Optical rotation of a uniformly, linearly polarized Bessel-like beam in free space

We report on the observation of a light beam in air whose polarization state is uniform and linear at the transverse region where the intensity is high and rotates uniformly with propagation distance. The beam is obtained by interference of two circularly polarized Bessel beams of opposite helicities and very similar, but not equal, cone angles. The measured rotation rate is in agreement with that expected from theory.