published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

The operation of a grating lateral shear heterodyne interferometer as a wavefront sensor for atmospherically perturbed wavefronts is analyzed. A novel wavefront sensor design is given and its feasibility is proven by laboratory experiments. The applications in mind are closed-loop active optical systems for compensating atmospheric perturbations and open-loop atmospheric wavefront measuring device. The optical properties of the turbulent atmosphere are summed up and the resulting wavefront sensor requirements are given. Among them are the property of sell-referencing, high white light efficiency, independence of scintillation effects, and high spatial and temporal sampling rates. Then the general heterodyne grating shearing interferometer is introduced. A description of the phase measurement by the heterodyne process in the frequency domain has been derived. The heterodyne process is interpreted as convolutions of the signal with a pair of filter functions, which isolate a particular harmonic term of the signal and provide its phase. The representation of the convolutions in the frequency domain provides an elegant way to analyse the systematic errors of the heterodyning with general, non-sinusoidal signals. Also the random phase errors of the heterodyne process have been determined using Gaussian error propagation. An algorithm is derived to carry out the wavefront reconstructions from the measured differences on a square array of discrete data points. It is based on a modal expansion in complex exponentials, leading to a simple filtering operation in the spatial frequency domain. The algorithm provides unbiased reconstructions over the finite data set. It has minimal error propagation in a least squares sense. It is computationally efficient in that the number of operations required for a reconstruction is approximately proportional to the number of wavefront points, if a Fast-Fourier-Transform algorithm is used. Finally, a compact wavefront sensor design is described fulfilling the requirements posed by the turbulent atmosphere. It determines wavefronts at 24 by 24 points at a sampling rate of 60 Hz. A rms-wavefront error of better than λ/20 can be achieved with astronomical light sources of sixth stellar magnitude. Laboratory experiments proved the feasibility of the design.

M.Ing. / Atmospheric turbulence (also referred to as optical or heat Scintillation, or heat shimmer) is a particular problem encountered in video surveillance, especially over distances where the target object focused on is over lkm in the distance. Images obtained from video surveillance are commonly required to be of a high quality for object identification and classification. Atmospheric turbulence causes degradation in the image quality through the blurring and a warping of the image, making object identification difficult. Algorithms have and still are being developed to suppress the image turbulence in digital video footage and enhance detail. There is a lack of reliable comparisons among algorithms to provide research direction, methods for identification of the best algorithms for particular applications, identification of useful image processing techniques and a full understanding of the problem. This need and lack of comparisons among the algorithms and atmospheric turbulence degraded videos is identified through the problem identification chapter. A literature study is undertaken in which the source of atmospheric turbulence and models are identified, image processing techniques discussed, filtering of electromagnetic waves reviewed, a review of some equipment, and a discussion of metrics. This is followed by the presentation of a number of atmospheric turbulence suppression algorithms developed by other authors. After a discussion of the algorithm implementations, the experimental design is described for algorithm image quality and performance investigation as well as the effect of optical filters. Experimental results are presented and discussed which provide repeatable results pertaining to the algorithms' image quality and processing requirements. The results allowed identification of the algorithms' strengths and weaknesses, how they compare, and their suitability for real and post processing environments. Efficient performing software components were also able to be identified, particularly Illuminance-Reflectance adjustment. The experiments and results provide a solution to this atmospheric turbulence comparison problem.

Atmospheric turbulence results taken on the Antarctic plateau are presented in this thesis. Covering two high sites: South Pole and Dome C, this work describes their seeing and meteorological conditions. Using an acoustic sounder to study the turbulence profile of the first kilo- metre of the atmosphere and a Differential Image Motion Monitor (DIMM) to investigate the integrated seeing we are able to deduce important at- mospheric parameters such as the Fried parameter (r0) and the isoplanatic angle (??0). It was found that at the two sites, the free atmosphere (above the first kilometer) was extremely stable and contributed between 0.2" and 0.3" of the total seeing with no evidence of jet or vortex peaks of strong turbulence. The boundary layer turbulence is what differentiates the two sites. Located on the Western flank of the plateau, the South Pole is prone to katabatic winds. Dome C on the other hand is on a local maximum of the plateau and the wind conditions are amongst the calmest in the world. Also linked to the topography is the vertical extent of the temperature in- version that is required to create optical turbulence. At the South Pole the inversion reaches 300 m and only 30 m at Dome C. This difference results in relatively poor seeing conditions at the South Pole (1.8") and excellent at Dome C (0.27"). The strong correlation between the seeing and the ground layer meteorological conditions indicates that even better seeing could be found at Dome A, the highest point of the plateau. Having most of the turbulence near the ground is also incredibly ad- vantageous for adaptive optics. The isoplanatic angle is respectively 3.3" and 5.7" for the South Pole and Dome C. This is significantly larger than at temperate sites where the average isoplanatic angle rarely exceeds 2". This means that wider fields can be corrected without the complication of conjugation to specific layers. For such purpose the potential is even more interesting. We show that ground conjugated adaptive optics would decrease the natural seeing to 0.22" for a wide field of 10 and 0.47" for a field of 1" at the South Pole. At Dome C the results are less impressive due to the already excellent seeing, but a gain of 0.1&quote can still be achieved over 1&quote. These results show that high angular resolution observations can be done better on the Antarctic plateau than any other known site.

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Lian-Ping Wang, Dept. of Mechanical Engineering. Includes bibliographical references.

Thesis (M.S.)--Oregon Graduate Center, 1976.

Ph.D. / Applied Physics & Electronic Science / Direct detection, photon counting receivers for use in optical communication through the clear air, turbulent atmosphere have been examined. In particular, spatial diversity arrays to overcome the effect off adding due to atmospheric turbulence are considered. Experimental results are compared with theoretical results for an optimum receiver structure based upon Bayes criterion of minimum probability of error. In addition, certain suboptimum receivers with simpler structures are derived directly from the optimum receiver. These receivers, along with an adaptive threshold receiver, are considered in order to examine the trade off between performance and complexity. The results indicate that the adaptive threshold receiver is a good compromise for information rates that are high with respect to scintillation frequencies, a relatively unrestrictive condition.

Thesis (Ph.D.)--North Carolina State University, 2008. / Title from PDF title screen. "01 JUL 2008"--Report documentation page. NTIS no: ADA 486101 Includes bibliographical references (p. 175-181). Also available online.

Thesis--M. Phil., University of Hong Kong, 1984.