A real-time turbulence simulator

Abstract

Optical communication and imaging systems that benefit from the transmission and detection of light propagated through the atmosphere have become essential for nu merous practical applications, e.g., for long distance communication, LIDAR systems or imaging. In many applications light is used for target tracking and distance mea surements or for telescopes used in astronomical observations. However, the irregular motion of air can distort optical light fields, therefore hindering the performance of optical systems. This happens due to density fluctuations which result in refractive index fluctuations that randomly perturb the optical phase of the light. The result is near field phase fluctuations, which in severe cases may lead to far field intensity variations, giving rise to unwanted distortions. For this reason, the analysis of such effects on optical systems remains topical and of practical relevance. In this dissertation we will focus on the impact of atmospheric turbulence on the transmission of spatial modes of light. We will explore techniques for characteris ing optical turbulence and simulating its phase distortions in the lab environment. We use the simulated turbulence to study its impact on laser beams propagating in freespace and turbulence. The laser beams studied here are eigen-modes of freespace that can carry orbital angular momentum (OAM). The modes are associated with spatially inhomogeneous polarisation fields, known as vector vortex modes, having spatial profiles that are characterised by the Laguerre-Gaussian (LG) modes. We will discuss the principle of generating and detecting such spatial modes by tailoring the dynamic phase of the spatial mode of light using Liquid Crystal (LC) displays and digital micromirror devices (DMDs). Subsequently, we study how both the polarisa tion and spatial components are affected by turbulence using the same tools. Finally, iii we will introduce a device called the sonic anemometer to extract velocity data, and we will use this data to calculate parameters that quantify the levels of atmospheric distortions due to optical turbulence. A modern digital micromirror device (DMDs) will then be used to execute a turbulence simulator and show that using digitally encoded phase screens we can accurately mimic realistic turbulence.