ABSTRACT
In previous work, we presented theory of how atmospheric turbulence can impart orbital angular momentum to propagating optical waves. In this paper we provide the first experimental demonstration of the detection of orbital angular momentum from distributed volume turbulence through the identification of well-defined, turbulence-induced, optical vortex trails in Shack-Hartmann wave front sensor measurements.
ABSTRACT
Previous work by Allen, demonstrated that optical beams possess orbital angular momentum. Other work has shown that a random, phase-only disturbance can impart ± 1 orbital angular momentum states to propagating waves. However, the field preceding the formation of these ± 1 states was unknown. In this paper, we identify the unique field that leads to the formation of a pair of branch points, indicators of orbital angular momentum. This field is then verified in a bench-top optical experiment.
Subject(s)
Light , Models, Theoretical , Nephelometry and Turbidimetry/methods , Scattering, Radiation , Computer SimulationABSTRACT
Light propagating through atmospheric turbulence acquires spatial and temporal phase variations. For strong enough turbulence conditions, interference from these phase variations within the optical wave can produce branch points; positions of zero amplitude. Under the assumption of a layered turbulence model, our previous work has shown that these branch points can be used to estimate the number and velocities of atmospheric layers. Key to this previous demonstration was the property of branch point persistence. Branch points from a single turbulence layer persist in time and through additional layers. In this paper we extend persistence to include branch point pairs. We develop an algorithm for isolating persistent pairs and show that through experimental data that they exist through time and through additional turbulence.
Subject(s)
Algorithms , Atmosphere , Interferometry/methods , Optics and Photonics/methods , Models, Theoretical , Nephelometry and TurbidimetryABSTRACT
This is the second of two papers demonstrating that photons with orbital angular momentum can be created in optical waves propagating through distributed turbulence. In the companion paper, it is shown that propagation through atmospheric turbulence can create non-trivial angular momentum. Here, we extend the result and demonstrate that this momentum is, at least in part, orbital angular momentum. Specifically, we demonstrate that branch points (in the language of the adaptive optic community) indicate the presence of photons with non-zero OAM. Furthermore, the conditions required to create photons with non-zero orbital angular momentum are ubiquitous. The repercussions of this statement are wide ranging and these are cursorily enumerated.
Subject(s)
Light , Models, Theoretical , Nephelometry and Turbidimetry/methods , Photons , Scattering, Radiation , Computer SimulationABSTRACT
This is the first in a series of papers demonstrating that photons with orbital angular momentum can be created in optical waves propagating through distributed turbulence. The scope of this first paper is much narrower. Here, we demonstrate that atmospheric turbulence can impart non-trivial angular momentum to beams and that this non-trivial angular momentum is highly localized. Furthermore, creation of this angular momentum is a normal part of propagation through atmospheric turbulence.
Subject(s)
Atmosphere/chemistry , Models, Theoretical , Nephelometry and Turbidimetry/methods , Refractometry/methods , Computer Simulation , Light , Scattering, RadiationABSTRACT
Optical waves propagating through atmospheric turbulence develop spatial and temporal variations in their phase. For sufficiently strong turbulence, these phase differences can lead to interference in the propagating wave and the formation of branch points; positions of zero amplitude. Under the assumption of a layered turbulence model, we show that these branch points can be used to estimate the number and velocities of atmospheric layers. We describe how to carry out this estimation process and demonstrate its robustness in the presence of sensor noise.
ABSTRACT
Astronomical seeing measurements are usually made at a single wavelength and extrapolated to other wavelengths of interest using scaling laws that assume a specific energy spectrum for the atmospheric turbulence. We describe a variation of the Differential Image Motion Monitor system that provides a measure of the seeing and evaluation of the scaling law by simultaneously measuring the image motion at three different wavelengths.
