Laser beam propagation through a turbid heated medium: analysis of the positional impacts of the heat source
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ORIGINAL PAPER
Laser beam propagation through a turbid heated medium: analysis of the positional impacts of the heat source N Chetty1, B Adeleye1, A Kumar1,2* and G Enoch1 1
School of Chemistry and Physics, University of KwaZulu-Natal, Pietermaritzburg 3201, South Africa 2
School of Physics and Material Sciences, Shoolini University, Solan 173212, India Received: 06 November 2019 / Accepted: 03 April 2020
Abstract: The propagation characteristics of laser beams through the atmosphere are severely hampered by temperature fluctuations, which result in fluctuations of the refractive index in the air along its pathway. In this work, we analysed the effect of fluctuations formed from a thermal turbulence source on a propagating laser beam for two different positions of the thermal source. An experimental technique to quantify the positional impact of the thermal source on a 532 nm Gaussian laser beam deviations (wander) and profile (spreading) fluctuations under laboratory thermal conditions such as temperature and pressure were setup. The laser beam propagated through a point diffraction interferometer (PDI) before it was incident on a detector. Optical wavefront data of the major atmospheric characteristics such as the turbulence strength, Rytov variance (scintillation) and the coherence diameter (Fried’s parameter) were successfully extracted from the produced interferograms. The index structure constant showed strong turbulence (Cn2 C 10-13 m-2/3), and it was stronger when the turbulence source is below the beam. The Rytov variances were in the weak scintillation range ðr2R 1Þ probably due to the short propagation distances. These results reveal that the location of the turbulence generator in the vicinity of a laser beam’s propagation path may strongly impact the physical characteristics including the beam wander, beam profile and intensity distribution. Keywords: Thermal turbulence; Positional impact; Laser beam; Turbulence strength List of symbols Refractive index structure function (m-2/3) Cn2 Temperature structure function (K2 m-2/3) CT2 DS Phase structure–function k Wavenumber L Propagation path length (m) P Atmospheric pressure (millibars) PDI Point diffraction interferometer PID Proportional integral derivative–– 2 Rytov variance rR r0 Coherence diameter (m)
r T1 and T2
Distance between the two reference points (m) Temperatures at two reference points (Kelvin)
1. Introduction Atmospheric optics are one of the branches of optics that describe light transmission, absorption, emission, refraction and reflection by the atmosphere [1, 2]. This includes an understanding of various optical phenomena such as the blue colour of the sky, rainbows, twinkling of stars and mirages involving visible and near-visible radiation as well as the propagation and distortion of electromagnetic signals through the air [2]. The propagation of light through the atmosphere depends to large extents on the physical composition of the atmosphere several optical interaction phenomena and the frequent changes in the atmosphere du
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