Orbital angular momentum modal crosstalk in Bessel-Gauss beams

Hunter RewUS Naval Research Laboratory NREIP Intern and College of William and Mary, Williamsburg, VA

Mentored by Dr. Abbie T. WatnikUS Naval Research Laboratory, Washington, D.C.

Communicating with Light

● Orbital angular momentum is the information carrier○ A property of optical vortices

● Bessel-Gauss beams are the medium○ Potentially helpful properties

● Turbulence is the enemy○ Mode hopping and crosstalk

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A. Dudley, M. Lavery, M. Padgett, and A. Forbes, "Unraveling Bessel Beams," Opt. Photon. News 24(6), 22-29 (2013).

Orbital Angular Momentum (OAM) and Optical Vortices

●○ determines mode order (...-1,0,1,2…)○ is the azimuthal coordinate

● A helical phase creates angular momentum

● Propagates as annular rings with a null in the center (vortex)

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Vortex Phase (L = 1)

Vortex Beam (L = 10)

Vortex Phase (L = 10)

Bessel-Gauss (BG) Beams

● A true Bessel beam self-heals and never diffracts○ requires infinite energy

● BG is a realization of the impossible Bessel beam using axicons

Credit: Egmason[https://en.wikipedia.org/wiki/Bessel_beam#/media/File:Bessel_beam_reform.svg]

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Beam Creation

●○ determines the slope of the axicon○ is the radial coordinate

● BG beams are created with a spiraled axicon phase map

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Vortex Phase Axicon Phase Bessel Vortex Phase

Atmospheric Turbulence

● Turbulence results from patches of varying heat and velocity in the air● Refractive index of air varies, inducing phase shifts and aberrations in light

Credit: Lawrence Livermore National Laboratory[https://tasc.llnl.gov/galleries/image-gallery]

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Turbulence Models

● Essentially random numbers● Strength determined by Fried’s parameter

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Vortex Phase Turbulence Phase Map Vortex Phase with Turbulence

Spatial Light Modulators (SLMs)

● High resolution displays that control the phase and/or intensity of incident light

● Our SLMs are binary and phase only○ Each pixel can be 0 or pi○ Forth Dimension Displays, 1280 x 1024 resolution

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Vortex Phase Tilted Plane WaveVortex Interfered with a Plane

WaveVortex Interfered with a Plane Wave

(Binary)

Experimental Setup

9M1

M2M3

Alignment

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Mode Multiplexing

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Vortex Phase (L = 1) Vortex Phase (L = -1) Null

Mode Multiplexing

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Vortex Phase (L = 10) Vortex Phase (L = -7) Vortex Phase (L = 3)

Data

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Analysis

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● Fraction of power in on-axis pixel● Normalized to sum to 1

Analysis with Turbulence

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Influence of Turbulence on Transmission

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Each point is an average of all modes and data runs

10 runs were performed for each turbulence strength, each with a unique, randomly generated, turbulence map

(D/r0)

Influence of Turbulence on Spread of Power

17(D/r0)

Future Work

● Further investigation of the effects of turbulence● Larger propagation distance to test the BG beams● Log-polar transformation for mode sorting

A. Dudley, T. Mhlanga, M. Lavery, A. McDonald, F. Roux, M. Padgett, and A. Forbes, "Efficient sorting of Bessel beams," Opt. Express 21, 165-171 (2013).

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Summary of Accomplishments

● Built table top setup for multiplexing/demultiplexing modes○ Designed SLM mounts for fabrication

● Interfaced SLMs and Camera with MATLAB● Automated data collection and analysis

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Acknowledgements

● ASEE● ONR● NRL● Tim Doster● Ryan Lindle● Abbie Watnik

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Orbital angular momentum modal crosstalk in Bessel-Gauss beams

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