ResearchSpace :: Browsing by Author "Naidoo, Darryl"

Browsing by Author "Naidoo, Darryl"

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Amplification of higher-order Poincaré sphere beams
(SPIE, 2019-09) Naidoo, Darryl; Sroor, H; Litvin, I; Forbes, A
The Higher order Poincar´e sphere (HOPS) is generally used to describe scalar and vector orbital angular momentum modes. These modes have found many applications to date; however, they are limited to low power levels. It has thus become topical to consider amplification of such structured light modes. Here, we study the purity of the HOPS beams in a master oscillator power amplifier configuration using recently developed characterization tools through birefringent and non-birefringent amplifiers. We outline a general theory for this problem where we consider both gain and vector perspectives, and confirm our theory by experiment.
Brightness enhancement in a solid-state laser by mode transformation
(Optical Society of America, 2018-07) Naidoo, Darryl; Litvin, Ihar; Forbes, A
Laser brightness is a measure of the ability to deliver intense light to a target and encapsulates both the energy content and the beam quality. High-brightness lasers require that both parameters be maximized, yet standard laser cavities do not allow this. For example, multimode beams, a mix of many transverse modes, have a high energy content but low beam quality, while single transverse mode Gaussian beams have a good beam quality, but their small mode volume means a low energy extraction. Here we overcome this fundamental limitation and demonstrate an optimal approach to realizing high-brightness lasers. We employ intra-cavity beam shaping to produce a single transverse mode that changes profile inside the cavity, Gaussian at the output end and flattop at the gain end, such that both energy extraction and beam quality are simultaneously optimized. This work should have a significant influence on the design of future high-brightness laser cavities.
Brightness enhancement in a solid-state laser by mode transformation
(Optical Society of America, 2018-07) Naidoo, Darryl; Litvin, Ihar; Forbes, A
Laser brightness is a measure of the ability to deliver intense light to a target and encapsulates both the energy content and the beam quality. High-brightness lasers require that both parameters be maximized, yet standard laser cavities do not allow this. For example, multimode beams, a mix of many transverse modes, have a high energy content but low beam quality, while single transverse mode Gaussian beams have a good beam quality, but their small mode volume means a low energy extraction. Here we overcome this fundamental limitation and demonstrate an optimal approach to realizing high-brightness lasers. We employ intra-cavity beam shaping to produce a single transverse mode that changes profile inside the cavity, Gaussian at the output end and flattop at the gain end, such that both energy extraction and beam quality are simultaneously optimized. This work should have a significant influence on the design of future high-brightness laser cavities.
Characterising laser beams with liquid crystal displays
(SPIE, 2016-09) Dudley, Angela L; Naidoo, Darryl; Forbes, A
We show how one can determine the various properties of light, from the modal content of laser beams to decoding the information stored in optical fields carrying orbital angular momentum, by performing a modal decomposition. Although the modal decomposition of light has been known for a long time, applied mostly to pattern recognition, we illustrate how this technique can be implemented with the use of liquid-crystal displays. We show experimentally how liquid crystal displays can be used to infer the intensity, phase, wavefront, Poynting vector, and orbital angular momentum density of unknown optical fields. This measurement technique makes use of a single spatial light modulator (liquid crystal display), a Fourier transforming lens and detector (CCD or photo-diode). Such a diagnostic tool is extremely relevant to the real-time analysis of solid-state and fibre laser systems as well as mode division multiplexing as an emerging technology in optical communication.
Constructing “petal” modes from the coherent superposition of Laguerre-Gaussian modes
(SPIE, 2011-01) Naidoo, Darryl; Forbes, A; Aït-Ameur, K; Brunel, M
An experimental approach in generating Petal-like transverse modes, which are similar to what is seen in porro-prism resonators, has been successfully demonstrated. We hypothesize that the petal-like structures are generated from a coherent superposition of Laguerre-Gaussian modes of zero radial order and opposite azimuthal order. To verify this hypothesis, visually based comparisons such as petal peak to peak diameter and the angle between adjacent petals are drawn between experimental data and simulated data. The beam quality factor of the Petal-like transverse modes and an inner product interaction is also experimentally compared to numerical results.
Controlled generation of higher-order Poincaré sphere beams from a laser
(Nature Publishing Group, 2016-03) Naidoo, Darryl; Roux, Filippus S; Dudley, Angela L; Litvin, Igor; Piccirillo, B; Marrucci, L; Forbes, A
The angular momentum of light can be described by positions on a higher-order Poincaré sphere, where superpositions of spin and orbital angular momentum states give rise to laser beams that have many applications, from microscopy to materials processing. Many techniques exist to create such beams but none so far allow their creation at the source. Here we report on a new class of laser that is able to generate all states on the higher-order Poincaré sphere. We exploit geometric phase control inside a laser cavity to map polarization to orbital angular momentum, demonstrating that the orbital angular momentum degeneracy of a standard laser cavity may be broken, producing pure orbital angular momentum beams, and that generalized vector vortex beams may be created with high purity at the source. This work paves the way to new lasers for structured light based on intracavity geometric phase control.
Demonstrating optical aberrations in the laboratory
(South African Institute of Physics, 2009-07) Naidoo, Darryl; Mafusire, C; Forbes, A
An optical aberration is a distortion of an image as compared to the object due to defects in an optical system
Detecting defects during powder deposition in additive manufacturing
(2018-11) Hendriks, Adriaan J; Ramokolo, Lesiba R; Ngobeni, Christopher M; Moroko, Matome C; Naidoo, Darryl
Additive manufacturing applications, in areas such as aerospace and medicine, are limited due to the lack of process stability and quality management. In particular, geometrical inaccuracies and the presence of mechanical defects hinder repeatability of the process. To break into industries with very high quality standards, an important issue to be addressed is in-situ quality control during a build. The work which will be presented here is focused on image based process monitoring of the powder bed after the deposition of a new powder layer. We will also discuss the effects these might have on consolidating the powder with the rest of the part. Preliminary results will be shown of defects identified after a new powder layer has been deposited.
Digital holograms for laser mode multiplexing
(SPIE, 2014-10-02) Mhlanga, T; Trichili, A; Dudley, Angela L; Naidoo, Darryl; Zghal, M; Forbes, A
High-capacity data transmission has been implemented using single channel optical systems. This technique is limited and soon it will be unable to ful ll the growing needs for higher bit rate data transmission. Hence multi-mode transmission has been recently given attention as a potential solution to the current problems. In this context, we demonstrate a method of multiplexing laser modes using spatial light modulators (SLMs). In our proposed technique, we use Laguerre Gaussian (LG) modes, which form a complete basis set; hence multi-mode masks can be created by taking a linear combination of the LG modes. Since LG modes are characterised by two degrees of freedom, the azimuthal index ` and radial index , this allows for multi-dimensional states. There are however some experimental challenges which include the sensitivity of the setup to misalignment, that leads to mode-coupling. It is also important that the injected modes ha a uniform power spectrum so that are weighted equally. The size of the multi-modes is highly dependent on the resolution of the SLM.
Doughnut laser beam as an incoherent superposition of two petal beams
(Optical Society of America, 2014-02) Litvin, IA; Ngcobo, Sandile; Naidoo, Darryl; Ait-Ameur, K; Forbes, A
Laguerre–Gaussian beams with a nonzero azimuthal index are known to carry orbital angular momentum (OAM), and are routinely created external to laser cavities. The few reports of obtaining such beams from laser cavities suffer from inconclusive evidence of the real electromagnetic field. In this Letter we revisit this question and show that an observed doughnut beam from a laser cavity may not be a pure Laguerre–Gaussian azimuthal mode but can be an incoherent sum of petal modes, which do not carry OAM. We point out the requirements for future analysis of such fields from laser resonators.
Fractal light from lasers
(American Physical Society, 2019-01) Sroor, H; Naidoo, Darryl; Miller, SW; Nelson, J; Courtial, J; Forbes, A
Fractals, complex shapes with structure at multiple scales, have long been observed in Nature: as symmetric fractals in plants and sea shells, and as statistical fractals in clouds, mountains and coastlines. With their highly polished spherical mirrors, laser resonators are almost the precise opposite of Nature, and so it came as a surprise when, in 1998, transverse intensity cross-sections of the eigenmodes of unstable canonical resonators were predicted to be fractals [Karman et al., Nature 402, 138 (1999)]. Experimental verification has so far remained elusive. Here we observe a variety of fractal shapes in transverse intensity cross-sections through the lowest-loss eigenmodes of unstable canonical laser resonators, thereby demonstrating the controlled generation of fractal light inside a laser cavity. We also advance the existing theory of fractal laser modes, first by predicting 3D self-similar fractal structure around the centre of the magnified self-conjugate plane, second by showing, quantitatively, that intensity cross-sections are most self-similar in the magnified self-conjugate plane. Our work offers a significant advance in the understanding of a fundamental symmetry of Nature as found in lasers.
The generation of flat-top beams by complex amplitude modulation with a phase-only spatial light modulator
(SPIE, 2012-08) Hendriks, A; Naidoo, Darryl; Roux, FS; Löpez-Mariscal, C; Forbes, A
Phase-only spatial light modulators are now ubiquitous tools in modern optics laboratories, and are often used to generate so-called structured light. In this work we outline the use of a phase-only spatial light modulator to achieve full complex amplitude modulation of the light, i.e., in amplitude and phase. We outline the theoretical concept, and then illustrate its use with the example of the laser beam shaping of Gaussian beams into flat-top beams. We quantify the performance of this approach for the creation of such fields, and compare the results to conventional lossless approaches to flat-top beam generation.
Generation of shape-invariant flat-top laser beams
(SPIE, 2015-02) Ait-Ameur, K; Naidoo, Darryl; Ngcobo, Sandile; Fromager, M; Litvin, I; Hasnaoui, A; Harfouche, A; Forbes, A
A great number of laser applications need in place of the usual Gaussian beam a flat-top intensity profile in the focal plane of a focusing lens. In general the transformation of the laser beam from the Gaussian to the flat-top shape is made by a diffractive beam shaping technique. It is worthwhile to note that this transformation occurs in the vicinity of the focal plane. If a flat top laser beam keeping its shape during propagation is needed then this can be obtained by a weighted incoherent mixing of LG(sub0)0 and LG(sub01) eigenmodes. Here, we consider the generation of these two transverse modes by a solid-state laser axially pumped by a laser diode. The idea is to design the laser cavity so as to make identical the losses of LG(sub00) and LG(sub01) modes. To reach this objective we have used two techniques. The first one called as diffractive lies to insert an adequate amplitude mask inside the cavity. The second one called as interferometric consisted to couple the laser to an external cavity. It is important to note that LG(sub00) and LG(sub01) modes are not spatially in concurrence, i.e. the peak of the LG(sub00) appears in the dip of the LG(sub01) mode. As a result, the energy extraction from the amplifying medium is improved increasing thus the laser slope efficiency. Theory and experimental verifications have been done for the diffractive and interferometric techniques allowing the generation of a flat-top laser beam keeping its shape from the near-field to the far-field.
Improving the laser brightness of a commercial laser system
(SPIE, 2017-02) Naidoo, Darryl; Litvin, Ihar; Forbes, A
We investigate the selection of a flat-top beam and a Gaussian beam inside a laser cavity on opposing mirrors. The concept is tested external to the laser cavity in a single pass and double pass regime where the latter mimics a single round trip in the laser. We implement this intra-cavity selection through the use of two 16 level diffractive optical elements. We consider a solid-state diode side-pumped laser resonator in a typical commercial laser configuration that consists of two planar mirrors where the DOEs are positioned at the mirrors. We out couple the Gaussian and flat-top distributions and we show that we improve the brightness of the laser with active mode control. We also demonstrate that the quality of the beam transformations determine the brightness improvement.
Intra-cavity decomposition of a dual-directional laser beam
(SPIE, 2011-01) Naidoo, Darryl; Forbes, A; Aït-Ameur, K; Fromager, M
A method of decomposing a dual-directional laser beam into a forward propagating field and a backward propagating field for an apertured plano-concave cavity is presented. An intra-cavity aperture is a simple method of laser beam shaping as higher-order transverse modes are discriminated. Two fundamental resonator theories, namely, Fox-Li and Laguerre-Gaussian decomposition are used in the determination of the respective beam profiles at a specific plane. A preliminary set-up is characterized for Gaussian propagation in an attempt to verify that the cavity is viable. A comparison of experimental data with the theories is presented.
Intra-cavity generation of superpositions of Laguerre-Gaussian beams
(Springer Verlag, 2012) Naidoo, Darryl; Aït-Ameur, K; Brunel, M; Forbes, A
In this paper we demonstrate experimentally the intra-cavity generation of a coherent superposition of Laguerre–Gaussian modes of zero radial order but opposite azimuthal order. The superposition is created with a simple intra-cavity stop that creates equal losses for the two azimuthal modes, and we show that by adjustment of the stop we can produce modes up to azimuthal order 8. The fact that we have a coherent superposition rather than an incoherent superposition is verified by intensity measurements, propagation measurements and a decomposition of the field by an inner product executed on a phase-only spatial light modulator. Such fields have relevance in quantum information and optical trapping.
Intra-cavity metamorphosis of a Gaussian beam to flat-top distribution
(SPIE Proceedings, 2014-02) Naidoo, Darryl; Litvin, I; Laskin, A; Forbes, A
We explore an intra-cavity beam shaping approach to generate a Gaussian distribution by the metamorphosis of a Gaussian beam into a flat-top distribution on opposing mirrors. The concept is tested external to the cavity through the use of two spatial light modulators (SLM), where the first SLM is used to transform a collimated Gaussian into a flat-top distribution and the second SLM is encoded with the conjugate phase of the flat-top for conversion back to a Gaussian. We implement this intra-cavity selection through the use of two optical elements of the refractive variant that are designed from the phase profiles addressed to the SLMs. We consider a solid-state diode side-pumped laser resonator that consists of two planar mirrors where the refractive optics are positioned at the mirrors. We out couple the Gaussian and show that the output beam size is comparable with the theoretical predictions and that we have an increase in optical brightness when compared to the cavity without any optics.
Intra-cavity vortex beam generation
(SPIE, 2011-08) Naidoo, Darryl; Aït-Ameur, K; Forbes, A
In this paper the authors explore vortex beams and in particular the generation of single LG0l modes and superpositions thereof. Vortex beams carry orbital angular momentum (OAM) and this intrinsic property makes them prevalent in transferring this OAM to matter and to be used in quantum information processing. The authors explore an extra-cavity and intra-cavity approach in LG0l mode generation respectively. The outputs of a Porro-prism resonator are represented by “petals” and they show that through a full modal decomposition, the “petal” fields are a superposition of two LG0l modes.
Laguerre Gaussian beam multiplexing through turbulence
(SPIE, 2014-08-17) Trichili, A; Mhlanga, T; Naidoo, Darryl; Dudely, A; Zghal, M; Forbes, A
We analyze the effect of atmospheric turbulence on the propagation of multiplexed Laguerre Gaussian modes. We present a method to multiplex Laguerre Gaussian modes using digital holograms and decompose the resulting field after encountering a laboratory simulated atmospheric turbulence. The proposed technique makes use of a single spatial light modulator for the generation of superimposed beam and a second spatial light modulator and a CCD camera for the modal decomposition. The obtained results demonstrate how sensitive the Laguerre Gaussian beams are to atmospheric distortions.
Layer-wise powder deposition defect detection in additive manufacturing
(SPIE, 2019-03) Hendriks, Adriaan J; Ramokolo, Lesiba R; Ngobeni, Christopher M; Moroko, Matome C; Naidoo, Darryl
Additive manufacturing applications, in areas such as aerospace and medicine, are limited due to the lack of process stability and quality management. In particular, geometrical inaccuracies and the presence of mechanical defects hinder repeatability of the process1. A great disadvantage of AM is that verifying the quality of AM produced parts are mainly done after part fabrication which does not allow the operator to act upon defects observed during the actual build. To break into industries with very high quality standards, an important issue to be addressed is in-situ quality control during a build2, 3. If defects on a new powder layer can be detected before laser melting occurs, a new layer may be suitably recoated or the process can be paused for user controlled rectification. The work which will be presented here is focused on image based process monitoring of a powder bed additive manufacturing system using a shadow casting method. As a proof of principle, a few main defects during recoating will be identified and analyzed to establish the severity and possible impact of the defects on metal powder consolidation. Preliminary results of defects identified before and after material consolidation will be shown. For this, a software package is in development to automatically detect defects. This is aimed towards developing a system which in the future will contribute to quality assurance.

Browsing by Author "Naidoo, Darryl"

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