http://hdl.handle.net/10204/455 2013-06-19T07:21:23Z 2013-06-19T07:21:23Z Schulze, C Flamm, D Dudley, A Forbes, A Duparre, M http://hdl.handle.net/10204/6780 2013-06-12T21:55:28Z 2013-02-01T00:00:00Z

Title: Modal decomposition for measuring the orbital angular momentum density of light Authors: Schulze, C; Flamm, D; Dudley, A; Forbes, A; Duparre, M Abstract: We present a novel technique to measure the orbital angular momentum (OAM) density of light. The technique is based on modal decomposition, enabling the complete reconstruction of optical fields, including the reconstruction of the beams Poynting vector and the OAM density distribution. The modal decomposition is performed using a computer-generated hologram (CGH), which allows fast and accurate measurement of the mode spectrum. The CGH encodes the modes of interest, whose powers and relative phase differences are measured from the far-field diffraction pattern of the illuminating optical field with the hologram transmission function. In combination with a classical measurement of Stokes parameters, including a polarizer and a quarter-wave plate in front of the hologram, the polarization state of each mode is measured. As a consequence, any arbitrary vector field can be reconstructed, including amplitude, phase, and polarization. Having all information on the optical field, the Poynting vector and the OAM density can be calculated directly. We applied our method to beams emerging from optical fibers, which allows us to investigate arbitrary coherent superposition of fiber modes with complexly shaped intensity and polarization distributions. The excitation of certain mode mixtures is done by appropriate input coupling and using diffractive phase masks to shape the input beam and hence enhance the excitation efficiency of distinct modes. The accuracy of the achieved results is verified by comparing the reconstructed with the directly measured beam intensity, revealing excellent agreement. Description: Proceedings of SPIE, San Francisco (USA), 3-6 February 2013. Published in SPIE Digital library

2013-02-01T00:00:00Z Dudley, A Litvin, I Roux, FS Forbes, A http://hdl.handle.net/10204/6775 2013-06-12T21:55:28Z 2013-02-01T00:00:00Z

Title: Complete azimuthal decomposition of optical fields Authors: Dudley, A; Litvin, I; Roux, FS; Forbes, A Abstract: By using digital holograms, we present a simple technique for performing a complete azimuthal decomposition of an arbitrary laser mode. The match-filter, used to perform the azimuthal decomposition, is bounded by an annular ring, allowing us to conduct a scale-independent decomposition on our selected mode. This technique therefore requires no prior knowledge of the mode structure, the mode phases, or the amplitude distribution. A basis comprising of the angular harmonics is used to express the spatial distribution of the selected mode in terms of spatially dependant coefficients. We use this to infer directly from the measured weightings of the azimuthally decomposed modes and their phase-delay measurements, the intensity of the selected field, its phase, and its orbital angular momentum (OAM) density. We illustrate the concept by executing a full decomposition of two examples: a superposition of two Bessel beams, with relative phase differences, and an off-axis vortex mode. We show a reconstruction of the amplitude, phase and OAM density of these fields with a high degree of accuracy. Description: Proceedings of SPIE, San Francisco (USA), 3-6 February 2013. Published in SPIE Digital library.

2013-02-01T00:00:00Z Mhlanga, T Dudley, A McDonald, A Roux, FS Lavery, M Padgett, M Forbes, A http://hdl.handle.net/10204/6774 2013-06-12T21:55:28Z 2013-02-01T00:00:00Z

Title: Efficient sorting of Bessel beams [Conference paper] Authors: Mhlanga, T; Dudley, A; McDonald, A; Roux, FS; Lavery, M; Padgett, M; Forbes, A Abstract: A procedure to efficiently sort orbital angular momentum (OAM) states of light, by performing a Cartesian to log-polar coordinate transformation which translates helically phased beams into a transverse phase gradient, currently exists1. We implement this mode transformer, which comprises of two custom refractive optical elements2, to efficiently sort Bessel beams carrying OAM. Introducing two cylindrical lenses, allows the focusing of each of the input OAM Bessel states to a different lateral position in the Fourier plane and separates the radial wave-vectors in the image-plane. We demonstrate the concept by separating over forty OAM states and radial wave-vectors. Description: SPIE Proceedings, Complex Light and Optical Forces VII, San Francisco (USA), 3-6 February 2013. Published in SPIE Digital library

2013-02-01T00:00:00Z Mahamood, RM Akinlabi, ET Shukla, M Pityana, RM http://hdl.handle.net/10204/6749 2013-05-28T21:55:15Z 2012-01-01T00:00:00Z

Title: Revolutionary additive manufacturing: an overview Authors: Mahamood, RM; Akinlabi, ET; Shukla, M; Pityana, RM Abstract: Consumer demands are moving away from standardized to customized products, as such, the evolution of alternative manufacturing technique has become imperative. Additive manufacturing (AM) is a process of building components layer by layer as against the traditional methods which are subtractive in nature. Though AM offers lots of advantages over traditional manufacturing techniques, its wide application is still however at the infancy phase. Despite all the benefits derived from AM technology, there are still a lot of unresolved issues with the technology that has hindered its performances thereby limiting its application to high tolerant jobs. This paper takes a look at some important AM technologies, some problems currently facing AM technology at large and proposes some solutions to these problems. A major known drawback in AM is poor dimensional accuracy and poor surface finish, only the layer height and melt pool temperature are controlled to solve this problem in the literature. The stair-stepping effect in adaptive manufacturing is rooted in a natural phenomenon of surface tension which is the cause of the poor surface finish and in combination with other factors is responsible for the poor dimensional accuracy. An adaptive controller is proposed for removing stair-stepping effect to improve the dimensional accuracy, the surface finish and the mechanical properties of the components. Successful implementation of these proposed controllers will greatly improve the performances of AM technologies and also aids its wide application for end use products. Further research works are also suggested to improve the overall AM performances. Description: Copyright: 2012 Old City Publishing. This is an ABSTRACT ONLY. The definitive version will be published in Lasers in Engineering, pp 1-18

2012-01-01T00:00:00Z