Experimental Study on the Effects of OAM Beams Propagating through Atmospheric Turbulence

Date of Award

2023

Degree Name

M.S. in Electro-Optics

Department

Department of Electro-Optics and Photonics

Advisor/Chair

Miranda van Iersel

Abstract

Free space optical (FSO) communication has gained interest for a long time due to its ability to have secure transmission and high data rates. Interest has increased in using beams carrying orbital angular momentum (OAM) for FSO communication, due to their (theoretically) infinite number of orthogonal modes and potential high resistance to atmospheric turbulence. Theory of OAM beams and atmospheric turbulence are discussed in detail in this study. In this experimental study, Laguerre-Gaussian (LG) beams are used as OAM beams and LG beams of different order are generated using a spatial light modulator (SLM). Moreover, the method used to generate a LG beam is also included. Besides, a wave-optics simulation method is used to generate phase screens containing simulated atmospheric turbulence, which in turn are used on two SLMs to generate atmospheric turbulence in our experimental set up. In this study, beams with different order of OAM are propagated through (simulated) atmospheric turbulence, using seven different strengths. The distorted beam is recorded using a CCD camera and the images are processed to determine their beam size, intensity, and on-axis scintillation. The least square method which is the mathematical analysis to determine the beam size and scintillation are explained in this thesis. The effect of the strength of atmospheric turbulence on different orders of LG beams and the behavior of different OAM mode in same atmospheric turbulence has been analyzed using these beam parameters. In addition, experimental results are compared with simulation results, which were obtained using a split-step method. Results have shown that in different strengths of atmospheric turbulence, the beam size shows similar behavior, and that a low OAM mode, due to its smaller beam size, is hardly detectable in stronger atmospheric turbulence. However, larger order OAM modes tend to be more resistant to atmospheric turbulence due to a larger beam size, it gives relatively low intensity fluctuations and high intensity contrast.

Keywords

free-space optical communication, orbital angular momentum, atmospheric turbulence, Laguerre-Gaussian beam

Rights Statement

Copyright © 2023, Author

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