Analysis of rotational Doppler effect based on radio waves carrying orbital angular momentum

The electromagnetic beam carrying orbital angular momentum (OAM) has a twisted phase front, which can induce a rotational Doppler frequency shift when it is illuminating on a spinning object. In this paper, the rotational Doppler effect induced by a radio wave carrying OAM is theoretically analyzed and experimentally verified. Compared with an optical vortex beam, the divergence angle of a radio wave carrying OAM is typically larger than that of an optical vortex beam, which results in the radio OAM-based sensing system with short-range detection distance. In this case, the receiver's location and the deflection of the rotating plane are important factors that affect the radio rotational Doppler effect. For an off-axis receiver or a deflected spinning object, a series of extra Doppler frequency shifts (L ± N)Ω/2π are induced when the spinning object is illuminated by a wave carrying OAM with topological charge L, even for L = 0, the plane wave. Hence, the high order OAM wave should be used to distinguish the real rotational frequency shift and improve detection sensitivity of spinning velocity. Combining the rotational Doppler effect with the linear Doppler effect, a vector velocity detecting scheme is discussed.