Experimental measurement of the fluctuations of a laser beam due to thermal turbulence.
In this work, we considered and developed a new method to detect and quantify the fluctuations of a laser beam due to thermal turbulence. The new method consisted of a single laser beam propagating in air and passing through a point diffraction interferometer (PDI). Stable interferograms were thus formed by diffraction of light at the PDI pinhole. Such interferograms underwent phase shifts due to the application of simulated thermal turbulence on the propagating laser beam. These phase shifts were then used to obtain atmospheric turbulence parameters such as the atmospheric turbulence strength, temperature near the propagating beam and the scintillation index. Chapter 1 of this thesis is an introduction and discussion of the theory on the propagation of laser beams in air. Gaussian beam propagation, turbulence detectors, Rytov’s theory and Kolmogorov’s theory of turbulence is also discussed in detail. artefact descriptive experimental procedure is then provided. This chapter focuses on the behavior of a laser beam propagating under the conditions of weak turbulence and relates the Rytov weak fluctuations to the Kolmogorov spectrum since the Rytov variance can be exactly equal to the scintillation index under the conditions of weak turbulence. Two unpublished scientific papers were submitted for publication to the Canadian Journal of Physics and Europian Journal of Remote Sensing. Chapter 2 consist of paper 1 which is based on the development of the experiment and it describes the apparatus in detail as well as it explains the experimental procedure. The preliminary results presented in paper 1 showed that a PDI can produce stable interferograms that can be used to extract the atmospheric turbulence parameters and thus, the PDI method can be used for atmospheric detection and ranging. In chapter 3, we discussed and analysed the experimental results, where the phase shifts were used to estimate the temperature that caused the purturbations on the interferograms. In chapter 4, we concluded about the use of a PDI as a remote sensing technique.