Free-space remote sensing of rotation at photon-counting level

TL;DR

This paper demonstrates a practical free-space remote sensing method using photon-counting to detect rotation speed and symmetry of objects via the rotational Doppler effect over a 120-meter link, even under atmospheric turbulence.

Contribution

It introduces a real-world free-space optical setup for remote rotational sensing using the rotational Doppler effect at photon-counting levels, analyzing effects of misalignments and turbulence.

Findings

Successful detection of rotation speed and symmetry over 120 meters

Quantitative analysis of turbulence and misalignment effects

Ability to deduce rotation parameters with partial object information

Abstract

The rotational Doppler effect associated with light's orbital angular momentum (OAM) has been found as a powerful tool to detect rotating bodies. However, this method was only demonstrated experimentally on the laboratory scale under well controlled conditions so far. And its real potential lies at the practical applications in the field of remote sensing. We have established a 120-meter 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 turbulences 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 efficiency does not exceed 50 %. Our work supports the feasibility of a practical sensor to remotely detect both the speed and symmetry of rotating bodies.