Vortex instability in turbulent free-space propagation

Martin P J Lavery

doi:10.1088/1367-2630/aaae9e

Abstract

The spatial structuring of optical fields is integral within many next generation optical metrology and communication techniques. A verifiable physical model of the propagation of these optical fields in a turbulent environment is important for developing effective mitigation techniques for the modal degradation that occurs in a free-space link. We present a method to simulate this modal degradation that agrees with recently reported experimental findings. A 1.5 km free-space link is emulated by decomposing the optical turbulence that accumulates over a long distance link, into many, weakly perturbing steps of 10 m. This simulation shows that the high-order vortex at the centre of the helical phase profiles in modes that carry orbital angular momentum of are unstable and fracture into many vortices when they propagate over the link. This splitting presents issues for the application of turbulence mitigation techniques. The usefulness of pre-correction, post-correction, and complex field conjugation techniques are discussed.

Highlights

The implementation of spatially structured optical fields have resulted in a wide array of scientific and technological advances [1,2,3]
Current models for free-space transmission of optical fields are based on theories developed for astronomical measurements, which generally assumed an input optical field with a flat wavefront [6, 7]. These theories have been extended for use with spatially structured optical fields [8,9,10]
Tip-tilt aberrations are generally the most commonly considered atmospheric turbulence effect impacting Gaussian optical modes, high-order aberrations can be present within long distance free-space links

Summary

Introduction

The implementation of spatially structured optical fields have resulted in a wide array of scientific and technological advances [1,2,3]. Recent studies have indicated that the intensity structure of these modes is largely persevered, the phase aberrations on these modes are considerable and result in the break-up of high-order OAM of l, which breaks up into a cluster og l, l = 1 modes [12]. Such a breakup in high order modes has not commonly been considered in the modelling of line-ofsight point-to-point links. In this paper a technique is presented for modelling the expected modal degradation of a high-order spatially structured mode that carry orbital angular momentum through simulated point-to-point link atmospheric turbulence similar to that experienced in an urban environment. Potential atmospheric mitigation techniques are compared, indicating single plane post-correction techniques are not appropriate for use in long distance free-space links

Channel mode

Relative stability of superpositions

Turbulence mitigation

Conclusion