Free-Space Remote Sensing of Rotation at the Photon-Counting Level

The rotational Doppler effect associated with light's orbital angular momentum has been found to be a powerful tool to detect rotating bodies. However, this method has only been demonstrated experimentally on the laboratory scale under well-controlled conditions so far. However, its real potential lies in practical applications in the field of remote sensing. We establish a 120-m-long free-space link between the rooftops of two buildings and show that both the rotation speed and the rotational symmetry of objects can be identified from the detected rotational Doppler frequency shift signal at photon-count level. Effects of possible slight misalignments and atmospheric turbulence are quantitatively analyzed in terms of mode power spreading to the adjacent modes as well as the transfer of rotational frequency shifts. Moreover, our results demonstrate that with the preknowledge of the object's rotational symmetry one may always deduce the rotation speed no matter how strong the coupling to neighboring modes is. Without any information of the rotating object, the deduction of the object's symmetry and rotational speed may still be obtained as long as the mode-spreading ratio does not exceed 100%. Our work supports the feasibility of a practical sensor to remotely detect both the speed and symmetry of rotating bodies.