Angle -of -arrival fluctuations of optical waves in the atmospheric surface layer
When an optical wave propagates through atmosphere, the wave experiences amplitude, phase, and angle-of-arrival (AOA) fluctuations which are mainly caused by the refractive-index fluctuations of the atmosphere. Thus, the wave fluctuations carry characteristics of the atmosphere. In this dissertation, the AOA fluctuations are studied theoretically and experimentally.
For the theoretical part, closed-form solutions of the AOA fluctuations for plane and spherical waves observed by a receiver with a finite aperture were developed. It was assumed that the waves propagate through homogeneous and isotropic media and that the Rytov approximation is valid. The existing closed-form solutions of the AOA fluctuations for the waves are valid only for the cases that the aperture size of the receiver is much larger or much smaller than the Fresnel length. The closed-form solutions developed in this dissertation, however, are valid for all ratios between the aperture diameter and the Fresnel length. The closed-form solutions were compared with the numerical solutions and the accuracy of the closed-form solutions is less than 0.2%.
For the experimental part, remote sensing of wind speed transverse to a propagation path using the frequency spectra of the AOA fluctuations was performed with a telescope and a CCD camera. The knee frequency, the intersection of the -2/3 and -8/3 power laws of the spectrum, is a function of wind speed and an effective baseline. If the knee frequency and the effective baseline are known, the transverse wind speed can be retrieved, and if the knee frequency and the transverse wind speed are known, the effective baseline can be estimated. From the measured knee frequency of the spectra of the AOA fluctuations and the aperture size of the telescope as initial guess for the effective baseline, the path-averaged transverse wind speed was retrieved. The effective baseline was calibrated based on wind speed measured by a anemometer. The rms difference between the path-averaged calibrated wind speed retrieved from the frequency spectra of the AOA fluctuations with 30 s of estimation time and the 30 s time-averaged transverse wind speed measured by the anemometer was 11 cm/s -1, while the wind speed varied between 0 and 80cm/s-1.
0799: Remote sensing