@article{Peters2023,
abstract = {Vectorial structured light, where the polarization is inhomogeneously distributed in space, has found a myriad of applications in both 2D and 3D optical fields. Here, we present an experimental study of the invariance and distortion of vectorial light through a real-world medium of atmospheric turbulence. We show that the amplitude and polarization structure are both severely distorted by the turbulent medium, yet the non-separability of these two degrees of freedom remains invariant. We monitor this invariance under a range of beam types and atmospheric conditions, over extended time periods, revealing the unitary nature of atmospheric turbulence in our experiment. Our results provide conclusive evidence that invariance and distortion are not mutually exclusive and that the degree of classical entanglement remains unaltered through such channels, and will be of interest to the large community interested in classical and quantum communication in free space.},
author = {Peters, Cade and Cox, Mitchell and Drozdov, Alice and Forbes, Andrew},
doi = {10.1063/5.0152065},
issn = {0003-6951},
journal = {Applied Physics Letters},
month = {jul},
number = {2},
pages = {21103},
publisher = {American Institute of Physics Inc.},
title = {{The invariance and distortion of vectorial light across a real-world free space link}},
url = {https://pubs.aip.org/apl/article/123/2/021103/2901625/The-invariance-and-distortion-of-vectorial-light},
volume = {123},
year = {2023}
}

Vectorial structured light, where the polarization is inhomogeneously distributed in space, has found a myriad of applications in both 2D and 3D optical fields. Here, we present an experimental study of the invariance and distortion of vectorial light through a real-world medium of atmospheric turbulence. We show that the amplitude and polarization structure are both severely distorted by the turbulent medium, yet the non-separability of these two degrees of freedom remains invariant. We monitor this invariance under a range of beam types and atmospheric conditions, over extended time periods, revealing the unitary nature of atmospheric turbulence in our experiment. Our results provide conclusive evidence that invariance and distortion are not mutually exclusive and that the degree of classical entanglement remains unaltered through such channels, and will be of interest to the large community interested in classical and quantum communication in free space.

@article{Niwareeba2023,
abstract = {In direct current optical orthogonal frequency division multiplexing (DCO-OFDM) systems, the high peak-to-average power ratio (PAPR) has been a significant challenge. Recently, lexicographical symbol position permutation (LSPP) using random permutations has been introduced as an efficient solution to reduce high PAPR. In this paper, we aim to evaluate the effectiveness of LSPP by comparing both adjacent and interleaved lexicographical permutation sequences with random lexicographical permutation sequences. Our findings demonstrate that random permutation yields superior PAPR reduction performance results when compared to adjacent and interleaved permutation. However, in scenarios with a limited number of sub-blocks, the use of adjacent and interleaved permutation becomes more favorable, as they can eliminate the possibility of generating identical permutation sequences, a drawback of random permutation. Additionally, we propose a novel algorithm to determine the optimal number of candidate permutation sequences that can achieve acceptable PAPR reduction performance while adhering to computational complexity constraints defined by the system requirements.},
author = {Niwareeba, Roland and Cox, Mitchell A and Cheng, Ling},
doi = {10.3390/electronics12132797},
issn = {2079-9292},
journal = {Electronics},
keywords = {CCDF,DCO,OFDM,PAPR,VLC,lexicographical,permutations},
month = {jun},
number = {13},
pages = {2797},
publisher = {Multidisciplinary Digital Publishing Institute},
title = {{Adaptive-Mode PAPR Reduction Algorithm for Optical OFDM Systems Leveraging Lexicographical Permutations}},
url = {https://www.mdpi.com/2079-9292/12/13/2797/htm https://www.mdpi.com/2079-9292/12/13/2797},
volume = {12},
year = {2023}
}

In direct current optical orthogonal frequency division multiplexing (DCO-OFDM) systems, the high peak-to-average power ratio (PAPR) has been a significant challenge. Recently, lexicographical symbol position permutation (LSPP) using random permutations has been introduced as an efficient solution to reduce high PAPR. In this paper, we aim to evaluate the effectiveness of LSPP by comparing both adjacent and interleaved lexicographical permutation sequences with random lexicographical permutation sequences. Our findings demonstrate that random permutation yields superior PAPR reduction performance results when compared to adjacent and interleaved permutation. However, in scenarios with a limited number of sub-blocks, the use of adjacent and interleaved permutation becomes more favorable, as they can eliminate the possibility of generating identical permutation sequences, a drawback of random permutation. Additionally, we propose a novel algorithm to determine the optimal number of candidate permutation sequences that can achieve acceptable PAPR reduction performance while adhering to computational complexity constraints defined by the system requirements.

@inproceedings{cox2023fio,
address = {Washington, D.C.},
author = {Cox, Mitchell A. and Dindar, Mikaeel and Iga, Fortune Kayala},
booktitle = {Frontiers in Optics + Laser Science 2023 (FiO, LS)},
doi = {10.1364/FIO.2023.JTu4A.72},
isbn = {978-1-957171-29-6},
pages = {JTu4A.72},
publisher = {Optica Publishing Group},
title = {{Low-cost Free-Space Optical Communications with Commodity Hardware}},
url = {https://opg.optica.org/abstract.cfm?URI=FiO-2023-JTu4A.72},
year = {2023}
}

@article{Singh2023,
abstract = {The use of nonlinear optics for the creation, control, and detection of structured light has gained prominence of late, but it is plagued by low efficiency and variable modal purity. Here, we show how to optimize the efficiency and modal purity in the nonlinear conversion of structured light modes. Using difference frequency generation of orbital angular momentum carrying Laguerre–Gaussian modes as an example, we achieved more than a 40% rise in efficiency while maintaining the modal purity above 90% compared to conventional methods. To demonstrate the versatility of our approach, we show the frequency conversion of perfect vortex beams up to ℓ =40 in a topological charge, achieving a new state of the art. We believe this theoretical treatment will facilitate our work's translation to other modal forms and nonlinear processes.},
author = {Singh, Sachleen and Sephton, Bereneice and Morland, Imogen and Cox, Mitchell A. and Leach, Jonathan and Forbes, Andrew and Buono, Wagner T},
doi = {10.1364/JOSAB.501917},
issn = {0740-3224},
journal = {Journal of the Optical Society of America B},
month = {dec},
number = {12},
pages = {3128},
title = {{Frequency conversion of orbital angular momentum with optimized efficiency and modal purity}},
url = {https://opg.optica.org/abstract.cfm?URI=josab-40-12-3128},
volume = {40},
year = {2023}
}

The use of nonlinear optics for the creation, control, and detection of structured light has gained prominence of late, but it is plagued by low efficiency and variable modal purity. Here, we show how to optimize the efficiency and modal purity in the nonlinear conversion of structured light modes. Using difference frequency generation of orbital angular momentum carrying Laguerre–Gaussian modes as an example, we achieved more than a 40% rise in efficiency while maintaining the modal purity above 90% compared to conventional methods. To demonstrate the versatility of our approach, we show the frequency conversion of perfect vortex beams up to ℓ =40 in a topological charge, achieving a new state of the art. We believe this theoretical treatment will facilitate our work's translation to other modal forms and nonlinear processes.

@article{Cox2023,
abstract = {This Lab Note introduces the “Stokes Camera,” a simple experimental arrangement for real-time measurement of spatial amplitude and polarization and thus spatially resolved Stokes parameters. It uses a polarization sensitive camera and a fixed quarter-wave plate, providing a one-shot, digital solution for polarization measurement that is only limited by the frame rate of the camera and the computation speed of the provided code. The note also provides background information on relevant polarization theory and vector vortex beams, which are used as a demonstration of the device.},
author = {Cox, Mitchell A. and Rosales-Guzm{\'{a}}n, Carmelo},
doi = {10.1364/AO.504249},
issn = {1559-128X},
journal = {Applied Optics},
month = {oct},
number = {29},
pages = {7828},
title = {{Real-time Stokes polarimetry using a polarization camera}},
url = {https://opg.optica.org/abstract.cfm?URI=ao-62-29-7828},
volume = {62},
year = {2023}
}

This Lab Note introduces the “Stokes Camera,” a simple experimental arrangement for real-time measurement of spatial amplitude and polarization and thus spatially resolved Stokes parameters. It uses a polarization sensitive camera and a fixed quarter-wave plate, providing a one-shot, digital solution for polarization measurement that is only limited by the frame rate of the camera and the computation speed of the provided code. The note also provides background information on relevant polarization theory and vector vortex beams, which are used as a demonstration of the device.

@article{Briantcev2023rnn,
abstract = {Among the problems that prevent free-space optical communication systems from becoming a truly mainstream technology is beam wander, which is especially important for structured light beams since beam misalignment introduces additional crosstalk at the receiver. The paper suggests a recurrent neural network-based (RNN) solution to predict beam wander in free space optics (FSO). The approach uses past beam center of mass positions to predict future movement, significantly outperforming various prediction types. The proposed approach is demonstrated using under-sampled experimental data over a 260 m link as a worst-case and over-sampled simulated data as a best-case scenario. In addition to conventional Gaussian beams, Hermite- and Laguerre-Gaussian beam wander is also investigated. With a 20 to 40% improvement in error over naive and linear predictions, while predicting multiple samples ahead in typical situations and overall matching or outperforming considered predictions across all studied scenarios, this method could help mitigate turbulence-induced fading and has potential applications in intelligent re-transmits, quality of service, optimized error correction, maximum likelihood-type algorithms, and predictive adaptive optics.},
author = {Briantcev, Dmitrii and Cox, Mitchell A. and Trichili, Abderrahmen and Ooi, Boon S. and Alouini, Mohamed-Slim},
doi = {10.1364/OE.496690},
issn = {1094-4087},
journal = {Optics Express},
month = {aug},
number = {18},
pages = {28859},
pmid = {37710696},
title = {{Beam wander prediction with recurrent neural networks}},
url = {https://opg.optica.org/abstract.cfm?URI=oe-31-18-28859},
volume = {31},
year = {2023}
}

Among the problems that prevent free-space optical communication systems from becoming a truly mainstream technology is beam wander, which is especially important for structured light beams since beam misalignment introduces additional crosstalk at the receiver. The paper suggests a recurrent neural network-based (RNN) solution to predict beam wander in free space optics (FSO). The approach uses past beam center of mass positions to predict future movement, significantly outperforming various prediction types. The proposed approach is demonstrated using under-sampled experimental data over a 260 m link as a worst-case and over-sampled simulated data as a best-case scenario. In addition to conventional Gaussian beams, Hermite- and Laguerre-Gaussian beam wander is also investigated. With a 20 to 40% improvement in error over naive and linear predictions, while predicting multiple samples ahead in typical situations and overall matching or outperforming considered predictions across all studied scenarios, this method could help mitigate turbulence-induced fading and has potential applications in intelligent re-transmits, quality of service, optimized error correction, maximum likelihood-type algorithms, and predictive adaptive optics.

@article{Singh2023,
abstract = {The use of nonlinear optics for the creation, control, and detection of structured light has gained prominence of late, but it is plagued by low efficiency and variable modal purity. Here, we show how to optimize the efficiency and modal purity in the nonlinear conversion of structured light modes. Using difference frequency generation of orbital angular momentum carrying Laguerre–Gaussian modes as an example, we achieved more than a 40% rise in efficiency while maintaining the modal purity above 90% compared to conventional methods. To demonstrate the versatility of our approach, we show the frequency conversion of perfect vortex beams up to ℓ =40 in a topological charge, achieving a new state of the art. We believe this theoretical treatment will facilitate our work's translation to other modal forms and nonlinear processes.},
author = {Singh, Sachleen and Sephton, Bereneice and Morland, Imogen and Cox, Mitchell A. and Leach, Jonathan and Forbes, Andrew and Buono, Wagner T},
doi = {10.1364/JOSAB.501917},
issn = {0740-3224},
journal = {Journal of the Optical Society of America B},
month = {dec},
number = {12},
pages = {3128},
title = {{Frequency conversion of orbital angular momentum with optimized efficiency and modal purity}},
url = {https://opg.optica.org/abstract.cfm?URI=josab-40-12-3128},
volume = {40},
year = {2023}
}

The use of nonlinear optics for the creation, control, and detection of structured light has gained prominence of late, but it is plagued by low efficiency and variable modal purity. Here, we show how to optimize the efficiency and modal purity in the nonlinear conversion of structured light modes. Using difference frequency generation of orbital angular momentum carrying Laguerre–Gaussian modes as an example, we achieved more than a 40% rise in efficiency while maintaining the modal purity above 90% compared to conventional methods. To demonstrate the versatility of our approach, we show the frequency conversion of perfect vortex beams up to ℓ =40 in a topological charge, achieving a new state of the art. We believe this theoretical treatment will facilitate our work's translation to other modal forms and nonlinear processes.

@article{Ng2023,
abstract = {A brief account of photonics research activities in the selected countries in the Middle East and Africa is presented in this article. Though not comprehensive, we hope to provide a glimpse of the research landscape in the region, and the collaboration and connection with each other and the international partners.},
author = {Ng, Tien Khee and Rjeb, Alaaeddine and Cox, Mitchell A. and Cordette, Steevy J. and Wan, Yating and Ashry, Islam and Gan, Qiaoqiang and Fratalocchi, Andrea and Ohkawa, Kazuhiro and Ooi, Boon S.},
doi = {10.1109/JPHOT.2023.3337312},
issn = {19430655},
journal = {IEEE Photonics Journal},
keywords = {Biomedical optical imaging,Optical devices,Optical fiber communication,Optical fiber sensors,Optical sensors,Optics,Photonics,globalization},
number = {1},
pages = {1--9},
publisher = {IEEE},
title = {{Globalization in Photonics Research and Development}},
volume = {16},
year = {2023}
}

A brief account of photonics research activities in the selected countries in the Middle East and Africa is presented in this article. Though not comprehensive, we hope to provide a glimpse of the research landscape in the region, and the collaboration and connection with each other and the international partners.

@article{Sephton2023,
abstract = {Information exchange between two distant parties, where information is shared without physically transporting it, is a crucial resource in future quantum networks. Doing so with high-dimensional states offers the promise of higher information capacity and improved resilience to noise, but progress to date has been limited. Here we demonstrate how a nonlinear parametric process allows for arbitrary high-dimensional state projections in the spatial degree of freedom, where a strong coherent field enhances the probability of the process. This allows us to experimentally realise quantum transport of high-dimensional spatial information facilitated by a quantum channel with a single entangled pair and a nonlinear spatial mode detector. Using sum frequency generation we upconvert one of the photons from an entangled pair resulting in high-dimensional spatial information transported to the other. We realise a d = 15 quantum channel for arbitrary photonic spatial modes which we demonstrate by faithfully transferring information encoded into orbital angular momentum, Hermite-Gaussian and arbitrary spatial mode superpositions, without requiring knowledge of the state to be sent. Our demonstration merges the nascent fields of nonlinear control of structured light with quantum processes, offering a new approach to harnessing high-dimensional quantum states, and may be extended to other degrees of freedom too.},
author = {Sephton, Bereneice and Vall{\'{e}}s, Adam and Nape, Isaac and Cox, Mitchell A. and Steinlechner, Fabian and Konrad, Thomas and Torres, Juan P. and Roux, Filippus S. and Forbes, Andrew},
doi = {10.1038/s41467-023-43949-x},
isbn = {4146702343949},
issn = {20411723},
journal = {Nature Communications},
number = {1},
pages = {1--9},
pmid = {38092724},
publisher = {Springer US},
title = {{Quantum transport of high-dimensional spatial information with a nonlinear detector}},
volume = {14},
year = {2023}
}

Information exchange between two distant parties, where information is shared without physically transporting it, is a crucial resource in future quantum networks. Doing so with high-dimensional states offers the promise of higher information capacity and improved resilience to noise, but progress to date has been limited. Here we demonstrate how a nonlinear parametric process allows for arbitrary high-dimensional state projections in the spatial degree of freedom, where a strong coherent field enhances the probability of the process. This allows us to experimentally realise quantum transport of high-dimensional spatial information facilitated by a quantum channel with a single entangled pair and a nonlinear spatial mode detector. Using sum frequency generation we upconvert one of the photons from an entangled pair resulting in high-dimensional spatial information transported to the other. We realise a d = 15 quantum channel for arbitrary photonic spatial modes which we demonstrate by faithfully transferring information encoded into orbital angular momentum, Hermite-Gaussian and arbitrary spatial mode superpositions, without requiring knowledge of the state to be sent. Our demonstration merges the nascent fields of nonlinear control of structured light with quantum processes, offering a new approach to harnessing high-dimensional quantum states, and may be extended to other degrees of freedom too.

@article{Briantcev2022,
abstract = {We present a fast and efficient simulation method of structured light free space optics (FSO) channel effects from propagation through a turbulent atmosphere. In a system that makes use of multiple higher order modes (structured light), turbulence causes crosstalk between modes. This crosstalk can be described by a channel matrix, which usually requires a complete physical simulation or an experiment. Current simulation techniques based on the phase-screen approximation method are very computationally intensive and are limited by the accuracy of the underlying models. In this work, we propose to circumvent these limitations by using a data-driven approach for the decomposition matrix simulation with a conditional generative adversarial network (CGAN) synthetic simulator.},
author = {Briantcev, Dmitrii and Cox, Mitchell A. and Trichili, Abderrahmen and Drozdov, Alice V. and Ooi, Boon S. and Alouini, Mohamed-Slim},
doi = {10.1364/OE.448899},
issn = {1094-4087},
journal = {Optics Express},
month = {feb},
number = {5},
pages = {7238},
publisher = {The Optical Society},
title = {{Efficient channel modeling of structured light in turbulence using generative adversarial networks}},
url = {https://opg.optica.org/abstract.cfm?URI=oe-30-5-7238},
volume = {30},
year = {2022}
}

We present a fast and efficient simulation method of structured light free space optics (FSO) channel effects from propagation through a turbulent atmosphere. In a system that makes use of multiple higher order modes (structured light), turbulence causes crosstalk between modes. This crosstalk can be described by a channel matrix, which usually requires a complete physical simulation or an experiment. Current simulation techniques based on the phase-screen approximation method are very computationally intensive and are limited by the accuracy of the underlying models. In this work, we propose to circumvent these limitations by using a data-driven approach for the decomposition matrix simulation with a conditional generative adversarial network (CGAN) synthetic simulator.

@article{Niwareeba2022,
author = {Niwareeba, Roland and Cox, Mitchell A. and Cheng, Ling},
doi = {10.1109/ACCESS.2021.3138193},
issn = {2169-3536},
journal = {IEEE Access},
pages = {1706--1713},
publisher = {IEEE},
title = {{PAPR Reduction in Optical OFDM Using Lexicographical Permutations With Low Complexity}},
url = {https://ieeexplore.ieee.org/document/9662343/},
volume = {10},
year = {2022}
}

@inproceedings{Drozdov2022,
abstract = {Structured light fields are highly susceptible to various aberrations, and so there is a large amount of research into the effects of turbulence on these fields and into methods of mitigating these effects. Much of this research is performed using simulated turbulence. Although simulated turbulence is useful, it is not always sufficient and sometimes a turbulent free-space link is required. Setting up such a link requires several nuanced considerations and potentially time-consuming steps. Tailored for beginners, in this tutorial-style paper we document our practical experience in building structured light free-space links.},
author = {Drozdov, Alice and Cox, Mitchell A.},
booktitle = {Complex Light and Optical Forces XVI},
doi = {10.1117/12.2626881},
editor = {Andrews, David L. and Galvez, Enrique J. and Rubinsztein-Dunlop, Halina},
isbn = {9781510649057},
issn = {1996756X},
keywords = {Atmospheric turbulence,Digital micromirror devices,Free space,Holograms,Mirrors,Receivers,Spatial light modulators,Structured light,Transmitters,Turbulence},
month = {mar},
pages = {201},
publisher = {SPIE},
title = {{Practical modal decomposition over turbulent free-space links}},
url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/12017/2626881/Practical-modal-decomposition-over-turbulent-free-space-links/10.1117/12.2626881.full},
volume = {12017},
year = {2022}
}

Structured light fields are highly susceptible to various aberrations, and so there is a large amount of research into the effects of turbulence on these fields and into methods of mitigating these effects. Much of this research is performed using simulated turbulence. Although simulated turbulence is useful, it is not always sufficient and sometimes a turbulent free-space link is required. Setting up such a link requires several nuanced considerations and potentially time-consuming steps. Tailored for beginners, in this tutorial-style paper we document our practical experience in building structured light free-space links.

@article{Niwareeba2022a,
author = {Niwareeba, Roland and Cox, Mitchell A. and Cheng, Ling},
doi = {10.1016/j.icte.2021.10.002},
issn = {24059595},
journal = {ICT Express},
keywords = {aco ofdm,ccdf,companding,papr,selective mapping},
month = {mar},
number = {1},
pages = {72--76},
publisher = {Elsevier B.V.},
title = {{Low complexity hybrid SLM for PAPR mitigation for ACO OFDM}},
url = {https://doi.org/10.1016/j.icte.2021.10.002 https://linkinghub.elsevier.com/retrieve/pii/S2405959521001363},
volume = {8},
year = {2022}
}

@article{Cox2022interf,
abstract = {Orbital angular momentum (OAM) modes are topical due to their versatility, and they have been used in several applications including free-space optical communication systems. The classification of OAM modes is a common requirement, and there are several methods available for this. One such method makes use of deep learning, specifically convolutional neural networks, which distinguishes between modes using their intensities. However, OAM mode intensities are very similar if they have the same radius or if they have opposite topological charges, and as such, intensity-only approaches cannot be used exclusively for individual modes. Since the phase of each OAM mode is unique, deep learning can be used in conjugation with interferometry to distinguish between different modes. In this paper, we demonstrate a very high classification accuracy of a range of OAM modes in turbulence using a shear interferometer, which crucially removes the requirement of a reference beam. For comparison, we show only marginally higher accuracy with a more conventional Mach–Zehnder interferometer, making the technique a promising candidate towards real-time, low-cost modal decomposition in turbulence.},
author = {Cox, Mitchell A. and Celik, Turgay and Genga, Yuval and Drozdov, Alice V.},
doi = {10.1364/AO.444954},
issn = {1559-128X},
journal = {Applied Optics},
month = {mar},
number = {7},
pages = {D1},
title = {{Interferometric orbital angular momentum mode detection in turbulence with deep learning}},
url = {https://www.osapublishing.org/abstract.cfm?URI=ao-61-7-D1},
volume = {61},
year = {2022}
}

Orbital angular momentum (OAM) modes are topical due to their versatility, and they have been used in several applications including free-space optical communication systems. The classification of OAM modes is a common requirement, and there are several methods available for this. One such method makes use of deep learning, specifically convolutional neural networks, which distinguishes between modes using their intensities. However, OAM mode intensities are very similar if they have the same radius or if they have opposite topological charges, and as such, intensity-only approaches cannot be used exclusively for individual modes. Since the phase of each OAM mode is unique, deep learning can be used in conjugation with interferometry to distinguish between different modes. In this paper, we demonstrate a very high classification accuracy of a range of OAM modes in turbulence using a shear interferometer, which crucially removes the requirement of a reference beam. For comparison, we show only marginally higher accuracy with a more conventional Mach–Zehnder interferometer, making the technique a promising candidate towards real-time, low-cost modal decomposition in turbulence.

@article{Nghatsane2021,
author = {Nghatsane, Xongile and Cheng, Ling and Cox, Mitchell A.},
doi = {10.1364/AO.414175},
issn = {1559-128X},
journal = {Applied Optics},
month = {mar},
number = {7},
pages = {1801},
title = {{Fritchman modeling for correlated turbulence-induced errors in FSO communication}},
url = {https://opg.optica.org/abstract.cfm?URI=ao-60-7-1801},
volume = {60},
year = {2021}
}

@inproceedings{Cox2021,
author = {Cox, Mitchell A. and Trichili, Abderrahmen and Ooi, Boon S. and Alouini, Mohamed-Slim},
booktitle = {2021 IEEE International Conference on Communications Workshops (ICC Workshops)},
doi = {10.1109/ICCWorkshops50388.2021.9473576},
isbn = {978-1-7281-9441-7},
month = {jun},
pages = {1--6},
publisher = {IEEE},
title = {{Higher-Order Spatial Modes in Turbulence: Alternatives to Orbital Angular Momentum}},
url = {https://ieeexplore.ieee.org/document/9473576/},
year = {2021}
}

@article{Cox2021dmdmod,
abstract = {Digital micro-mirror devices (DMDs) are a popular alternative to liquid crystal spatial light modulators for laser beam shaping due to their relatively low cost, high speed, and polarization and wavelength independence. Here we describe in detail how to convert a low-cost digital light projector (DLP) evaluation module that uses a Texas Instruments DLP4710 DMD into a spatial light modulator using a 3D printed mount. The resulting device is shown to accurately shape Laguerre-Gauss modes, is able to operate in real-time over HDMI without modification with a 180 Hz hologram refresh rate, and has a resolution of 1920×1080 pixels and diagonal screen size of 0.47 inches (11.9 mm).},
author = {Cox, Mitchell A. and Drozdov, Alice V.},
doi = {10.1364/AO.412729},
issn = {1559-128X},
journal = {Applied Optics},
month = {jan},
number = {2},
pages = {465},
pmid = {33448973},
title = {{Converting a Texas Instruments DLP4710 DLP evaluation module into a spatial light modulator}},
url = {https://www.osapublishing.org/abstract.cfm?URI=ao-60-2-465},
volume = {60},
year = {2021}
}

Digital micro-mirror devices (DMDs) are a popular alternative to liquid crystal spatial light modulators for laser beam shaping due to their relatively low cost, high speed, and polarization and wavelength independence. Here we describe in detail how to convert a low-cost digital light projector (DLP) evaluation module that uses a Texas Instruments DLP4710 DMD into a spatial light modulator using a 3D printed mount. The resulting device is shown to accurately shape Laguerre-Gauss modes, is able to operate in real-time over HDMI without modification with a 180 Hz hologram refresh rate, and has a resolution of 1920×1080 pixels and diagonal screen size of 0.47 inches (11.9 mm).

@article{cox2020slt,
abstract = {Optical communication is an integral part of the modern economy, having all but replaced electronic communication systems. Future growth in bandwidth appears to be on the horizon using structured light, encoding information into the spatial modes of light, and transmitting them down fibre and free-space, the latter crucial for addressing last mile and digitally disconnected communities. Unfortunately, patterns of light are easily distorted, and in the case of free-space optical communication, turbulence is a significant barrier. Here we review recent progress in structured light in turbulence, first with a tutorial style summary of the core concepts, before highlighting the present state-of-the-art in the field. We support our review with new experimental studies that reveal which types of structured light are best in turbulence, the behaviour of vector versus scalar light in turbulence, the trade-off of diversity and multiplexing, and how turbulence models can be exploited for enhanced optical signal processing protocols. This comprehensive treatise will be invaluable to the large communities interested in free-space optical communication with spatial modes of light.},
archivePrefix = {arXiv},
arxivId = {2005.14586},
author = {Cox, Mitchell A. and Mphuthi, Nokwazi and Nape, Isaac and Mashaba, Nikiwe and Cheng, Ling and Forbes, Andrew},
doi = {10.1109/JSTQE.2020.3023790},
eprint = {2005.14586},
issn = {1077-260X},
journal = {IEEE Journal of Selected Topics in Quantum Electronics},
month = {mar},
number = {2},
pages = {1--21},
title = {{Structured Light in Turbulence}},
url = {https://ieeexplore.ieee.org/document/9198098/},
volume = {27},
year = {2021}
}

Optical communication is an integral part of the modern economy, having all but replaced electronic communication systems. Future growth in bandwidth appears to be on the horizon using structured light, encoding information into the spatial modes of light, and transmitting them down fibre and free-space, the latter crucial for addressing last mile and digitally disconnected communities. Unfortunately, patterns of light are easily distorted, and in the case of free-space optical communication, turbulence is a significant barrier. Here we review recent progress in structured light in turbulence, first with a tutorial style summary of the core concepts, before highlighting the present state-of-the-art in the field. We support our review with new experimental studies that reveal which types of structured light are best in turbulence, the behaviour of vector versus scalar light in turbulence, the trade-off of diversity and multiplexing, and how turbulence models can be exploited for enhanced optical signal processing protocols. This comprehensive treatise will be invaluable to the large communities interested in free-space optical communication with spatial modes of light.

@article{Pinnell2020md,
author = {Pinnell, Jonathan and Nape, Isaac and Sephton, Bereneice and Cox, Mitchell and Rodriguez-Fajardo, Valeria and Forbes, Andrew},
doi = {10.1364/josaa.398712},
issn = {1084-7529},
journal = {Journal of the Optical Society of America A},
number = {11},
pages = {146--160},
title = {{Modal analysis of structured light with spatial lightmodulators: a practical tutorial}},
volume = {37},
year = {2020}
}

@article{Cox2020Roadmap,
author = {Trichili, Abderrahmen and Cox, Mitchell A. and Ooi, Boon S. and Alouini, Mohamed-Slim},
doi = {10.1364/JOSAB.399168},
issn = {0740-3224},
journal = {Journal of the Optical Society of America B},
month = {nov},
number = {11},
pages = {A184},
title = {{Roadmap to free space optics}},
url = {https://www.osapublishing.org/abstract.cfm?URI=josab-37-11-A184},
volume = {37},
year = {2020}
}

@article{Abadi2019,
abstract = {Free-Space Optical (FSO) systems offer the ability to distribute high speed digital links into remote and rural communities where terrain, installation cost or infrastructure security pose critical hurdles to deployment. A challenge in any point-to-point FSO system is initiating and maintaining optical alignment from the sender to the receiver. In this paper we propose and demonstrate a low-complexity self-aligning FSO prototype that can completely self-align with no requirement for initial manual positioning and could therefore form the opto-mechanical basis for a mesh network of optical transceivers. The prototype utilises off-the-shelf consumer electrical components and a bespoke alignment algorithm. We demonstrate an eight fibre spatially multiplexed link with a loss of 15 dB over 210 m.},
author = {Abadi, Mojtaba Mansour and Cox, Mitchell A. and Alsaigh, Rakan E. and Viola, Shaun and Forbes, Andrew and Lavery, Martin P.J.},
doi = {10.1038/s41598-019-55670-1},
issn = {20452322},
journal = {Scientific Reports},
number = {1},
pages = {1--8},
publisher = {Springer US},
title = {{A space division multiplexed free-space-optical communication system that can auto-locate and fully self align with a remote transceiver}},
url = {http://dx.doi.org/10.1038/s41598-019-55670-1},
volume = {9},
year = {2019}
}

Free-Space Optical (FSO) systems offer the ability to distribute high speed digital links into remote and rural communities where terrain, installation cost or infrastructure security pose critical hurdles to deployment. A challenge in any point-to-point FSO system is initiating and maintaining optical alignment from the sender to the receiver. In this paper we propose and demonstrate a low-complexity self-aligning FSO prototype that can completely self-align with no requirement for initial manual positioning and could therefore form the opto-mechanical basis for a mesh network of optical transceivers. The prototype utilises off-the-shelf consumer electrical components and a bespoke alignment algorithm. We demonstrate an eight fibre spatially multiplexed link with a loss of 15 dB over 210 m.

@article{cox2019dmd,
author = {Cox, Mitchell A. and Toninelli, Ermes and Cheng, Ling and Padgett, Miles J. and Forbes, Andrew},
doi = {10.1109/ACCESS.2019.2925972},
issn = {2169-3536},
journal = {IEEE Access},
pages = {85860--85866},
title = {{A High-Speed, Wavelength Invariant, Single-Pixel Wavefront Sensor With a Digital Micromirror Device}},
url = {https://ieeexplore.ieee.org/document/8751958/},
volume = {7},
year = {2019}
}

@article{Kamwangala2018,
author = {Kamwangala, Christian and Cox, Mitchell A. and Cheng, Ling},
doi = {10.1002/ett.3453},
issn = {21613915},
journal = {Transactions on Emerging Telecommunications Technologies},
month = {feb},
number = {2},
pages = {e3453},
publisher = {Wiley-Blackwell},
title = {{Transmitter power control for a multicarrier visible light communication system}},
url = {http://doi.wiley.com/10.1002/ett.3453 https://onlinelibrary.wiley.com/doi/10.1002/ett.3453},
volume = {30},
year = {2019}
}

@phdthesis{Cox2019,
abstract = {The work presented extends and contributes to research in mitigating the effects of turbulence in long-range Free Space Optical (FSO) links using structured modes of light. While there is an extensive body of existing research in the use of Orbital Angular Momentum (OAM) modes for mode division multiplexed communication channels, there has not been an investigation into the use of alternative mode sets and their resilience to atmospheric turbulence. In this thesis several different approaches are investigated, each with a corresponding journal publication. In the research presented it is found that a carefully chosen subset of Hermite- Gauss modes are significantly more resilient to propagation in turbulence than the more commonly used Laguerre-Gauss modes. Knowledge of this independence is exploited in a “modal diversity” proof-of-principle system which is shown to be effective - an intriguing result which exposes several questions about the propaga- tion of higher order modes in turbulence. In this research it is also found that the resilience of vector vortex modes over scalar vortex modes is in fact a misconception and the choice of mode set for FSO should not be based on its vectorial nature. Motivated by the predominant effects of beam wander in the previous investiga- tions, it is shown that turbulence-induced beam wander is neither a memoryless nor a first order memory system as expected when Taylor's frozen-turbulence model is invoked. A more accurate modelling approach for the time-varying nature of turbu- lence induced beam wander is presented which may be used to design optimised mode sets and digital signal processing schemes in future FSO systems},
author = {Cox, Mitchell Arij},
school = {University of the Witwatersrand, Johannesburg},
title = {{Improving the Resilience of Free-Space Optical Links using Structured Modes of Light}},
type = {PhD},
year = {2019}
}

The work presented extends and contributes to research in mitigating the effects of turbulence in long-range Free Space Optical (FSO) links using structured modes of light. While there is an extensive body of existing research in the use of Orbital Angular Momentum (OAM) modes for mode division multiplexed communication channels, there has not been an investigation into the use of alternative mode sets and their resilience to atmospheric turbulence. In this thesis several different approaches are investigated, each with a corresponding journal publication. In the research presented it is found that a carefully chosen subset of Hermite- Gauss modes are significantly more resilient to propagation in turbulence than the more commonly used Laguerre-Gauss modes. Knowledge of this independence is exploited in a “modal diversity” proof-of-principle system which is shown to be effective - an intriguing result which exposes several questions about the propaga- tion of higher order modes in turbulence. In this research it is also found that the resilience of vector vortex modes over scalar vortex modes is in fact a misconception and the choice of mode set for FSO should not be based on its vectorial nature. Motivated by the predominant effects of beam wander in the previous investiga- tions, it is shown that turbulence-induced beam wander is neither a memoryless nor a first order memory system as expected when Taylor's frozen-turbulence model is invoked. A more accurate modelling approach for the time-varying nature of turbu- lence induced beam wander is presented which may be used to design optimised mode sets and digital signal processing schemes in future FSO systems

@article{cox2019hglg,
abstract = {Vast geographical distances in Africa are a leading cause for the so-called "digital divide" due to the high cost of installing fibre. Free-Space Optical (FSO) communications offer a convenient and higher bandwidth alternative to point-to-point radio microwave links, with the possibility of re-purposing existing infrastructure. Unfortunately, the range of high bandwidth FSO remains limited. While there has been extensive research into an optimal mode set for FSO to achieve maximum data throughput by mode division multiplexing, there has been relatively little work investigating optical modes to improve the resilience of FSO links. Here we experimentally show that a carefully chosen subset of Hermite-Gaussian modes is more resilient to atmospheric turbulence than similar Laguerre-Gauss beams, theoretically resulting in a 167% theoretical increase of propagation distance at a mode dependent loss of 50%.},
archivePrefix = {arXiv},
arxivId = {1901.07203},
author = {Cox, Mitchell A. and Maqondo, Luthando and Kara, Ravin and Milione, Giovanni and Cheng, Ling and Forbes, Andrew},
doi = {10.1109/JLT.2019.2905630},
eprint = {1901.07203},
issn = {0733-8724},
journal = {Journal of Lightwave Technology},
month = {aug},
number = {16},
pages = {3911--3917},
title = {{The Resilience of Hermite– and Laguerre–Gaussian Modes in Turbulence}},
url = {https://ieeexplore.ieee.org/document/8668467/},
volume = {37},
year = {2019}
}

Vast geographical distances in Africa are a leading cause for the so-called "digital divide" due to the high cost of installing fibre. Free-Space Optical (FSO) communications offer a convenient and higher bandwidth alternative to point-to-point radio microwave links, with the possibility of re-purposing existing infrastructure. Unfortunately, the range of high bandwidth FSO remains limited. While there has been extensive research into an optimal mode set for FSO to achieve maximum data throughput by mode division multiplexing, there has been relatively little work investigating optical modes to improve the resilience of FSO links. Here we experimentally show that a carefully chosen subset of Hermite-Gaussian modes is more resilient to atmospheric turbulence than similar Laguerre-Gauss beams, theoretically resulting in a 167% theoretical increase of propagation distance at a mode dependent loss of 50%.

@misc{Cox2019a,
abstract = {One of the major challenges for long range, high speed Free-Space Optical (FSO) communication is turbulence induced beam wander. Beam wander causes fluctuations in the received intensity as well as crosstalk in mode division multiplexed systems. Existing models for beam wander make use of probability distributions and long term averages and are not able to accurately model time-dependent intensity fluctuations such as deep fading, where the received intensity is too low to maintain reliable communication for an extended period of time. In this work we present an elegant new memory model which models the behaviour of beam wander induced intensity fluctuations with the unique capability to accurately simulate deep fading. This is invaluable for the development of optimised error correction coding and digital signal processing in order to improve the throughput and reliability of FSO systems.},
author = {Cox, M.A. and Gailele, L. and Cheng, L. and Forbes, A.},
booktitle = {arXiv},
issn = {23318422},
keywords = {Beam Wander,Free Space Optical Communication,Mode Division Multiplexing,Turbulence},
title = {{Modelling the memory of turbulence-induced beam wander}},
year = {2019}
}

One of the major challenges for long range, high speed Free-Space Optical (FSO) communication is turbulence induced beam wander. Beam wander causes fluctuations in the received intensity as well as crosstalk in mode division multiplexed systems. Existing models for beam wander make use of probability distributions and long term averages and are not able to accurately model time-dependent intensity fluctuations such as deep fading, where the received intensity is too low to maintain reliable communication for an extended period of time. In this work we present an elegant new memory model which models the behaviour of beam wander induced intensity fluctuations with the unique capability to accurately simulate deep fading. This is invaluable for the development of optimised error correction coding and digital signal processing in order to improve the throughput and reliability of FSO systems.

@article{Toninelli2019a,
abstract = {Waves can carry both linear and angular momentum. When the wave is transverse (e.g. light), the angular momentum can be characterised by the “spin” angular momentum associated with circular polarisation, and the “orbital” angular momentum (OAM) arising from the phase cross-section of the beam. When the wave is longitudinal (e.g. sound) there is no polarization and hence no spin angular momentum. However, a suitably phase-structured sound beam can still carry OAM. Observing the transfer of OAM from sound to a macroscopic object provides an excellent opportunity to study the exchange of energy between waves and matter. In this paper we show how to build a compact free-space acoustic spanner based on a 3D-printed sound-guiding structure and common electronic components. We first characterise the sound fields by measuring both phase and amplitude maps, and then show a video of our free-space acoustic spanner in action, in which macroscopic objects spin in a circular motion and change direction of rotation according to the handedness of the OAM acoustic field.},
author = {Toninelli, Ermes and Cox, Mitchell A. and Gibson, Graham M. and Brown, Stuart D. and Edgar, Matthew P. and Forbes, Andrew and Padgett, Miles J.},
doi = {10.1038/s41598-019-43046-4},
issn = {2045-2322},
journal = {Scientific Reports},
keywords = {Acoustics,Electrical and electronic engineering},
month = {dec},
number = {1},
pages = {6757},
publisher = {Nature Publishing Group},
title = {{A compact acoustic spanner to rotate macroscopic objects}},
url = {http://www.nature.com/articles/s41598-019-43046-4},
volume = {9},
year = {2019}
}

Waves can carry both linear and angular momentum. When the wave is transverse (e.g. light), the angular momentum can be characterised by the “spin” angular momentum associated with circular polarisation, and the “orbital” angular momentum (OAM) arising from the phase cross-section of the beam. When the wave is longitudinal (e.g. sound) there is no polarization and hence no spin angular momentum. However, a suitably phase-structured sound beam can still carry OAM. Observing the transfer of OAM from sound to a macroscopic object provides an excellent opportunity to study the exchange of energy between waves and matter. In this paper we show how to build a compact free-space acoustic spanner based on a 3D-printed sound-guiding structure and common electronic components. We first characterise the sound fields by measuring both phase and amplitude maps, and then show a video of our free-space acoustic spanner in action, in which macroscopic objects spin in a circular motion and change direction of rotation according to the handedness of the OAM acoustic field.

@article{lavery2018,
abstract = {Innovations in ‘sustainable' photonics technologies such as free-space optical links and solar-powered equipment provide developing countries with new cost-effective opportunities for deploying future-proof telecommunication networks.},
author = {Lavery, Martin P. J. and Abadi, Mojtaba Mansour and Bauer, Ralf and Brambilla, Gilberto and Cheng, Ling and Cox, Mitchell A. and Dudley, Angela and Ellis, Andrew D. and Fontaine, Nicolas K. and Kelly, Anthony E. and Marquardt, Christoph and Matlhane, Selaelo and Ndagano, Bienvenu and Petruccione, Francesco and Slav{\'{i}}k, Radan and Romanato, Filippo and Rosales-Guzm{\'{a}}n, Carmelo and Roux, Filippus S. and Roux, Kobus and Wang, Jian and Forbes, Andrew},
doi = {10.1038/s41566-018-0162-z},
issn = {1745-4885},
journal = {Nature Photonics},
keywords = {Fibre optics and optical communications,Scientific community and society},
month = {may},
number = {5},
pages = {249--252},
publisher = {Nature Publishing Group},
title = {{Tackling Africa's digital divide}},
url = {http://www.nature.com/articles/s41566-018-0162-z},
volume = {12},
year = {2018}
}

Innovations in ‘sustainable' photonics technologies such as free-space optical links and solar-powered equipment provide developing countries with new cost-effective opportunities for deploying future-proof telecommunication networks.

@article{ndagano2017b,
author = {Ndagano, Bienvenu and Nape, Isaac and Cox, Mitchell A. and Rosales-Guzman, Carmelo and Forbes, Andrew},
doi = {10.1109/JLT.2017.2766760},
issn = {0733-8724},
journal = {Journal of Lightwave Technology},
month = {jan},
number = {2},
pages = {292--301},
title = {{Creation and Detection of Vector Vortex Modes for Classical and Quantum Communication}},
url = {http://ieeexplore.ieee.org/document/8085118/},
volume = {36},
year = {2018}
}

@article{cox2018diversity,
author = {Cox, Mitchell A. and Cheng, Ling and Rosales-Guzm{\'{a}}n, Carmelo and Forbes, Andrew},
doi = {10.1103/PhysRevApplied.10.024020},
issn = {2331-7019},
journal = {Physical Review Applied},
month = {aug},
number = {2},
pages = {024020},
publisher = {American Physical Society},
title = {{Modal Diversity for Robust Free-Space Optical Communications}},
url = {https://link.aps.org/doi/10.1103/PhysRevApplied.10.024020},
volume = {10},
year = {2018}
}

@article{Cox2016,
abstract = {Free-space optical communication with spatial modes of light has become topical due to the possibility of dramatically increasing communication bandwidth via Mode Division Multiplexing (MDM). While both scalar and vector vortex modes have been used as transmission bases, it has been suggested that the latter is more robust in turbulence. Using orbital angular momentum as an example, we demonstrate theoretically and experimentally that the crosstalk due to turbulence is the same in the scalar and vector basis sets of such modes. This work brings new insights about the behaviour of vector and scalar modes in turbulence, but more importantly it demonstrates that when considering optimal modes for MDM, the choice should not necessarily be based on their vectorial nature.},
annote = {[27-29]},
author = {Cox, Mitchell A. and Rosales-Guzm{\'{a}}n, Carmelo and Lavery, Martin P. J. and Versfeld, Daniel J. and Forbes, Andrew},
doi = {10.1364/OE.24.018105},
issn = {1094-4087},
journal = {Optics Express},
keywords = {Atmospheric propagation,Free-space optical communication,Polarization,Turbulence},
month = {aug},
number = {16},
pages = {18105},
publisher = {Optical Society of America},
title = {{On the resilience of scalar and vector vortex modes in turbulence}},
url = {https://www.osapublishing.org/abstract.cfm?URI=oe-24-16-18105 https://opg.optica.org/abstract.cfm?URI=oe-24-16-18105},
volume = {24},
year = {2016}
}

Free-space optical communication with spatial modes of light has become topical due to the possibility of dramatically increasing communication bandwidth via Mode Division Multiplexing (MDM). While both scalar and vector vortex modes have been used as transmission bases, it has been suggested that the latter is more robust in turbulence. Using orbital angular momentum as an example, we demonstrate theoretically and experimentally that the crosstalk due to turbulence is the same in the scalar and vector basis sets of such modes. This work brings new insights about the behaviour of vector and scalar modes in turbulence, but more importantly it demonstrates that when considering optimal modes for MDM, the choice should not necessarily be based on their vectorial nature.