Vortex instability in turbulent free-space propagation

The spatial structuring of optical fields is integral within many next generation optical metrology and communication techniques. A verifiable physical model of the propagation of these optical fields in a turbulent environment is important for developing effective mitigation techniques for the modal degradation that occurs in a free-space link. We present a method to simulate this modal degradation that agrees with recently reported experimental findings. A 1.5 km free-space link is emulated by decomposing the optical turbulence that accumulates over a long distance link, into many, weakly perturbing steps of 10 m. This simulation shows that the high-order vortex at the centre of the helical phase profiles in modes that carry orbital angular momentum of $| {\ell }| \geqslant 2{\hslash }$ are unstable and fracture into many vortices when they propagate over the link. This splitting presents issues for the application of turbulence mitigation techniques. The usefulness of pre-correction, post-correction, and complex field conjugation techniques are discussed

Efficient sorting of free electron orbital angular momentum

We propose a method for sorting electrons by orbital angular momentum (OAM). Several methods now exist to prepare electron wavefunctions in OAM states, but no technique has been developed for efficient, parallel measurement of pure and mixed electron OAM states. The proposed technique draws inspiration from the recent demonstration of the sorting of OAM through modal transformation. We show that the same transformation can be performed on electrons with electrostatic optical elements. Specifically, we show that a charged needle and an array of electrodes perform the transformation and phase correction necessary to sort OAM states. This device may enable the analysis of the spatial mode distribution of inelastically scattered electrons

Measuring orbital angular momentum superpositions of light by mode transformation

We recently reported on a method for measuring orbital angular momentum (OAM) states of light based on the transformation of helically phased beams to tilted plane waves [Phys. Rev. Lett.105, 153601 (2010)]. Here we consider the performance of such a system for superpositions of OAM states by measuring the modal content of noninteger OAM states and beams produced by a Heaviside phase plate

Demonstration of a 280-Gbit/s free-space SDM communications link utilizing plane-wave spatial multiplexing

We demonstrate a 280-Gbit/s free-space SDM communications link incorporating a set of independent tilted truncated plane-waves, each generated by a single mode fiber placed at the back-focal plane of a spherical lens. Each of the 7 tilted plane-wave channels are encoded with a 40-Gbit/s 16-QAM signal. Our approach comprises two identical linear fiber-arrays placed approximately 5 m apart. As each fiber array is placed at the back-focal-plane of a spherical lens, each fiber array is effectively placed in a conjugate image plane of the other. A channel crosstalk less than 26 dB is shown, with a bit-error-rate below the FEC threshold of 3.8 × 10−3

A deterministic detector for vector vortex states

Encoding information in high-dimensional degrees of freedom of photons has led to new avenues in various quantum protocols such as communication and information processing. Yet to fully benefit from the increase in dimension requires a deterministic detection system, e.g., to reduce dimension dependent photon loss in quantum key distribution. Recently, there has been a growing interest in using vector vortex modes, spatial modes of light with entangled degrees of freedom, as a basis for encoding information. However, there is at present no method to detect these non-separable states in a deterministic manner, negating the benefit of the larger state space. Here we present a method to deterministically detect single photon states in a four dimensional space spanned by vector vortex modes with entangled polarisation and orbital angular momentum degrees of freedom. We demonstrate our detection system with vector vortex modes from the |[Formula: see text]| = 1 and |[Formula: see text]| = 10 subspaces using classical and weak coherent states and find excellent detection fidelities for both pure and superposition vector states. This work opens the possibility to increase the dimensionality of the state-space used for encoding information while maintaining deterministic detection and will be invaluable for long distance classical and quantum communication

Free-space propagation of high dimensional structured optical fields in an urban environment

Spatially structured optical fields have been used to enhance the functionality of a wide variety of systems that use light for sensing or information transfer. As higher-dimensional modes become a solution of choice in optical systems, it is important to develop channel models that suitably predict the effect of atmospheric turbulence on these modes. We investigate the propagation of a set of orthogonal spatial modes across a free-space channel between two buildings separated by 1.6 km. Given the circular geometry of a common optical lens, the orthogonal mode set we choose to implement is that described by the Laguerre-Gaussian (LG) field equations. Our study focuses on the preservation of phase purity, which is vital for spatial multiplexing and any system requiring full quantumstate tomography. We present experimental data for the modal degradation in a real urban environment and draw a comparison to recognized theoretical predictions of the link. Our findings indicate that adaptations to channel models are required to simulate the effects of atmospheric turbulence placed on high-dimensional structured modes that propagate over a long distance. Our study indicates that with mitigation of vortex splitting, potentially through precorrection techniques, one could overcome the challenges in a real point-to-point free-space channel in an urban environment

Multi-layer light trapping structures for enhanced solar collection

Light trapping is a commonly used technique for enhancing the efficiency of solar collection in many photovoltaic (PV) devices. In this paper, we present the design of multi-layer light trapping structures that can potentially be retrofitted, or directly integrated, onto crystalline or amorphous silicon solar panels for enhanced optical collection at normal and extreme angle of incidence. This approach can improve the daily optical collection performance of solar panel with and without internally integrated light trapping structure by up to 7.18% and 159.93%, respectively. These improvements predict an enhancement beyond many research level and commercially deployed light trapping technologies. We further enhance this performance by combining our multi-layer optics with high refractive index materials to achieve a daily optical collection of up to 32.20% beyond leading light trapping structures. Our additive light trapping designs could enable the upgradeability of older PV technologies and can be tailored to optimally operate at unique angular ranges for building exteriors or over a wide range of incidence angle for applications such as unmanned aerial vehicles

Twisting waves increase the visibility of nonlinear behaviour

Nonlinear behaviour for acoustic systems is readily measured at high acoustic pressures in gasses or bulk materials. However, at low acoustic pressures non-linear effects are not commonly observed. We find that by phase structuring acoustics beams, one observes evidence of non-linear behaviour at an acoustic pressure of 66.78 dB lower than non-structured beams in room temperature air. A bespoke 28-element ultrasonic phased array antenna was developed to generate short pulses that carry orbital angular momentum and are propagated over a short air channel. When sampling small areas of the wavefront, we observed a distinctive change in the frequency components near phase singularities. At these phase singularities the local propagation path is screwed, resulting in the collection signals from pulses traveling along different paths across the aperture of a microphone. The usually negligible frequency chirping that arises from nonlinear behaviour in air interfere at these singularity points and produce a distinctive distortion of the acoustic pulse. Simple physical movement in the system or super-sonic wave speeds do not yield similar results. Such distortions in measured frequency response near phase singularities could lead to errors for SONAR or acoustic communication systems, where received signals are integrated over a finite-area detector. With further development this behaviour could potentially lead to accurate measurement techniques for determining a material's nonlinear properties at lower acoustic pressure

Vector space-time wave packets

Space-time wave packets (STWPs) are propagation-invariant pulsed beams whose characteristics stem from the tight association between their spatial and temporal degrees of freedom. Until recently, only scalar STWPs have been synthesized in the form of light sheets. Here we synthesize vector STWPs that are localized in all dimensions by preparing polarization-structured spatiotemporal spectra and unveil the polarization distribution over the STWP volume via time-resolved complex field measurements. Such vector STWPs are endowed with cylindrically symmetric polarization vector structures, which require joint manipulation of the spatial, temporal, and polarization degrees of freedom of the optical field. These results may be useful in particle manipulation, and in nonlinear and quantum optics

Structured light enhanced machine learning for fiber bend sensing

The intricate optical distortions that occur when light interacts with complex media, such as few- or multi-mode optical fiber, often appear random in origin and are a fundamental source of error for communication and sensing systems. We propose the use of orbital angular momentum (OAM) feature extraction to mitigate phase-noise and allow for the use of intermodal-coupling as an effective tool for fiber sensing. OAM feature extraction is achieved by passive all-optical OAM demultiplexing, and we demonstrate fiber bend tracking with 94.1% accuracy. Conversely, an accuracy of only 14% was achieved for determining the same bend positions when using a convolutional-neural-network trained with intensity measurements of the output of the fiber. Further, OAM feature extraction used 120 times less information for training compared to intensity image based measurements. This work indicates that structured light enhanced machine learning could be used in a wide range of future sensing technologies