Frequency changes during the propagation of a light beam

Abstract

The energy and therefore the frequency of electromagnetic fields are usually conserved quantities in free space propagation. However here, we observe a frequency varying optical wave in free space propagation. The light comes from the Arago spot and this change of frequency is based on the rotational Doppler effect. More precisely, the light diffracted by a two-dimensional asymmetric object carries orbital angular momentum that varies with the distance. As the object rotates, the frequency of the diffracted light experiences different rotational Doppler shifts along propagation. It varies in a discrete quantized manner as it propagates and can be adjusted, without violation of the conservation of energy. Since this phenomenon is deeply rooted in the diffraction process, it shines some new light in this still open issue.

The Arago–Poisson Spot: New Applications for an Old Concept

“…This indeed means that since the topological charge changes during propagation, the optical frequency of the spot must also change during propagation. This counterintuitive assumption has been verified experimentally [102] and is evidenced in Figure 13. The frequency has been measured with a self-heterodyne technique, with a beat signal against a reference signal that has been shifted thanks to acousto-optic modulators.…”