Scattering of guided waves in thick gratings at extreme angles

Kurth, Martin Lyndon (2006) Scattering of guided waves in thick gratings at extreme angles. Masters by Research thesis, Queensland University of Technology.

Abstract

The aim of this project was to develop a passive optical compensating arrangement that would allow the formation and continued stability of interference patterns over a long timescale and also to investigate optical wave scattering in thick gratings at extreme angles of scattering. A novel passive arrangement based on a Sagnac interferometer is described that produces interference patterns more stable than those produced by a conventional arrangement. An analysis of the arrangement is presented that shows it to be an order of magnitude more stable than an equivalent conventional approach. The excellent fringe stability allowed holographic gratings with small periods (~ 0.5 μm) to be written in photorefractive lithium niobate with low intensity writing fields (~mW/cm2) produced by a He:Ne laser, despite long grating fabrication times (~ 1000 s). This was possible because the optical arrangement compensated for phase shifts introduced by translational and rotational mirror motion caused by environmental perturbations. It was shown that the rapid introduction of a phase shift in one of the writing fields can change the direction of energy flow in the two-wave mixing process. It was found that the improvement in stability of the modified Sagnac arrangement over a conventional interferometer decreased when the crossing angle was increased and that the point about which the mirrors are rotated greatly affects the stability of the arrangement. For a crossing angle of 12 degrees, the modified Sagnac arrangement is more than twice as stable when the mirrors are rotated about their midpoints, rather than their endpoints. Investigations into scattering in the extremely asymmetrical scattering (EAS) geometry were undertaken by scattering light from a 532nm Nd:YAG laser off gratings written in photorefractive barium titanate and lithium niobate. Despite the difficulties posed by background noise, there was very good agreement between the observed scattered field and that predicted by a previously established theoretical model. Thus, this work represents the first experimental observation of EAS in the optical part of the spectrum.

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ID Code: 16347
Item Type: QUT Thesis (Masters by Research)
Supervisor: Jaatinen, Esa & Gramotnev, Dmitri
Keywords: holographic grating fabrication, Sagnac interferometer, photorefractive effect, extremely asymmetrical scattering, passive stabilisation, stable interference patterns
Divisions: Past > QUT Faculties & Divisions > Faculty of Science and Technology
Past > Schools > School of Physical & Chemical Sciences
Department: Faculty of Science
Institution: Queensland University of Technology
Copyright Owner: Copyright Martin Lyndon Kurth
Deposited On: 03 Dec 2008 04:01
Last Modified: 22 Mar 2016 06:33

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