Integration of Gaussian-mode emission and multimode fiber reception with Doppler-shifted interval analysis for rotational velocity detection.

Rotational Doppler velocimetry (RDV) has emerged as a key technique for sensing and identifying spinning objects. However, existing fiber-based RDV systems generally rely on customized mode-selective couplers for orbital-angular-momentum generation or mode filtering, resulting in increased complexity and cost. Here, we propose a fiber-assisted RDV scheme that combines Gaussian-mode emission, multimode-fiber (MMF) reception, and Doppler-interval analysis without requiring such devices. The scheme is theoretically analyzed and experimentally demonstrated over a rotational-speed range of 0-5000 r/min. Spectral autocorrelation enables robust interval extraction, yielding relative errors below 0.2% and standard deviations below 5 r/min. Robust operation is further verified under lateral displacement, angular misalignment, and varying surface roughness. Moreover, larger MMF core diameters are shown to enhance optical collection and improve signal-to-noise ratio. These results demonstrate a simple, low-cost, and practical route toward engineering-oriented rotational Doppler LiDAR systems.