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O. Wallner:
"Modal Filtering of Optical Waves";
Supervisor, Reviewer: W. Leeb, P. Winzer; Institut für Nachrichtentechnik und Hochfrequenztechnik, 2004.

This thesis investigates theoretical and practical aspects of modal wavefront filtering. It also introduces an interferometric method for measuring the performance of a modal wavefront filter based on a single-mode waveguide and describes practical testing of prototype fibers in the mid-infrared.
Modal filtering of optical waves is a type of wavefront filtering which relies on the unique properties of single-mode waveguides. As, by definition, single-mode waveguides only propagate the fundamental mode, they provide -- at their output -- a field with predefined transverse distribution which is independent of the input field. The input field only determines the output field`s complex amplitude. Modal wavefront filtering therefore allows for equalizing optical fields with respect to the transverse profile which, e.g., is of particular importance in astronomical interferometry.
The first part of this thesis is concerned with theoretical aspects of wavefront filtering in general and modal wavefront filtering in particular. We elaborate the fundamental difference between spatial wavefront filtering and modal wavefront filtering. The first is blocking of spatial frequency components of a field by, e.g. focussing the field onto a pinhole, the latter is projecting the field onto a field with predefined transverse amplitude and phase distribution, e.g. by coupling the field into a single-mode waveguide. We discuss the essential requirements on modal wavefront filters which are uniqueness and spatial stationarity of the transverse outputfield distribution. In view of practical application of modal wavefront filters, we investigate broadband performance and the significance of the coupling phase. We compare the waveguiding properties of single-mode waveguides, such as step-index fibers, integrated optical waveguides, and photonic crystal fibers, and of lossy multi-mode waveguides, like hollow dielectric and hollow metallic fibers, which -- at least in theory -- can serve as modal wavefront filters. Today modal wavefront filters are only available at telecommunications wavelengths, i.e. for wavelengths between 0.6\µm and 1.55µm. However, astronomical applications ask for an operation between 4µm and 20µm, hence we review materials and fabrication technologies for mid-infrared fibers. We conclude the theoretical part with the analysis of a multi-axial single-mode beam recombination scheme, a setup incorporating the functionality of beam recombination into the modal wavefront filter.
The second part is devoted to wavefront filter performance evaluation and describes measurements with prototype silver-halide fibers in the mid-infrared. We review several conventional methods which allow to determine whether a waveguide is single-mode or not and propose an interferometric measurement to determine the waveguides` filter action, i.e. the increase in interferometer output power contrast by applying a modal wavefront filter. An increase in output power contrast by orders of magnitudes can only be achieved with single-mode waveguides. The setup allows to quantify the waveguide`s modal wavefront filter performance. We describe a corresponding measurement setup which incorporates a Mach-Zehnder interferometer operating at a wavelength of 10.6µm and discuss the results obtained for prototype silver-halide fibers, developed within an ESA-funded project and to be used as wavefront filter for the DARWIN mission.