Remote angular velocity measurement by the cascaded rotational Doppler effect

The rotational Doppler effect has attracted extensive attention, caused by the angular momentum and energy exchange between rotating objects and waves. However, most previous works used a simple rotation frame, which made use of only a single-round angular momentum and energy exchange. We propose and demonstrate a frame containing a spiral phase plate cascaded with rotating targets to make an amplification of the traditional Doppler shift, and reduce the diffusion of orbital angular momentum modes by half, which means the distance of practical application is doubled theoretically. To this end, an experiment is carried out to verify the frame. It shows a more practical, convenient, and non-destructive method to measure the rotational speed of a remote target.