the adjacent frequency peak gaps in echo light signals, the rotational speed of a rotating target can be accurately measured, with the maximum average relative measurement error being 0.812%. This method paves the way for the development of miniature and compact RDE-based metrology devices, which are particularly suitable for applications with limited volume and payload, thereby enhancing the practical implementation of RDE-based metrology.