Depolarization of Vector Light Beams on Propagation in Free Space

Nikolai Petrov

doi:10.3390/photonics9030162

Abstract

Nonparaxial propagation of the vector vortex light beams in free space was investigated theoretically. Propagation-induced polarization changes in vector light beams with different spatial intensity distributions were analyzed. It is shown that the hybrid vector Bessel modes with polarization-OAM (orbital angular momentum) entanglement are the exact solutions of the vector Helmholtz equation. Decomposition of arbitrary vector beams in the initial plane z = 0 into these polarization-invariant beams with phase and polarization singularities was used to analyze the evolution of the polarization of light within the framework of the 2 × 2 coherency matrix formalism. It is shown that the 2D degree of polarization decreases with distance if the incident vector beam is not the modal solution. The close relationship of the degree of polarization with the quantum-mechanical purity parameter is emphasized.

Highlights

The polarization of light reflects the vector nature of electromagnetic fields and plays a very important role in optics and physics of the light–matter interaction [1–3]
Degree of polarization of pure states, which are the modal solutions of the Maxwell equations, remains invariant during propagation, i.e., it does not change on propagation
New hybrid vector Bessel beams with polarization-orbital angular momentum (OAM) entanglement, which are the modal solutions of the Maxwell equations, are proposed to study the evolution of vector beams in free space

Summary

Introduction

The polarization of light reflects the vector nature of electromagnetic fields and plays a very important role in optics and physics of the light–matter interaction [1–3]. Significant changes in the state of polarization and the degree of polarization occur during the propagation of radiation in inhomogeneous media. Depolarization takes place in optical waveguides without birefringence It was shown in [11] that the polarization degree of the linearly polarized light in a graded-index isotropic optical fiber decreases with increasing distance. In [12], it was shown theoretically that the polarization degree of the linearly polarized light in an isotropic optical fiber with parabolic distribution of refractive index decreases with increasing distance due to the Rytov rotation of the polarization vector, but the degree of polarization of circularly polarized light is retained with increasing distance. It was shown that the effects of diffraction and spin–orbit interaction are responsible for depolarization of light in an isotropic graded-index medium. Intrafibre rotation of the plane of polarization was demonstrated in [17]

Results

Discussion

Conclusion