The apparent heterogeneity of the national stem landscape : does it reflect reality or is it an illusion?

Kurtz K. Miller


Experimental data obtained from recently conducted long-range laser beam propagation experiments has revealed inconsistencies with analytic and numeric simulations results based on classical Kolmogorov turbulence theory. This inconsistency may be related with not accounting for refraction effects caused by refractive index variation with elevation and presence of large-scale atmospheric structures which introduce refractive index gradients and can alter the trajectory of optical wave energy flux. In this thesis, atmospheric refraction effects are studied using a ray tracing technique. Due to refraction a ray propagating in the atmosphere doesn't follow a straight line and may not arrive to a desired location. In this thesis the ray tracing technique was applied for analysis of optical propagation over a 150 km propagation path. It was shown that due to refraction the ray trajectory may deviate from the geometric straight line by 60m in the middle of the path. We also considered the impact of refraction on atmospheric propagation of laser beams with different wavelengths (λ=0.532µm, λ=1.064µm, and λ=1.550µm) which were launched at the same angle. Due to the difference in refractive index of air for different wavelengths, the ray's paths follow different trajectories. It was shown that at the end of the propagation path, the distance between ray trajectories can be as long as ~4.1m for the 0.532µm and the 1.064µm rays, and ~4.3m for 0.532µm and 1.550µm rays. Besides traditional ray tracing technique we also introduced a new computational method that allows analysis of combined refraction and turbulence effects on laser beam propagation. In this method, traditional beam propagation using the well-known split step operator method is combined with ray tracing. In this technique the atmospheric volume is represented as a set of thin phase screens that obey Kolmogorov turbulence statistics. The ray tracing technique is applied to describe optical wave propagation between phase screens. At each screen, the turbulence-induced random tip and tilt wave-front phase component is added to the ray angle. In this way, the ray trajectory is no longer deterministic, but it has a turbulence induced uncertainty. It was shown that at the end of a 150km propagation path, the turbulence induced deviation on ray trajectory can be on the order of 5m. These results show that for correct analysis of laser beam propagation over long distances in the atmosphere, refraction and turbulence effects should be considered jointly. The proposed numerical simulation technique allows this joint analysis.