Rotational speed measurement under misalignment conditions based on rotational Doppler effect

The rotational Doppler effect (RDE) occurs when a vortex beam carrying orbital angular momentum (OAM) is normally incident on the center of a rotating target, which is widely applied for the angular velocity measurement of the rotating target and OAM detection. The combined vortex beam based on coherent beam combining technology has many unique advantages, such as high power and excellent quality. In this paper, the combined vortex beam is used as the detection source to measure the rotating target at a distance of 1 kilometer, we prove the feasibility of the combined vortex beam can be used to measure the rotational speed. However, there are some misaligned incidence conditions such as lateral displacement and oblique angle between the optical axis and the rotation axis. We analyze the change of the OAM spectrum and characteristic peak’s intensity with the increase of misalignment. The results show the OAM mode of the probing beam will expand to the adjacent modes, resulting in a series of discrete frequency-shifted signals including the characteristic peaks, when the optical axis does not coincide with the rotation axis. When deviation exceeds a certain value, the dispersion is too large to cause the frequency characteristic peaks are gradually submerged in the scattering signals. These analyses provide a reference for the practical detection of rotational speed in remote sensing.