Beam-spreading and topological charge of vortex beams propagating in a turbulent atmosphere

We theoretically and experimentally investigate the beam-spreading of a vortex beam propagating in a turbulent atmosphere. It is found that the vortex beam is less affected by turbulence than a non-vortex one. The topological charge of the vortex beam on propagation has also been investigated experimentally. It is shown that the topological charge of vortex beam will exhibit fluctuating behavior as the beam propagates through a turbulent atmosphere.     

Microscale vortex laser with controlled topological charge

A microscale vortex laser is a new type of coherent light source with small footprint that can directly generate vector vortex beams. However, a microscale laser with controlled topological charge, which is crucial for virtually any of its application, is still unrevealed. Here we present a microscale vortex laser with controlled topological charge. The vortex laser eigenmode was synthesized in a metamaterial engineered non-Hermitian micro-ring cavity system at exceptional point. We also show that the vortex laser cavity can operate at exceptional point stably to lase under optical pumping.The microscale vortex laser with controlled topological charge can serve as a unique and general building block for nextgeneration photonic integrated circuits and coherent vortex beam sources. The method we used here can be employed to generate lasing eigenmode with other complex functionalities.     

Characterizing topological charge of optical vortex using non-uniformly distributed multi-pinhole plate

We propose an efficient method for characterizing the orbital angular momentum (OAM) of an optical vortex with a large topological charge (TC) through distinguishing the interference pattern of the non-uniformly-distributed multi-pinholes using fewer pinholes. This method overcomes the limit on large TC detection by multi-point interferometer and can be used to probe optical vortices with arbitrary sizes. In addition, it also has potential application in measuring light beam with OAM from astronomical sources.     

Settled fast measurement of topological charge by direct extraction of plane wave from vortex beam

A method of measuring the vortex beam topological charge(TC) is proposed based on a device that can directly extract the plane wave form from the vortex beam in which the different propagation angles of the plane waves are uniquely related to the different TCs. Then the TC can be obtained by simply comparing the energy values perceived by two fixed sensors in the detection location with the help of twin omnidirectional energy absorbers(OEAs). Because the settled detection relies only on the simple quantitative value at two fixed positions, neither pattern recognition nor field analysis procedure is applied, thus allowing faster measurement. Some features of the methodology are investigated, and the numerical simulations verify the feasibility and robustness of the system.     

Irrational charge from topological order

Topological or deconfined phases of matter exhibit emergent gauge fields and quasiparticles that carry a corresponding gauge charge. In systems with an intrinsic conserved U(1) charge, such as all electronic systems where the Coulombic charge plays this role, these quasiparticles are also characterized by their intrinsic charge. We show that one can take advantage of the topological order fairly generally to produce periodic Hamiltonians which endow the quasiparticles with continuously variable, generically irrational, intrinsic charges. Examples include various topologically ordered lattice models, the three-dimensional resonating valence bond liquid on bipartite lattices as well as water and spin ice. By contrast, the gauge charges of the quasiparticles retain their quantized values.     

Detecting the topological charge of vortex beams using an annular triangle aperture

The Fraunhofer diffraction pattern of a vortex beam by an annular triangle aperture is analyzed theoretically and experimentally. It is found that the pattern of the far-field diffraction intensity distribution exhibits a triangular lattice array, which becomes much clearer with the increase of the ratio of the inner to the outer side of the annular triangle aperture. The number of spot points of any external side of the triangular lattice array minus one is just equal to the topological charge value of the measured optical vortex. For the vortex beam with negative topological charge, the triangular diffraction pattern after the annular triangle aperture will be rotated by 180° in relation to the case of the positive topological charge. Based on the above properties, we propose a simple and feasible method to determine the magnitude and sign of the topological charge of an optical vortex beam.     

Measuring the topological charge of vortex beams by using an annular ellipse aperture

We theoretically and experimentally investigate the Fraunhofer diffraction pattern of a vortex beam passing through an annular ellipse aperture. It is found that the pattern of the far-field diffraction intensity distribution exhibits some dark spots, which become clearer with increasing the value of the ellipticity factor of the annular ellipse aperture. The diffraction phenomenon is more obvious with increasing the value of the ratio of the inner long axis (or short axis) to the outer side of the annular ellipse aperture. The number of the dark spots in the Fraunhofer diffraction intensity distribution is just equal to the topological charge value of the measured optical vortex, and the centre of each dark spot is just a phase-singularity point. Based on this property, we can measure the topological charge of an optical vortex beam.     

Measuring the topological charge of an optical vortex by using a tilted convex lens

We demonstrate, analytically and experimentally, a simple, but effective method to determine the topological charge of an optical vortex by using a spherical bi-convex lens, a ubiquitous optical element found in any optics laboratory. Just by tilting the lens and recording the intensity distribution of a propagating vortex at a predicted position past the lens, we have been able to measure both the sign and the magnitude of the topological charge m   up to m=±14$m=\pm 14$. Our experimental results are in excellent agreement with analytical predictions.     

Tunable optical vortex arrays from a single nematic topological defect

We report on the generation of tunable structured light fields endowed with various sets of phase singularities from a single topological defect in a nematic liquid crystal mesophase. The experimental demonstration relies on the use of electric field-induced nonsingular topological defects called "umbilics."     

A Study of premixed propagating flame vortex interaction

Experimental data is presented for the interaction between a propagating flame and a simple vortex flow field structure generated in the wake of solid obstacles. The interaction between gas movement and obstacles creates vortex shedding forming a simple flow field recirculation. The presence of the simple turbulent structure within the gas mixture curls the flame front increasing curvature and enhancing burning rate. A novel twin camera Particle Image Velocimetry, PIV, was employed to characterise the flow field recirculation and the interaction with the flame front. The technique allowed the quantification of the flame/vortex interaction. The twin camera technique provides data to define the spatial variation of both the velocity of the flow field and flame front. Experimentally obtained values of local flame displacement speed and flame stretch rate are presented for simple flame/vortex interactions.     

The topological structure of vortex in BEC

We find that there exists an elementary topological current in Bose-Einstein condensation. Based on the -mapping topological current theory, the implicit function theorem and the Taylor expansion, the topological structure of vortex lines is detailed in the neighborhoods of the bifurcation points of the condensate wave function. Received: 9 April 1998 / Revised: 28 August 1998 / Accepted: 31 August 1998     

Abrikosov vortex escape from a columnar defect as a topological electronic transition in a vortex core

The microscopic scenario of vortex escape from a columnar defect under the influence of a transport current has been studied. For defect radii smaller than the superconducting coherence length the depinning process is shown to be a consequence of two subsequent topological electronic transitions in a trapped vortex core. The first transition at a critical current j _L is associated with the opening of Fermi surface segments corresponding to the creation of a vortex-antivortex pair bound to the defect. The second transition at a certain current j _d > j _L is caused by merging of different Fermi surface segments, which accompanies the formation of a freely moving vortex.     

Instantons with fractional topological charge

We introduce and discuss the properties of multivalent gauges for Yang-Mills fields, and demonstrate the existence of instantons with fractional topological charges in such gauges.     

Partially coherent vortex beams propagating through slant atmospheric turbulence and coherence vortex evolution

Using the extended Huygens-Fresnel principle and the quadratic approximation of the phase structure function, and taking the Gaussian Schell-model (GSM) vortex beam as a typical example of partially coherent vortex beams, the explicit expressions for the cross-spectral density function and average intensity of GSM vortex beams with topological charge m=+1 propagating through slant atmospheric turbulence are derived, and used to study the propagation properties of GSM vortex beams in atmospheric turbulence along a slant path and evolution behavior of coherence vortices. It is shown that the spreading of GSM vortex beams along a horizontal path is larger than that along a slant path in the long atmospheric propagation. The propagation through horizontal atmospheric turbulence can be treated as a special case of the altitude-independent structure constant. The position of coherence vortices in slant atmospheric turbulence does not coincide with that in horizontal atmospheric turbulence, and the dependence of position of coherence vortices on the zenith angle, wavelength and reference point is illustrated by numerical examples. A comparison with the previous work is also made.     

The new method of topological charge determination of optical vortices in the interference field of the optical vortex interferometer

In this paper, the new method of determination of the topological charge of vortex points in the interference field obtained by three plane waves interference is presented. Such optical fields are used in the optical vortex interferometer (OVI) and the determination of vortex points’ topological charge allows of unique determination of the relative phase between interfering waves (phase unwrapping problem). The new method uses additional plane wave, which produce a characteristic fork-like fringe structure in the neighbourhood of vortex points. By analysing the orientation of these fork-like patterns one can read the sign of the topological charge of the given vortex point. The method is simple and can be used for OVI calibration performed before the measurements.     

Dynamics of a topological soliton propagating over a Frenkel-Kontorova chain

An approximate analytical expression describing the motion of a topological soliton along a Frenkel-Kontorova chain at relatively high speeds is derived with the help of the Langevin equation. Comparison with numerical solutions shows a good quality of the resulting approximation.     

One can have noninteger topological charge

It is argued that the recently found fractionally charged self-dual solution is relevant.     

Recognizing the orbital angular momentum(OAM)of vortex beams from speckle patterns

The orbital angular momentum(OAM)of vortex beams offers a new degree for information encoding,which has been applied to optical communications.OAM measurement is essential for these applications,and has been realized in free space by several methods.However,these methods are inapplicable to estimate the OAM of vortex beams directly from the speckle patterns in the exit end of a multimode fiber(MMF).To tackle this issue,we design a convolutional neural network(CNN)to realize 100%accuracy recognition of two orthogonally polarized OAM modes from speckle patterns.Moreover,we demonstrate that even when the speckle patterns are cropped to only 1/64 of the original patterns,the recognition accuracy of the designed neural network is still higher than 98%.We also study the recognition accuracy of cropped speckles in different areas of speckle patterns to verify the feasibility of OAM recognition after cropping.The results demonstrate that recognizing the OAMs of two orthogonally polarized vortex beams from only a portion of speckle patterns in the exit end of an MMF is feasible,offering the potential to construct a 1×N data transmission scheme.