Detection of Rotational Doppler Frequency and Measurement With Broadened Spectrum

The Doppler effect can be divided into two types: the linear Doppler effect and the rotational Doppler effect (RDE). The RDE of a vortex beam has attracted extensive attention in rotational measurement due to its advantages, such as the ability to detect rotation perpendicular to the beam axis and a frequency shift independent of wavelength. In the application of RDE, the measurement of rotational direction, detection of weak scattered signals, and spectrum broadening pose several major challenges. This article expounds on the theoretical and experimental basis of the RDE and summarizes the recent progress in signal detection methods, including advancements in rotational direction detection, weak signal detection, and signal subdivision. The spectrum broadening analysis method based on mode decomposition is introduced in this article, and several common reasons leading to Doppler frequency spectrum broadening are provided, along with the introduction of the rotational shaft localization methods based on spectrum broadening. Finally, the existing challenges and promising future directions are discussed.