For decades, singular beams carrying angular momentum have been a topic of considerable interest. Their intriguing applications are ubiquitous in a variety of fields, ranging from optical manipulation to photon entanglement, and from microscopy and coronagraphy to free-space communications, detection of rotating black holes, and even relativistic electrons and strong-field physics. In most applications, however, singular beams travel naturally along a straight line, expanding during linear propagation or breaking up in nonlinear media. Here, we design and demonstrate diffraction-resisting singular beams that travel along arbitrary trajectories in space. These curved beams not only maintain an invariant dark "hole" in the center but also preserve their angular momentum, exhibiting combined features of optical vortex, Bessel, and Airy beams. Furthermore, we observe three-dimensional spiraling of microparticles driven by such fine-shaped dynamical beams. Our findings may open up new avenues for shaped light in various applications.

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Curved singular beams for three-dimensional particle manipulation

Underwater optical wireless communication is an emerging field that can provide reliable connectivity for future generation Internet of Underwater Things devices. In this article, we propose a communication system based on single and superposition of Laguerre–Gaussian modes to transfer information and rely on a convolutional neural network for the mode identification in an underwater environment. A 100% recovery fidelity is reported at clear and turbid water. Beyond 90% of identification, accuracy is achieved under different laboratory-emulated underwater turbulence conditions. The practical implementation of the proposed spatial-mode-based communication scheme is further discussed.

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A CNN-Based Structured Light Communication Scheme for Internet of Underwater Things Applications

High brightness lasers

The invention relates to optical communication methods and systems. In particular, the invention relates to an optical communication method and system which is configured to create a multiplexed beam from an incident beam, wherein the multiplexed beam comprises a predetermined number of spatial modes simultaneously generated and multiplexed together in a fashion that is independent of wavelength. The spatial modes have two degrees of spatial freedom. The multiplexed beam is de-multiplexed downstream from multiplexing thereof in the communication system in a simultaneous fashion independent of wavelength to yield the predetermined number of spatial mode. The modes are used in optical communication as channels or as bits in a bit (de) encoding scheme.

Optical communication method and system

Investigation on the propagation of an optical beam carrying phase singularities through atmospheric turbulence attracts researcher involved in the laser projection applications. In this paper, a group of Custom Designed Beams (CDB) are designed by radially encoding the multiple singular phases at the predefined locations to study the turbulence resistant property of these beams. Generation and propagation of these CDB are investigated in free space and the presence of atmospheric turbulence. The CDB are generated by a computer-generated hologram employing on the phase-only SLM. The ensquared energy of the CDB are calculated numerically and validated with the experimental results in free-space. Also, the effect of atmospheric turbulence on the propagation of the CDB is comprehensively investigated with the help of two types of turbulence simulators, atmospheric turbulence simulator and the turbulence cell, in the laboratory. A MATLAB based modified offline video processing method is used to compute all the relevant performance parameters. The effectiveness of these beams is determined by analyzing the beam deformation/spreading as beam spreading ratio and sensitiveness towards intensity fluctuation as scintillation index with reference to the conventional Gaussian beam. It is also reported that the CDB having multiple phase singularities at predefined configuration shows a significant decrease in the percentage of intensity fluctuations as compared to the Gaussian beam. Out of all the analyzed beams, the CDB of cases C14 and C15 show a significant reduction in scintillation index by 20–30% with more energy intact at the center. Thus, CDB of cases C14 and C15 would be proposed as the preferred design for laser projection applications.

Investigations on the turbulence resistant characteristics of Custom Designed Beams embedded with multiple phase singularities

We analyze the eect 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 eld 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.

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Laguerre Gaussian beam multiplexing through turbulence

High-capacity data transmission has been implemented using single channel optical systems. This technique is limited and soon it will be unable to fulﬁll 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 ρ, 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.

Digital holograms for laser mode multiplexing

Bessel-Gaussian (BG) modes possess unique characteristics that have been exploited in the classical world and which may also o er advantages over other modes in the quantum regime. We use the reconstruction property of BG modes to recover the degree of entanglement of our quantum state after encountering an obstruction. For BG modes, there exists a minimum distance behind an obstruction before reconstruction of the mode occurs. By moving the obstruction along the path of propagation of the entangled photon pairs, we quantitatively show a increase in the degree of entanglement as the obstruction moved beyond that minimum distance.

Recovery of quantum-entanglement after encountering an obstruction

Unravelling Bessel beams: Presentation

We propose a 2-dimensional method for Bessel Gaussian beam azimuthal and radial decomposition using digital holograms. We illustrate the reconstruction of a Bessel Gaussian beam after encountering an obstruction. From the measured decomposition we show the reconstruction of the amplitude, phase and azimuthal index of the field with high degree of accuracy.

Thandeka Mhlanga

Papers

Laguerre Gaussian beam multiplexing through turbulence

Digital holograms for laser mode multiplexing

Recovery of quantum-entanglement after encountering an obstruction

Unravelling Bessel beams: Presentation

Detecting Bessel beams