Impact of Measured Turbulence on Laser Beam Propagation

Abstract

This study investigates the effects of atmospheric turbulence on laser beam propagation. It compares the effects of Kolmogorov turbulence to those of measured turbulence. A previous experiment took measurements of atmospheric turbulence using thermosondes beneath a weather balloon. The data shows elevated layers of increased turbulence strength. Within each layer, the power spectral density of the turbulence exhibits a power law differing from the -11/3 power law of Kolmogorov theory. To understand how these departures from theory impact optical propagation, we import the turbulence data into a wave-optics model. Additionally, we simulate two common types of turbulence measurement devices to test their usefulness against the non-Kolmogorov turbulence. One device is a scintillometer, while the other measures the Modulation Transfer Function (MTF). These simulated measurements produce a much simpler and lower resolution description of the turbulence compared to the data from the thermosondes, which we consider to be truth in this work. The wave-optics model uses the traditional split-step propagation method with phase screens that represent the turbulence for each segment of the path. We generate the phase screens using either the thermosondes' truth measurements or the Kolmogorov statistics derived from the scintillometer and the MTF method. We propagate laser beams and point sources through both types of phase screens and compare the results. The metrics for the comparisons include scintillation index, Strehl ratio, and power-in-the-bucket. The results show that the Kolmogorov turbulence prescribed by the MTF method underestimates the strength of the turbulence. On the other hand, the scintillometer prescribes turbulence that is a good surrogate for the true non-Kolmogorov turbulence. These results will lead to a better understanding of optical turbulence, how it can be measured, and how it affects optical beams during propagation.