Helical Phase Front Research Articles

Evolution of orbital angular momentum in three-dimensional structured light

Light beams with an azimuthal phase dependency of $e^{i\ell\phi}$ have helical phase fronts and thus carry orbital angular momentum (OAM), a strictly conserved quantity with propagation. Here, we engineer quasi three-dimensional (3D) structured light fields and demonstrate unusual scenarios in which OAM can vary locally in both sign and magnitude along the beam's axis, in a controlled manner, under free-space propagation. To reveal the underlying mechanisms of this phenomenon, we perform full modal decomposition and reconstruction of the generated beams to describe the evolution of their intrinsic OAM and topological charge with propagation. We show that topological transition and the associated variation in local OAM rely on the creation, movement, and annihilation of local vortex charges without disturbing the global net charge of the beam, thus conserving the global OAM while varying it locally. Our results may be perceived as an experimental demonstration of the Hilbert Hotel paradox, while advancing our understanding of topological deformations in general.

Broadband efficient vortex beam generation with metallic helix array

Vortex beams with orbital angular momentum are characterized by a helical phase front and a phase singularity at the beam center. Such beams have various intriguing applications, such as optical communications, particle trapping, and high resolution imaging. Generating a vortex beam by exploiting the geometric phase has attracted great interest, due to its spin-to-orbital conversion feature and device versatility. To meet the requirements of applications such as optical communications and make the devices more user-friendly, vortex beam generation needs to be efficient in a large frequency range. The efficient bandwidth of the optical element is critical here, since the geometric phase is frequency independent. In this work, we design and fabricate a broadband efficient reflective vortex beam generator based on a metallic helix array, which has a uniform efficient co-circular-polarization reflection property. Vortex beams with arbitrary topological charge can be generated with such devices. The measured efficiency of our device is above 58.4%, with the relative bandwidth of 64.52%.

Directly using 88-km conventional multi-mode fiber for 6-mode orbital angular momentum multiplexing transmission

Twisted light carrying orbital angular momentum (OAM), which featuring helical phase front, has shown its potential applications in diverse areas, especially in optical communications in free space and specially designed fibers, e.g. a vortex fiber. Instead of specially designed fibers extensively used in the reported OAM-based fiber transmission experiments, here we demonstrate the viability of a conventional graded-index multi-mode fiber (MMF) for OAM multiplexing transmission with less digital signal processing (DSP) complexity. We demonstrate a 120-Gbit/s quadrature phase-shift keying (QPSK) signal transmission in an 8.8-km OM4 MMF by using OAM mode multiplexing with all the modes in the first two mode-groups (OAM01L, OAM01R, OAM+11L, OAM+11R, OAM-11L, OAM-11R) with only 2×2 and 4×4 multiple-input-multiple-output (MIMO) equalization. Moreover, we demonstrate the data-carrying two OAM mode groups multiplexing transmission over the 8.8-km MMF without MIMO equalization. These demonstrations may open up new perspectives to enable the realistic use of OAM-based MMF solution in data centers and super-computers.

Equivalence between orbital angular momentum and multiple‐input multiple‐output in uniform circular arrays: Investigation by eigenvalues

AbstractIt had been shown that array weight vectors using uniform circular arrays (UCAs) in multiple‐input multiple‐output (MIMO) can generate helical phase distribution used in orbital angular momentum (OAM) communication system. In this letter, the MIMO channel, or OAM multiplexing characteristic, was investigated by means of eigenvalues. It was also found that the OAM is not effective as the distance between transmitting and receiving antennas increases because the radiation pattern along the direction of propagation is null with helical phase front. It was found that MIMO and/or OAM performance improve in near‐field region. It was also found that the performance improves when radius of UCAs becomes large.

Flexible and high-efficiency generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere

We propose theoretically and verify experimentally a compact optical configuration to directly generate arbitrary vector vortex beams on a hybrid-order Poincare sphere with good flexibility and high efficiency based on a reflective phase-only liquid crystal spatial light modulator (LC-SLM). The conversion system, consisting of an LC-SLM and a quarter-wave plate, can be considered a flexible dielectric metasurface to simultaneously modulate inhomogeneous polarization and helical phase-front. This approach has some advantages, including a simple experimental setup, good flexibility, and high efficiency. Orthogonally polarized modes alignment and an explicit superposition existing in the conventional method are not necessary in the proposed method, which exhibits potential applications in many advanced domains.

Performance evaluation of underwater optical communications using spatial modes subjected to bubbles and obstructions

Spatial modes have attracted increasing interest in free-space and fiber-based optical communications. Underwater wireless optical communication is becoming a promising technique in marine exploration. Here we investigate the underwater wireless optical communications using different spatial modes, i.e., traditional Gaussian modes, orbital angular momentum modes having helical phase fronts, and diffraction-free and obstruction-tolerant Bessel modes. We evaluate the underwater transmission performance of three spatial modes subjected to dynamic bubbles, which cause similar power fluctuations, regardless of spatial modes. We also demonstrate an underwater transmission link subjected to static obstructions using three spatial modes carrying 1.4Gbaud orthogonal frequency division multiplexing 16-ary quadrature amplitude modulation (16-QAM) signals. The Bessel mode shows the best performance against obstructions.

Orbital angular momentum mode groups multiplexing transmission over 2.6-km conventional multi-mode fiber.

Twisted light carrying orbital angular momentum (OAM) is a special kind of structured light that has a helical phase front, a phase singularity, and a doughnut intensity profile. Beyond widespread developments in manipulation, microscopy, metrology, astronomy, nonlinear and quantum optics, OAM-carrying twisted light has seen emerging application of optical communications in free space and specially designed fibers. Instead of specialty fibers, here we show the direct use of a conventional graded-index multi-mode fiber (MMF) for OAM communications. By exploiting fiber-compatible mode exciting and filtering elements, we excite the first four OAM mode groups in an MMF. We demonstrate 2.6-km MMF transmission using four data-carrying OAM mode groups (OAM0,1, OAM+1,1/OAM-1,1, OAM+2,1, OAM+3,1). Moreover, we demonstrate two data-carrying OAM mode groups multiplexing transmission over the 2.6-km MMF with low-level crosstalk free of multiple-input multiple-output digital signal processing (MIMO-DSP). The demonstrations may open up new perspectives to fiber-based OAM communication/non-communication applications using already existing conventional fibers.

Order-Controllable Cylindrical Vector Vortex Beam Generation by Using Spatial Light Modulator and Cascaded Metasurfaces

Cylindrical vector vortex (CVV) beam, which possesses both helical phase front and spatially inhomogeneous polarization, is a promising structured light for its various applications ranging from optical communication to optical field manipulation and optical microscopy. However, approaches to generate CVV beams with switchable and tunable polarization order and topological charge are still immature, which hinders the wide application of CVV beams. In this paper, we have experimentally demonstrated that order-controllable CVV beams can be produced by using spatial light modulator (SLM) and equivalent <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -plate system at wavelength of 1550.8 nm. It is shown that the topological charge of the CVV beam can be switched by directly programming the SLM. We have also demonstrated that the polarization order of the CVV beam can be tuned to as high as eight by employing an equivalent <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -plate system, which consists of two cascaded metasurfaces and a half-wave plate. To further verify the helical phase of the CVV beam, we have proposed a novel measurement method based on first removing the vector property and then interfering the remaining helical phase with plane wave or spherical wave.

Characterization of Red/Green/Blue Orbital Angular Momentum Modes in Conventional G.652 Fiber

The space domain of lightwaves has attracted increasing interest in optical communications. Lightwaves having helical phase front and carrying orbital angular momentum (OAM) have seen potential applications both in free-space and fiber-based optical communications. The widely deployed conventional ITU-T G.652 fiber is single mode fiber at 1550 nm, which, however, might support high-order modes as well as OAM modes at short wavelength. In this paper, we present detailed theoretical analyses on the mode properties of circularly polarized OAM modes in conventional G.652 fiber at three RGB wavelengths (red at 632.8 nm, green at 532 nm, and blue at 476.5 nm). The G.652 fiber has a 4.6-μm core radius, a 62.5-μm cladding radius, and 0.277% refractive index difference between the core and the cladding. We first study all the cylindrical vector modes (fiber eigenmodes) and synthesize the circular OAM modes by proper linear combination of degenerate fiber eigenmodes supported in the G.652 fiber at three RGB wavelengths. We then calculate the effective mode area and nonlinearity and discuss the tolerance to fiber ellipticity and bending (n eff differences, 2π walk-off length, 10-ps walk-off length, loss, and OAM crosstalks) for red, green, and blue OAM modes. Additionally, we investigate the OAM modes properties at another three wavelengths of conventional semiconductor lasers, i.e., 780, 650, and 405 nm. Moreover, we also estimate the fiber attenuation, propagation loss, and communication bandwidth/capacity. The obtained results may stimulate wide interesting OAM related applications using conventional G.652 fiber, especially for hundreds of meters or km-scale short reach/distance applications.

Generation and Transmission of OAM-Carrying Vortex Beams Using Circular Antenna Array

Electromagnetic orbital angular momentum (OAM) has attracted a lot of interest recently in the field of communication systems. OAM-carrying radio beams can be generated and detected by several devices. In this paper, we use a circular phased antenna array to generate OAM-carrying beams that possess ring-shaped intensities and helical phase fronts. We also propose and validate a communication system that uses OAM-based multiplexing. A circular antenna array is used to generate the superimposed OAMmode by using appropriate excitation settings. The transmission characteristics of OAM modes with respect to different transmission distances are investigated. We find that the OAM-based multiplexing technology is only feasible within a certain distance range. The obtained experimental results are consistent with simulation and numerical calculation results.

Orbital angular momentum 25 years on [Invited

Twenty-five years ago Allen, Beijersbergen, Spreeuw, and Woerdman published their seminal paper establishing that light beams with helical phase-fronts carried an orbital angular momentum. Previously orbital angular momentum had been associated only with high-order atomic/molecular transitions and hence considered to be a rare occurrence. The realization that every photon in a laser beam could carry an orbital angular momentum that was in excess of the angular momentum associated with photon spin has led both to new understandings of optical effects and various applications. These applications range from optical manipulation, imaging and quantum optics, to optical communications. This brief review will examine some of the research in the field to date and consider what future directions might hold.

Design and implementation of planar reflection spiral phase plate for beams with orbital angular momentum

In this study the authors report on a design of an offset-fed reflection planar spiral phase plate (SPP) to generate electromagnetic waves with orbital angular momentum (OAM). Detailed design guidelines and measurement procedures are given, respectively in this study. Through the use of a phase tuning technique, the proposed planar SPP converts incident spherical wave to beams with desired helical phase front. An experimental mode-2 OAM beam generation setup working at 94 GHz has been performed to verify the design method. Typical OAM phase and intensity patterns can be observed in the experiment. The OAM spectrum calculated from this experiment phase information proves the high purity performance of the OAM beams that this approach generated.

Nondestructive Measurement of Orbital Angular Momentum for an Electron Beam.

Free electrons with a helical phase front, referred to as "twisted" electrons, possess an orbital angular momentum (OAM) and, hence, a quantized magnetic dipole moment along their propagation direction. This intrinsic magnetic moment can be used to probe material properties. Twisted electrons thus have numerous potential applications in materials science. Measuring this quantity often relies on a series of projective measurements that subsequently change the OAM carried by the electrons. In this Letter, we propose a nondestructive way of measuring an electron beam's OAM through the interaction of this associated magnetic dipole with a conductive loop. Such an interaction results in the generation of induced currents within the loop, which are found to be directly proportional to the electron's OAM value. Moreover, the electron experiences no OAM variations and only minimal energy losses upon the measurement, and, hence, the nondestructive nature of the proposed technique.

Generation and propagation characteristics of electromagnetic vortices in radio frequency

Electromagnetic vortices, which describe the orbital angular momentum (OAM) carrying waves with a helical phase front, have recently attracted much interest in a radio frequency domain due to their potential applications in many diverse areas. In an OAM-based scenario, the antenna for OAM mode multiplexing/demultiplexing plays an essential role in controlling the overall system performance. In this paper, we demonstrated theoretically and experimentally an easily realized OAM antenna based on the traveling-wave circular loop structure for efficiently multiplexing/demultiplexing multiple OAM modes; in addition, its general propagation characteristics including the polarization, divergence, and radiation pattern are mathematically analyzed. Schemes for antenna size reduction and various radiation pattern manipulations have also been discussed to realize a more flexible and compact system.

Study on the theory and method of vortex‐electromagnetic‐wave‐based radar imaging

Helical phase fronts and orbital angular momentum of the vortex electromagnetic (EM) wave are applied to radar imaging at microwave frequencies and comprehensive simulations are conducted. Results demonstrate that an incrementally phased uniform circular array can be exploited to generate vortex EM wave at microwave frequencies. The imaging model based on the linear frequency modulation signal is established and the pulse-compression and fast Fourier transform methods are used to image the target in both range and azimuth dimensions. Furthermore, antenna pattern optimisation methods for imaging are studied to enhance the directivity of the emitting signal. The work can promote the significant development of novel information-rich radar imaging technology and provide staring imaging with substantial technical supports.

On the natures of the spin and orbital parts of optical angular momentum

The modern field of optical angular momentum began with the realisation by Allen et al in 1992 that, in addition to the spin associated with polarisation, light beams with helical phase fronts carry orbital angular momentum. There has been much confusion and debate, however, surrounding the intricacies of the field and, in particular, the separation of the angular momentum into its spin and orbital parts. Here we take the opportunity to state the current position as we understand it, which we present as six perspectives: (i) we start with a reprise of the 1992 paper in which it was pointed out that the Laguerre–Gaussian modes, familiar from laser physics, carry orbital angular momentum. (ii) The total angular momentum may be separated into spin and orbital parts, but neither alone is a true angular momentum. (iii) The spin and orbital parts, although not themselves true angular momenta, are distinct and physically meaningful, as has been demonstrated clearly in a range of experiments. (iv) The orbital part of the angular momentum in the direction of propagation of a beam is not simply the azimuthal component of the linear momentum. (v) The component of spin in the direction of propagation is not the helicity, although these are related quantities. (vi) Finally, the spin and orbital parts of the angular momentum correspond to distinct symmetries of the free electromagnetic field and hence are separately conserved quantities.

High-dimensional structured light coding/decoding for free-space optical communications free of obstructions.

Bessel beams carrying orbital angular momentum (OAM) with helical phase fronts exp(ilφ)(l=0;±1;±2;…), where φ is the azimuthal angle and l corresponds to the topological number, are orthogonal with each other. This feature of Bessel beams provides a new dimension to code/decode data information on the OAM state of light, and the theoretical infinity of topological number enables possible high-dimensional structured light coding/decoding for free-space optical communications. Moreover, Bessel beams are nondiffracting beams having the ability to recover by themselves in the face of obstructions, which is important for free-space optical communications relying on line-of-sight operation. By utilizing the OAM and nondiffracting characteristics of Bessel beams, we experimentally demonstrate 12 m distance obstruction-free optical m-ary coding/decoding using visible Bessel beams in a free-space optical communication system. We also study the bit error rate (BER) performance of hexadecimal and 32-ary coding/decoding based on Bessel beams with different topological numbers. After receiving 500 symbols at the receiver side, a zero BER of hexadecimal coding/decoding is observed when the obstruction is placed along the propagation path of light.

Manipulation of metallic nanoparticle with evanescent vortex Bessel beam.

In this work, we propose a novel strategy to optically trap and manipulate metallic nanoparticles using evanescent vortex Bessel beam (EVBB). A versatile method is presented to generate evanescent Bessel beam with tunable optical angular momentum by focusing a radially polarized vortex beam onto a one-dimensional photonics band gap structure. The behavior of a metallic nanoparticle in the EVBB is numerically studied. We show that such particle can be stably trapped near the surface. The orbital angular momentum drives the metallic nanoparticle to orbit around the beam axis, and the direction of the orbital motion is controlled by the handedness of the helical phase front. The technique demonstrated in this work may open up new avenues for optical manipulation, and the non-contact tunable orbiting dynamics of the trapped particle may find important applications in higher resolution imaging techniques.

Helicon modes in uniform plasmas. III. Angular momentum

Helicons are electromagnetic waves with helical phase fronts propagating in the whistler mode in magnetized plasmas and solids. They have similar properties to electromagnetic waves with angular momentum in free space. Helicons are circularly polarized waves carrying spin angular momentum and orbital angular momentum due to their propagation around the ambient magnetic field B0. These properties have not been considered in the community of researchers working on helicon plasma sources, but are the topic of the present work. The present work focuses on the field topology of helicons in unbounded plasmas, not on helicon source physics. Helicons are excited in a large uniform laboratory plasma with a magnetic loop antenna whose dipole axis is aligned along or across B0. The wave fields are measured in orthogonal planes and extended to three dimensions (3D) by interpolation. Since density and B0 are uniform, small amplitude waves from loops at different locations can be superimposed to generate complex antenna patterns. With a circular array of phase shifted loops, whistler modes with angular and axial wave propagation, i.e., helicons, are generated. Without boundaries radial propagation also arises. The azimuthal mode number m can be positive or negative while the field polarization remains right-hand circular. The conservation of energy and momentum implies that these field quantities are transferred to matter which causes damping or reflection. Wave-particle interactions with fast electrons are possible by Doppler shifted resonances. The transverse Doppler shift is demonstrated. Wave-wave interactions are also shown by showing collisions between different helicons. Whistler turbulence does not always have to be created by nonlinear wave-interactions but can also be a linear superposition of waves from random sources. In helicon collisions, the linear and/or orbital angular momenta can be canceled, which results in a great variety of field topologies. The work will be contrasted to the research on helicon plasma sources.

Helicons in Unbounded Plasmas.

Helicons are whistler modes with helical phase fronts. They have been studied in solid state plasmas and in discharge tubes where boundaries and nonuniformities are ever present. The present work shows that helicons also exist in unbounded and uniform plasmas, thereby bridging the fields of laboratory and space plasma physics. First measurements of helicon field lines in three dimensional space are presented. Helicons with negative and positive mode numbers can propagate with equal amplitudes.