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Vortex beams (VB) are structured light beams with helical wavefront and carrying orbital angular momentum (OAM). Compared with Gaussian beams, VBs possess the rotational Doppler effect (RDE), which is anticipated to make up the deficiency of traditional detection methods in the spin motion of the target object. However, in practical applications, the rotational speed measurement technology based on the VB is faced with challenges, for example, weak echo signal intensity due to low vortex beam light power and OAM spectrum expansion due to off-axis incidence of the vortex beam. These problems above directly limit the accuracy and application range of rotational speed measurement. To expand the application range of detection scheme based on the VB， we make a study of the measurement scheme of the target rotational speed based on the combined vortex beam (CVB), which is on the basis of the experimental device for rotational speed measurement with CVB generated by fibre laser arrays. Firstly, the OAM spectra of the off-axis incidence situation are simulated. Based on the simulation results, we derive the general model of the distribution of echo signal peaks in the off-axis incidence situation, and propose a rotational speed measurement scheme based on the frequency interval between adjacent spectral peaks. Secondly, we carry out the target rotational speed measurement experiment in off-axis incidence situation, and the frequency difference between two adjacent spectral peaks is obtained from the spectrum map of the echo signal to measure the rotational speed of the target object. The results show that， the target rotational speed can be accurately measured regardless of the lateral displacement and angular deflection under the case of off-axis incidence, which confirms the validity of the universal model for rotational speed measurement. The rotational speed measurement scheme proposed in this study considers the off-axis incidence situation prevalent in practical application， thereby improving the applicability in the target object rotational speed measurement, and providing technical reference for remote sensing detection application based on the VB.
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Keywords:
- Rotational Doppler effect /
- Vortex beam /
- Off-axis incident /
- Rotational speed measurement
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