Baghsiahi, SH; (2012) Multimode optical waveguides and lightguides for backplane interconnection and laser illuminated display systems. Doctoral thesis , UCL (University College London).

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Abstract

The aim of the research in this thesis was to design, model, analyse and experimentally test multimode optical waveguides and lightguides for manipulating infrared light for optical backplane interconnections and visible light for laser illuminated display systems. Optical Input/Output Coupling loss at the entry and exit of polymer waveguides depends on optical scattering due to end facet roughness. The input/output coupling loss was measured for different end facet roughness magnitudes and the waveguide surface profiles due to different cutting methods (dicing saw and three milling routers) were compared. The effect of the number of cutting edges on the router, the rotation rate and translation (cutting) speed of the milling routers on the waveguide end facet roughness was established. A further new method for reducing the end facet roughness and so the coupling loss, by curing a layer of core material at the end of the waveguide to cover the roughness fluctuations, was proposed and successfully demonstrated giving the best results reported to date resulting in an improvement of 2.8 dB, even better than those obtained by use of index matching fluid which is impractical in commercial systems. The insertion loss due to waveguide crossing having various crossing angles was calculated using a beam propagation method and ray tracing simulations and compared to experimental measurements. Differences between the results were resolved leading to an understanding that only low order waveguide modes at no more than 6 degrees to the axis were propagating inside the waveguide. Several different optical designs of multimode waveguide for the light engine of a 3D autostereoscopic laser illuminated display system were proposed. Each design performed the functions of laser beam combining, beam shaping and beam homogenizing and the best method was selected, designed, modelled, tested, and implemented in the system. The waveguide material was inspected using spectroscopy to establish the effect of high power optical density on the material performance showing an increased loss particularly in the shorter wavelengths. The effect of waveguide dimensions on the speckle pattern was investigated experimentally and the speckle contrast was reduced to below the threshold of human perception. Speckle contrast was also recorded for the first time along the axis of the 3D display system and normal to it in the viewing area and the speckle characteristics at each stage were investigated. New algorithms for analysing speckle were used and the perceptual ability of human eyes to detect speckle size and contrast were taken into account to minimise perceived speckle patterns. The effect of the core diameter of optical fibres on the speckle pattern was investigated and it was shown that the speckle spot diameter is reduced by increasing the fibre core diameter. Based on this experiment, it was suggested that speckle reduction is more effective if the optical fibre used in the display system has larger diameter. Therefore, a slab waveguide of 1 mm thickness and 20 m width was used for laser beam combining, homogenising and beam shaping and a uniformity of 84% was achieved with just 75 mm length. The speckle was also completely removed at the output of the waveguide

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