Dual-point noncoaxial rotational Doppler effect towards synthetic OAM light fields for real-time rotating axis detection

Probing the axis of a rotator is important in astrophysics, aerospace, manufacturing, machinery, automation, and virtual reality, etc. Existing optical solutions commonly require multiple sequential measurements via symmetry-broken light fields, which make them time-consuming, inefficient, and prone to accumulated errors. Herein, we propose the concept of a dual-point noncoaxial rotational Doppler effect (DNRDE) and demonstrate a one-shot detection technique to solve this problem. An on-demand synthetic orbital angular momentum (OAM) light beam impinges on a rotating scatterer surface, supporting dual-point rotational Doppler shifts, in which the information of the rotating axis is acquired by comparing these two frequency shifts with a prescribed threshold. The existence of arbitrary dual-point Doppler shifts enables the one-time direct identification of rotating axis orientations, which is fundamentally inaccessible in single-point detection. This robust detection technique is compatible with generalised synthetic OAM light fields by utilising optical modal filters. Compared with traditional approaches, our DNRDE-driven detection approach exhibits a four-fold enhancement in measurement speed, higher energy efficiency, and superior accuracy with a maximal absolute measurement error of 2.23°. The proposed dual-point detection method holds great promise for detecting rotating bodies in various applications, such as astronomical surveys and industrial manufacturing.