TITLE: Correlations of Optical with Non-Optical Measurements of
Turbulence for the Airborne Laser Program
AUTHOR: James H. Brown
SCHOOL: Wellesley High School
SCHOOL ADDRESS: 50 Rice Street, Wellesley, MA 02181
The United States Air Force plans to deploy high-energy lasers on 747 Boeing jetliners. During an engagement, the 747 would detect a missile launch with advanced radar and fire the laser beam at the missile target. The energy from the laser beam would fracture the casing of the missile and explode (U.S. GAO report, "Theater Missile Defense, Significant Technical Challenges Face the Airborne Laser (ABL) Program", October, 1997: 13.) A major factor in determining the success of the ABL to destroy a missile will be the Air Force's ability to predit (and hopefully correct or mitigate) the beam size and energy at the target. A major problem is beam spreading due to optical turbulence, which is caused mainly by small temperature variations in the atmosphere (thermal turbulence). Temperature variations refract or bend rays of light thus spreading the energy of the beam. If the laser beam spread too much the concentration of the energy will be insufficient to penetrate the target thus making the ABL ineffective. The GAO recommended that the Air Force measure the correlation between optical and non-optical turbulence. Once the spread is predicted accurately, the Air Force could use adaptive optics to correct the spread or increase the energy of the laser sufficiently to accomplish the mission. Given the GAO recommendation, the object of this study was to determine the correlation between the spot size of the laser beam with differences in the temperature of the atmosphere through which the laser beams passes, even when all the fine scale temperature variations are not well known. The experiment consisted of having a narrow laser beam propagate through air heated in a tube into air at ambient temperatures. Results indicate a positive correlation between optical turbulence (beam spread) and the temperature gradient. A "power law" function was found to fit the data. In fact, the correlation is represented by u r = 0.2 u T0.71, where u r represents the change in beam radium and u T represents the change in temperature. Raw data, scaled data, empirical fits, error estimates, and confidence levels are presented. Results indicate that the ABL program can predict the beam spread, and therefore energy loss, by measuring atmospheric temperature gradients along the propagation path.