Rotational Doppler effect of superimposed elliptical optical vortices for oblique rotational axis detection.

The rotational Doppler effect (RDE) of structured light carrying orbital angular momentum (OAM) has attracted widespread interest in optical sensors and OAM spectrum detection. However, the motion parameters of spinning objects are the main focus of the RDE-based research, and other equally significant attitude properties have rarely been considered. In the experiments, we noticed an intriguing phenomenon: the rotational Doppler spectral distribution is affected by both the ellipticity of the elliptical optical vortex (EOV) and the tilt angle between the rotational axis and the optical axis. This led us to wonder whether it is feasible to detect an oblique rotational axis or compensate for the rotational Doppler broadening effect induced by oblique incidence by utilizing the EOV. Here, we revealed the RDE quantitative relationship with tilt angle and ellipticity and proposed a novel measuring method for the tilt angle. Based on the specific relationship between the tilt angle and ellipticity of EOV, the tilt angle can be obtained accurately by employing a series of EOV with periodically varying ellipticity to illuminate the rotating object and analyzing the time-frequency spectral distribution of scattered light. Furthermore, the spectrum-broadening effect caused by oblique incidence in the real scenario may be addressed through our scheme. The method may find applications in industrial manufacturing and target attitude measurement, and our results provide new insights for obtaining more information about objects.