Researchers demonstrate the ability to multiplex and transfer data between twisted beams of light with different amounts of orbital angular momentum — a development that provides new opportunities for increasing the data capacity of free-space optical communications links.

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Terabit free-space data transmission employing orbital angular momentum multiplexing

to be done in this area. Face recognition is a problem that scarcely clamoured for attention before the computer age but, having surfaced, has involved a wide range of techniques and has attracted the attention of some fine minds (David Mumford was a Fields Medallist in 1974). This singular application of mathematical modelling to a messy applied problem of obvious utility and importance but with no unique solution is a pretty one to share with students: perhaps, returning to the source of our opening quotation, we may invert Duncan's earlier observation, 'There is an art to find the mind's construction in the face!'.

Linear and nonlinear waves, by G. B. Whitham. Pp.636. £50. 1999. ISBN 0 471 35942 4 (Wiley).

As they travel through space, some light beams rotate. Such light beams have angular momentum. There are two particularly important ways in which a light beam can rotate: if every polarization vector rotates, the light has spin; if the phase structure rotates, the light has orbital angular momentum (OAM), which can be many times greater than the spin. Only in the past 20 years has it been realized that beams carrying OAM, which have an optical vortex along the axis, can be easily made in the laboratory. These light beams are able to spin microscopic objects, give rise to rotational frequency shifts, create new forms of imaging systems, and behave within nonlinear material to give new insights into quantum optics.

/pdf/orbital-angular-momentum-origins-behavior-and-applications-1sv8qfuhrs.pdf

Orbital angular momentum: origins, behavior and applications

The fact that light carries both linear and angular momentum is well-known to physicists. One application of the linear momentum of light is for optical tweezers, in which the refraction of a laser beam through a particle provides a reaction force that draws the particle towards the centre of the beam. The angular momentum of light can also be transfered to particles, causing them to spin. In fact, the angular momentum of light has two components that act through different mechanisms on various types of particle. This Review covers the creation of such beams and how their unusual intensity, polarization and phase structure has been put to use in the field of optical manipulation.

Tweezers with a twist

Orbital angular momentum (OAM), which describes the “phase twist” (helical phase pattern) of light beams, has recently gained interest due to its potential applications in many diverse areas. Particularly promising is the use of OAM for optical communications since: (i) coaxially propagating OAM beams with different azimuthal OAM states are mutually orthogonal, (ii) inter-beam crosstalk can be minimized, and (iii) the beams can be efficiently multiplexed and demultiplexed. As a result, multiple OAM states could be used as different carriers for multiplexing and transmitting multiple data streams, thereby potentially increasing the system capacity. In this paper, we review recent progress in OAM beam generation/detection, multiplexing/demultiplexing, and its potential applications in different scenarios including free-space optical communications, fiber-optic communications, and RF communications. Technical challenges and perspectives of OAM beams are also discussed.

Optical communications using orbital angular momentum beams

We present a new characterization of partially coherent electric and magnetic wave vector fields.This characterization is based on the 36 auto/cross correlations of the 3+3 complex Cartesian components of the electric and magnetic wave fields and is particularly suited for analyzing electromagnetic wave data on board spacecraft. Data from spacecraft based electromagnetic wave instruments are usually processed as data arrays. These data arrays however do not have a physical interpretation in themselves; they are simply a convenient storage format. In contrast, the characterization proposed here contains exactly the same information but are in the form of manifestly covariant space-time tensors. We call this data format the Canonical Electromagnetic Observables (CEO) since they correspond to unique physical observables. Some of them are already known, such as energy density, Poynting flux, stress tensor, etc, while others should be relevant in future space research. As an example we use this formalism to analyze data from a chorus emission in the mid-latitude magnetosphere, as recorded by the STAFF-SA instrument on board the Cluster-II spacecraft.

Canonical Electromagnetic Observables for Systematic Characterization of Electric and Magnetic Wave Field Data on board Spacecraft

Intense and localized burst of electrostatic emissions have been observed by the Whisper instrument on Cluster at frequency close to the electron plasma frequency, in the magnetopause boundary layer. The good time resolution of Whisper when in burst mode telemetry as well as the combination of the observations performed by the four Cluster spacecraft at short separation distance (~100 km) have allowed to determine in one case the dimension of these structure, typically a few tens of kilometers, close to the electron scale. We browse Whisper data in search for such events and present here some examples of their observations. We present the main characteristics of the spectra and power profile of the emissions.

A search for electron scale structures close to the magnetopause

Antennanordning för användning av tredimensionell elektromagnetisk fältinformation inherent i en radiovåg

We derive the second-order sampling properties of certain autocovariance and autocorrelation estimators for sequences of independent and identically distributed samples. Specifically, the estimators we consider are the classic lag windowed correlogram, the correlogram with subtracted sample mean, and the fixed-length summation correlogram. For each correlogram we derive explicit formulas for the bias, covariance, mean square error and consistency for generalised higher-order white noise sequences. In particular, this class of sequences may have non-zero means, be complexed valued and also includes non-analytical noise signals. We find that these commonly used correlograms exhibit lag dependent covariance despite the fact that these processes are white and hence by definition do not depend on lag.

Deriving the sampling errors of correlograms for general white noise

https://hal.archives-ouvertes.fr/hal-00157870/document

Tobia Carozzi

Papers

Canonical Electromagnetic Observables for Systematic Characterization of Electric and Magnetic Wave Field Data on board Spacecraft

A search for electron scale structures close to the magnetopause

Antennanordning för användning av tredimensionell elektromagnetisk fältinformation inherent i en radiovåg

Deriving the sampling errors of correlograms for general white noise

A search for electron scale structures close to the magnetopause.