http://hdl.handle.net/10204/918 2013-06-19T13:26:38Z http://hdl.handle.net/10204/6780 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 http://hdl.handle.net/10204/6775 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 http://hdl.handle.net/10204/6774 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 http://hdl.handle.net/10204/6693 Title: Fingerprint pores extractor Authors: Mngenge, NA; Nelufule, NN; Nelwamondo, FV; Msimang, M Abstract: Automatic Fingerprint Recognition Systems (AFRSs) rely on minutiae position and orientation within the fingerprint image for matching. Minutiae information is highly accurate provided that the fingerprint image matched is of high quality. However, this is not always the case because of diseases and hash working conditions that affect fingerprints. In order to maintain high level of security independent of varying fingerprint image quality research suggests the use of other fingerprint features to compliment minutiae. These are things like ridge contours, sweat pores, dots, and incipient ridges. Sweat pores have been proven as one of the most distinctive among these feature. Thus in order to improve accuracy of AFRSs pores can be fused with minutiae or used alone. Sweat pores have been less utilized in the past due to constraints imposed by fingerprint scanning devices and resolution standards. Recently, progress has been made on both scanning devices and resolution standards to support the use of pores in AFRSs. However, very few techniques exist for extracting, matching and fusing them with minutiae. Matching and fusion can only be possible if pores are available. Some techniques have been proposed to reliable extract pores. However, existing techniques can only work on one resolution i.e. an algorithm proposed and tested on 500dpi cannot work on 1000dpi without minor modifications because pores size change if resolution changes. In addition, existing pore extraction techniques are computationally expensive. In this paper an algorithm to extract feature level 3 (pores) is proposed. The algorithm uses Laplacian of Gaussian (LoG) in Fourier domain in order to reduce computation. The performance of the proposed algorithm is tested on two distinct databases with different resolutions in order to validate its accuracy. The accuracy of the proposed algorithm is further measured using false detection rate (FDR) and true detection rate (TDR). Results show that FDR ranges from 10-35% while TDR ranges from 65-90%. Description: 2012 National Conference on Computing and Communication Systems, Durgapur, West Bengal, India, 21- 22 November 2012. To be published in IEEE Xplore 2012-11-01T00:00:00Z