Creation and characterization of vector vortex modes for classical and quantum communication

Vector vortex beams are structured states of light that are non-separable in their polarisation and spatial mode, they are eigenmodes of free-space and many fibre systems, and have the capacity to be used as a modal basis for both classical and quantum communication. Here we outline recent progress in our understanding of these modes, from their creation to their characterization and detection. We then use these tools to study the propagation behaviour of such modes in free-space and optical fibre and show that modal cross-talk results in a decay of vector states into separable scalar modes, with a concomitant loss of information. We present a comparison between probabilistic and deterministic detection schemes showing that the former, while ubiquitous, negates the very benefit of increased dimensionality in quantum communication while reducing signal in classical communication links. This work provides a useful introduction to the field as well as presenting new findings and perspectives to advance it further

Piracy and Maritime Crime

As modern nation-states emerged from feudalism, privateering for both profit and war supplemented piracy at the margins of national sovereignty. More recently, an ocean enclosure movement under the aegis of the United Nations Convention on the Law of the Sea 1982 has granted states access to maritime resources far beyond their territorial limits.https://digital-commons.usnwc.edu/usnwc-newport-papers/1034/thumbnail.jp

A space division multiplexed free-space-optical communication system that can auto-locate and fully self align with a remote transceiver

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

Robust interferometer for the routing of light beams carrying orbital angular momentum

We have developed an interferometer requiring only minimal angular alignment for the routing of beams carrying orbital angular momentum. The Mach–Zehnder interferometer contains a Dove prism in each arm where each has a mirror plane around which the transverse phase profile is inverted. One consequence of the inversions is that the interferometer needs no alignment. Instead the interferometer defines a unique axis about which the input beam must be coupled. Experimental results are presented for the fringe contrast, reaching a maximum value of 93±1%

Modelling the advection of vertically migrating shrimp larvae

The role of larval advection in determining the complex, large-scale patterns of immigration of penaeid postlarvae in the Gulf of Carpentaria is investigated by modelling the interaction between diurnal vertical migration of larvae with wind-forced and tidal currents. Eight vertical migration schemes are modelled in which both the timing of the migration and the position of the larvae in the water column are varied. These schemes are then coupled with both two-dimensional and three-dimensional models of the currents of the Gulf, to examine horizontal advection of larvae. When the larvae migrate vertically with a diurnal period, their horizontal advection is enhanced. The largest horizontal advection distances occur when the larvae move diurnally from the water column into the bottom boundary layer. Advection distances of up to 165 km are possible during the two to three week planktonic larval period. This distance corresponds to, and may determine, the offshore extent of the adult distribution. The onshore advection pattern of larvae varies in both space and time (on a seasonal scale) and is consistent with the observed spatial and temporal recruitment patterns seen by sampling postlarval immigation into nursery areas. During the period of highest reproductive activity (March) in the southeastern corner of the Gulf, the area of the largest fishery, the advection of larvae is offshore and little recruitment of postlarvae to the nursery grounds is accomplished. Six months later, during the next period of reproductive activity (October), when the number of spawning female shrimp is much lower, the phase of the tidal currents, relative to the day-night cycle, has progressed 180° and the larvae are moved onshore allowing postlarvae access to their estuarine nursery grounds several months prior to the main fishing season (March)

Structural and magnetic properties of E-Fe_{1-x}Co_xSi thin films deposited via pulsed laser deposition

We report pulsed laser deposition synthesis and characterization of polycrystalline Fe1-xCox Si thin films on Si (111). X-ray diffraction, transmission electron, and atomic force microscopies reveal films to be dense, very smooth, and single phase with a cubic B20 crystal structure. Ferromagnetism with significant magnetic hysteresis is found for all films including nominally pure FeSi films in contrast to the very weak paramagnetism of bulk FeSi. For Fe1-xCoxSi this signifies a change from helimagnetism in bulk, to ferromagnetism in thin films. These ferromagnetic thin films are promising as a magnetic-silicide/silicon system for polarized current production, manipulation, and detection.Comment: 12 pages, 4 figures accepted in the Applied Physics Letter