Airborne Measurement of Atmospheric-Induced Beam Jitter

When a laser beam propagates through a field of varying index of refraction, its wavefront becomes optically distorted. The distortions imposed onto the wavefront resulting from these index-of-refraction non-uniformities can be categorized into the mean, tip, and tilt (tip/tilt) components, as well as variations over the laser beam diameter referred to as higher-order disturbances. While higher-order disturbances cause the laser beam to diffract and spread energy out in the far-field, tip/tilt causes the beam to deflect from its original path. This is caused by propagating the laser through optical turbulence structures larger than or on the order of the size of the beam diameter. The dynamic, net deflection of the beam in the far-field caused by tip/tilt is referred to as jitter. Beam jitter poses a serious problem for tracking in directed energy applications. The work presented in this dissertation sought to make in-flight experimental measurements of the tip/tilt imposed onto a laser beam by the atmosphere. The measurement of tip/tilt is challenging since the optical distortions caused by mechanical contamination in the form of aircraft platform vibration and aerodynamic buffeting also manifest as tip/tilt. A procedure referred to as the stitching method was used to remove the influence of mechanical contamination and quantify the atmospheric-induced component of tip/tilt. In order to generalize the results, the strength of the atmospheric optical turbulence environment through which the beam propagated was also quantified. From these results, it was found that the tip/tilt imposed onto the beam aligned with what analytic solutions predict and that the optical turbulence environment had Kolmogorov-like characteristics. Using both simulated and experimentally-measured turbulence-induced tilt time series, realistic tilt compensation approaches could be studied. It was shown that current approaches underpredict the bandwidth required to sufficiently compensate for tip/tilt imposed onto the beam. Based on the developments presented in this dissertation, suggestions are provided on how tracking can be improved in realistic digital control systems.