The implementation of spatial multiplexing has become an area of great interest for free-space communication links, particularly for its use in last-mile links within larger optical networks. Light carrying orbital angular momentum (OAM) has emerged as a potential candidate that could be utilised for multiplexing independent channels. We will present measured inter-channel crosstalk for a set of 11-OAM modes propagating through 3m of slowly flowing water, similar to that found in oceanic flow conditions
Light transmission of Laguerre Gaussian (LG) vortex beams with different orbital angular momentum
(OAM) values (L) in scattering beads and mouse brain tissue media were experimentally investigated
for the first time in comparison with Gaussian (G) beams. The LG beams with different OAM were
generated using a spatial light modulator (SLM) in reflection mode. The scattering beads media consist
of various sizes and concentrations of latex beads in water solutions. The transmissions of LG and G
beams through scattering beads and brain tissue media were measured with different ratios of sample
thicknesses (z) to scattering mean free path (l<sub>s</sub>) of the turbid media, z/l<sub>s</sub>. The results indicate that within
the ballistic region where z/l<sub>s</sub> is small, the LG and G beams show no significant difference, while in the
diffusive region where z/l<sub>s</sub> is higher, the vortex beams show higher transmission than G beams. In the
diffusive region, the LG beams with higher L values show higher transmission than the beams with
lower L values due to the eigen channels in the media. The transition points from the ballistic to
diffusive regions for different scattering beads and brain tissue media were studied.
The linear Doppler shift forms the basis of various sensor types for the measurement of linear velocity, ranging from speeding cars to fluid flow. Recently, a rotational analogue was demonstrated, enabling the measurement of angular velocity using light carrying orbital angular momentum (OAM). If measurement of the light scattered from a spinning object is restricted to a defined OAM state, then a frequency shift is observed that scales with the rotation rate of the object and the OAM of the scattered photon. In this work we measure the rotational Doppler shift from micron-sized calcite particles spinning in an optical trap at tens of Hz. In this case the signal is complicated by the geometry of the rotating particle, and the effect of Brownian motion. By careful consideration of these influences, we show how the signal is robust to both, representing a new technique with which to probe the rotational motion of micro-scale particles.
In this work we will present two techniques for the measurement of superimposed higher-order Bessel beams. In the first technique we will outline a simple approach using only a spatial light modulator and a Fourier transforming lens to decompose the OAM spectrum of an optical field. We test this approach on symmetric and non-symmetric superpositions of non-diffracting higher-order Bessel beams. Our second procedure consists of two refractive optical elements which perform a Cartesian to log-polar coordinate transformation, translating helically phased beams into a transverse phase gradient. By introducing two cylindrical lenses we can focus each of the azimuthal modes associated with each Bessel beam to a different lateral position in the Fourier plane, while separating the radial wave-vectors in the image-plane.
Previously we have demonstrated that the orbital angular momentum (OAM) of the light beam may be measured by image transformation that maps the azimuthal to linear transverse co-ordinate (Berkhout et al 2010 Phys. Rev. Lett. 105 153601). For each input OAM state the transmitted light is focused to a different transverse position enabling simultaneous measurement over many states. We present a significant improvement to our earlier design, extending the measurement bandwidth to greater than 50 OAM states and showing simultaneous measurement of the radial co-ordinate. We further demonstrate the transformation working in reverse, potentially allowing for the rapid switching of OAM modes.
A procedure to efficiently sort orbital angular momentum (OAM) states of light, by performing a Cartesian to log-polar
coordinate transformation which translates helically phased beams into a transverse phase gradient, currently exists1. We
implement this mode transformer, which comprises of two custom refractive optical elements2, to efficiently sort Bessel
beams carrying OAM. Introducing two cylindrical lenses, allows the focusing of each of the input OAM Bessel states to
a different lateral position in the Fourier plane and separates the radial wave-vectors in the image-plane. We demonstrate
the concept by separating over forty OAM states and radial wave-vectors.
The desire to increase the amount of information that can be encoded onto a single photon has driven research into many areas of optics. One such area is optical orbital angular momentum (OAM) . These beams have helical phasefronts and carry an orbital angular momentum of mbar per photon, where the integer m is unbounded, giving a large state space in which to encode information.
We recently developed a telescope system comprising two bespoke refractive optical elements to transform OAM states into transverse momentum states . This is achieved by mapping the azimuthal position of the input plane to the lateral position in the output . A mapping of this type transforms a set of concentric rings at the input plane into a set of parallel lines in the output plane. A lens can then separate the resulting transverse momentum states into specified lateral positions, allowing for the efficient measurement of multiple OAM states simultaneously.
Separating OAM states in this way presents an opportunity for this larger alphabet to improve the data capacity of a free space link and has potential application in both the classical and quantum regimes.
We will present our latest design, increasing the bandwidth of measurable states to over 50 OAM modes. In such a system we study the crosstalk introduced by a thin phase turbulence, showing that turbulence similarly degrades the purity of all the modes within this range.
The information carried by a photon can be encoded in one or more of many different degrees of freedom. Beyond the two-dimensional space of polarisation (spin angular momentum) our interest lies in the unbounded yet discrete state space of Orbital Angular Momentum (OAM). We examine how photon pairs can be generated and measured over a large range of OAM states.
We report a new simple optical system for the highly efficient measurement of the Orbital Angular Momentum States of
Light. It uses an image reformatter to map each input state onto a different lateral position in the output aperture. This,
near perfect, separation of states potentially makes available the high information capacity of OAM in both classical and