Visualization of internal energy flows in optical fields carrying a pair of fractional vortices

The internal energy flows resulting from the azimuthal component of optical currents in beams carrying a pair of fractional vortices during propagation have been experimentally detected in a knife edge test. Two optical vortices of fractional charge in a beam pair up by means of a connecting dark intensity line between intensity zeros of the vortices. Independent of the polarity of the fractional vortices, this pairing up process occurs. This is in contrast to the isolated intensity null points of vortices of integer charges and thus calls for a study on the internal energy flows in such beams.     

The birth, annihilation, and evolution of nonparaxial optical vortices: 2. Topological dipole

The events of the birth and annihilation of optical vortices in topologically neutral nonparaxial combined mode beams (topological dipoles) were studied. It is shown that there exists an extended domain of parameters of the elementary beams, for which the optical vortices of the dipole annihilate near the focal plane, thus restoring a wave field free of the wavefront dislocations.     

A class of partially coherent beams carrying optical vortices

A new class of partially coherent beams with a separable phase, which carry optical vortices, is introduced. It is shown that any member of the class can be represented as an incoherent superposition of fully coherent Laguerre-Gauss modes of arbitrary order, with the same azimuthal mode index. The free-space propagation properties of such partially coherent beams are studied analytically, and their M2 quality factor is investigated numerically.     

Higher-order extrema in two-dimensional wave fields

Higher-order extrema with topological indices greater than unity are discussed. Explicit constructions are given for their wave functions, and simple geometric rules are presented for analysis of their topological indices. Experimental means for verifying the theory with use of Gaussian laser beams are considered, unusual properties of optical vortices constructed from this new type of critical point are described, and applications to topologically based optical arithmetic are suggested.     

Quadrefringence of optical vortices in a uniaxial crystal

The splitting of a single optical vortex into four separate ones in a singular beam is theoretically and experimentally described for the propagation of obliquely incident light in a uniaxial crystal. We also find the condition under which the generated vortices in each of the four individual beams propagate independently without changing their structure and have different locations in all beams for any crystal lengths.     

Topological activity in Bragg elliptical twisted fibers

We have theoretically shown that Bragg twisted elliptical fibers manifest, in certain spectral regions, the property of topological activity--the ability to change in the reflected field the topological charge of incoming optical vortices and fundamental modes by two units. This property could be used for narrowband generation of optical vortices from Gaussian beams and for changing the topological charge of incoming optical vortices.     

Topological defects in moiré fringes with spiral zone plates

We present a study of spatial structures created by superposition of spiral zone plates used for generating optical beams with phase singularities. Moiré fringes are observed that show topological defects similar to those appearing in interference patterns of optical vortices. A brief theoretical discussion is included that supports the similarities between the two phenomena. Our results may lead to interesting applications to digital information processing by optical means.     

Method for exploring the orbital angular momentum of an optical vortex beam with a triangular multipoint plate

An efficient method for exploring the orbital angular momentum of an optical vortex beam is provided. The method, based on a triangular multipoint plate, can easily determine both the sign and the magnitude of the topological charge of the optical vortices. We demonstrate its feasibility by measuring the orbital angular momentum of Laguerre-Gaussian laser beams.     

Generation and conversion of optical vortices in long-period twisted elliptical fibers

We theoretically demonstrate that long-period twisted elliptical fibers have the ability to change in a certain wavelength range the topological charge of the incoming field by two units. We also show that such fibers can generate charge-2 optical vortices from the incoming Gaussian beams.     

Controllable elliptical dark-hollow beams

A new (to our knowledge) kind of light beam called the controllable elliptical dark-hollow beam (CEDHB), is introduced to describe dark-hollow beams with axially rotational asymmetry by using the tensor method. The propagation formulas of CEDHBs through paraxial aligned and misaligned nonsymmetrical optical systems are derived through vector integration. With the derived formulas, the propagation properties of CEDHBs in free-space propagation and through a misaligned thin lens are studied graphically. The CEDHBs provide a convenient model to describe and treat dark-hollow beams with axially rotational asymmetry and can be used conveniently to analyze atoms manipulated with a dark-hollow beam.     

Composite coherence vortices in coherent and incoherent superpositions of two off-axis partially coherent vortex beams

The composite coherence vortices by coherently and incoherently superimposing two parallel, off-axis partially coherent vortex beams and their evolution in free space are studied. It is shown that the superposition scheme, off-axis distance, coherence parameter and propagation distance affect the position and number of composite coherence vortices. The motion, creation and annihilation of composite coherence vortices appear by varying the off-axis distance, coherence parameter and propagation distance. The coherent and incoherent superpositions result in the different position and number of composite coherence vortices and their different evolution behavior in the coherent limit.     

Spatial correlation properties and correlation vortices of partially coherent vortex beams diffracted by an aperture

Taking the Gaussian–Schell model vortex beam as a typical example of partially coherent vortex beams, the spatial correlation properties and correlation vortices of partially coherent vortex beams diffracted by an aperture are studied. It is shown that the off-axis displacement and spatial coherence affects the spectral degree of coherence. The number and position of correlation vortices depend on the off-axis displacement, spatial coherence, aperture truncation and propagation distance, where the effect of aperture diffraction on the correlation vortices is stressed. The number of correlation vortices decrease as the truncation parameter increases. The correlation vortices in the diffracted field result from the vortex embedded in partially coherent beams at the source plane rather than from the aperture diffraction. The correlation vortices in the diffracted field appear even when the vortex core is stopped by the aperture.     

Characterization of optical fields with quantized orbital angular momentum by invariants of higher order moments of radial coordinates

We show that the skewness and kurtosis parameters of optical fields with quantized orbital angular momenta (OAM) and integer topological charge, which depend on the propagation distance only through normalized transverse coordinates, remain invariant at propagation through axially symmetric first-order optical systems, if defined in terms of higher-order moments of the radial coordinate. The values of these parameters, which characterize the shape of optical fields, depend on the type of OAM beams (Gaussian, Laguerre–Gauss or spiral phase plates in far-field) and the topological charge. As a result, the skewness and kurtosis can be used to identify the type of OAM beam and the absolute value of the topological charge for Gaussian and Laguerre–Gauss vortices encountered in most applications.     

High-power efficient multiple optical vortices in a single beam generated by a kinoform-type spiral phase plate

We propose using a solitary kinoform-type spiral phase plate structure to generate an array of vortices located in a single beam. Kinoform-type spiral surfaces allow each wavelength component of the phase modulation value to be wrapped back to its 2 pi equivalent for optical vortices of high charge. This allows the surface-relief profiles of high-charge vortices to be microfabricated with the same physical height as spiral phase plates of unity-charged optical vortices. The m-charged optical vortex obtained interacts with the inherent coherent background, which changes the propagation dynamics of the optical vortex and splits the initial m charge into /m/ unity-charged optical vortices within the same beam. Compared to a hologram, a multistart spiral phase plate is more efficient in the use of available spatial frequencies and beam energy and also is computationally less demanding. Furthermore, using microfabrication techniques will allow for greater achievable tolerances in terms of smaller feature sizes.     

Polarization-splitting common-path interferometer based on a zero-twist liquid crystal display

We present a compact optical polarization-splitting common-path interferometer based on a zero-twist liquid crystal display (LCD). The LCD is encoded with a diffraction grating pattern and illuminated with a polarization state with both horizontal and vertical components. The polarization component perpendicular to the director axis of the liquid crystal molecules is not affected by the LCD and forms the reference beam. However, the polarization component parallel to the director axis is diffracted at an angle determined by the period of the grating. By imposing an analyzer polarizer, these two beams create an interferogram that can either display retardance patterns encoded onto the LCD or analyze external birefringent optical elements. The programmability of the system allows new ways of increasing the utility of the interferograms. Experimental results are provided, including the visualization of optical vortices with different and opposite topological charges.     

Non-diffracting beams with controlled spatial coherence

We investigate the propagation properties of partially coherent non-diffracting beams. They are obtained as a superposition of plane waves possessing a single radial frequency and interfering with controlled angular correlation. The theoretical model is adapted to the experiments based on the use of the Fourier spatial filter illuminated by the Gaussian shell-model source. As an example, the influence of the partial coherence on the intensity distribution and phase properties of optical vortices is examined.     

OAM光通信技术研究进展

涡旋光束携带的轨道角动量(OAM)为光波的空间域提供了新的维度资源,吸引了越来越多研究人员的关注。由于具有不同OAM模式值的涡旋光束相互正交,因此将OAM模式引入传统光通信领域,衍生出两种新的应用机制--OAM键控(OAM-SK)与OAM复用(OAM-DM),这为未来实现高速、大容量及高频谱效率的光通信技术提供了潜在的解决方案。本文将从OAM光束的类别和产生方法等基本概念理论出发,对这两种通信应用机制相关的典型研究案例做简要概述,并重点论述三种关键技术,包括OAM光束复用技术、OAM光束解调技术以及OAM光通信的大气湍流效应抑制技术。最后,对OAM光通信技术的未来发展趋势及其前景进行了分析与展望。     

A few-mode optical fiber retaining optical vortices

The ability of an optical fiber with axial losses to selectively suppress the fundamental HE ₁₁ mode, as well as the TE and TM waveguide modes, and, simultaneously, to transmit optical vortices with almost zero energy losses is considered. The attenuation coefficients for the corresponding eigenmodes and vortices are determined. It is shown that such a fiber operates as a mode filter for the feeding beam.     

Dipole influence on Shack-Hartmann vortex detection in scintillated beams

Optical vortices can appear in an optical beam that propagates over a long distance through a turbulent atmosphere. A Shack-Hartmann wavefront sensor can be used to detect such vortices. However, the morphology of these vortices, which changes with beam propagation, and nearby oppositely charged vortices will affect this vortex detection. The influence of the morphology and the separation distance from oppositely charged vortices on the Shack-Hartmann vortex detection is studied. Numerical simulations for vortex detection under these turbulent atmospheric circumstances are also provided.