Remote Vector Velocimetry with Fiber‐Delivered Scalar Fields

Abstract The Doppler effect reveals the law that light waves undergo frequency changes in interacting with motion, which is highly significant in velocity detection and has applications in fields such as astrophysics, aerospace, and advanced manufacturing. A typical Doppler velocimetry involves illuminating a moving object with interference fringes generated based on phase gradients while detecting the frequency shift of scattered light to determine the velocity. Beyond the spatial phase distributions, the spatial amplitude is a unique dimension of light fields that can be directly controlled, but its application prospects in motion detection are rarely revealed, particularly in both the magnitude and orientation of velocity measurements. In this work, a remote vector velocimeter based on spatially structured amplitude fields is proposed for monitoring angular velocities of objects in situ. Guided through a 40 km seven‐core fiber, the structured beams with spatially‐distributed amplitude are constructed at the remote fiber facet by adjustable mode excitation in outer cores, and the Doppler signals reflected by the target are collected and transmitted back by the inner core, enabling the remote measurement of rotational motion vectors with a probe‐signal‐integrated configuration. These results suggest the great potential of spatial amplitude fields in motion detection, the cost‐efficient and compact velocimetry may contribute to the communities of optical sensing and engineering.