Orbital Angular Momentum Generation Research Articles

Reconfigurable OAM antenna based on sub‐wavelength phase modulation structure

A reconfigurable orbital angular momentum (OAM) antenna is studied based on PIN diode loaded multi-layer phase modulation structure. The PIN diodes on each layer are controlled on and off by changing their bias voltages, thus the phase range of the multi-layer antenna unit can cover 0-2 π . Considering the phase superposition of the OAM carrying beam and lens's beam converging, different OAM generation together with beam focusing can be designed by tuning the number of conductive PIN diodes of each layer. The reconfigurable OAM antenna with modes of ±1, ± 2, ±3 and ±4 is investigated via theoretical analysis as well as software simulation. The simulation results show that a single OAM state can be generated at a given time and eight OAM beams are switchable at 10 GHz by changing the bias voltage of each diode. The authors' work may give a new sight for the reconfigurable OAM antenna design and will widen the applications of the OAM-based electromagnetic sensing.

Two-dimensional tunable orbital angular momentum generation using a vortex fiber.

We demonstrate the two-dimensional tunable orbital angular momentum (OAM) generation in a ring-core (vortex) fiber. The LP11 mode generated by an all fiber fused coupler is coupled into a vortex fiber. Because the vector modes of the LP11 mode group in the vortex fiber are no longer degenerate, the mode status will change between linearly polarized modes (LPMs) and complex OAM modes periodically during propagation. The generated OAM can be tuned smoothly by filtering the mixed mode with different polarization directions or changing the wavelength at a certain polarization directions. The two-dimensional tuning of OAM from l=-1 to l=+1 is experimentally demonstrated in an all fiber OAM generator.

Optically Controlled Orbital Angular Momentum Generation in a Polaritonic Quantum Fluid.

Applications of the orbital angular momentum (OAM) of light range from the next generation of optical communication systems to optical imaging and optical manipulation of particles. Here we propose a micron-sized semiconductor source that emits light with predefined OAM pairs. This source is based on a polaritonic quantum fluid. We show how in this system modulational instabilities can be controlled and harnessed for the spontaneous formation of OAM pairs not present in the pump laser source. Once created, the OAM states exhibit exotic flow patterns in the quantum fluid, characterized by generation-annihilation pairs. These can only occur in open systems, not in equilibrium condensates, in contrast to well-established vortex-antivortex pairs.

Orbital angular momentum beams generated by passive dielectric phase masks and their performance in a communication link.

We demonstrate the generation of orbital angular momentum (OAM) beams using high-efficient polarization-insensitive phase masks. The OAM beams generated by the phase masks are characterized in terms of their tolerance to misalignment (lateral displacement or tilt) between the incident beam and phase mask. For certain scenarios, our results show that (a)when the tilt angle is within the range of -20 to +20 deg, the crosstalk among modes is less than -15 dB; and (b)lateral displacement of 0.3mm could cause a large amount of power leaked to adjacent modes. Finally, OAM beams generated by the phase masks are demonstrated over a two-channel OAM-multiplexing link, each channel carrying a 40Gbit/s data stream. An optical signal-to-noise-ratio (OSNR) penalty of ∼1 dB is measured without crosstalk at the bit error rate (BER) of 3.8×10-3. With crosstalk, an OSNR penalty of <1.5 dB is observed at the same BER.

Tunable Orbital Angular Momentum Generation Based on Two Orthogonal LP Modes in Optical Fibers

We present a method to generate tunable orbital angular momentum (OAM) based on linearly polarized (LP) modes in optical fibers. The tunable OAM is produced by combining two even (odd) LP modes with orthogonal polarization directions. The tunability of OAM is realized by controlling the power proportion of the two LP modes through a polarizer. From another perspective, the generated mixed vortex can also be regarded as a result of overlapping two orthogonal optical vortices of equal helicity but opposite chirality, then the tunable OAM can be achieved through filtering the mixed mode using a polarizer.

Theoretical analysis of hollow ring-core optical fibre for transmitting orbital angular momentum modes

Ring-core optical fibre has been successfully demonstrated to have good performance for lifting mode degeneracy, and maintaining orbital angular momentum (OAM) modes in the first radial order, making it promising for applications of OAM mode multiplexing and generation of cylindrical vector beams. The degeneracy lift can be intensified using a hollow ring-core fibre, i.e. when the innermost layer of waveguide is air. The cut-off ring thickness for supporting the lowest OAM modes in radial order are obtained under various refractive index differences between the transport layer and the cladding. To optimize the design, our analysis addresses the dependence of modal properties on the fibre structure parameters, such as the effective area (Aeff) of respective OAM mode, and the differential mode delay.

Radiation pattern synthesis for the generation of vortex electromagnetic wave

Radiation pattern synthesis methods for the generation of orbital angular momentum (OAM) beams based on circular arrays are proposed. The concentric-ring arrays are designed to suppress the side lobes of the radiation pattern firstly. Based on the genetic algorithm, two objective functions are defined to calculate the signal amplitude of each ring. Simulation results show that the proposed method can effectively suppress the side lobes, lower than -30 dB. Subsequently, for electromagnetic vortex imaging, the beam steering technique is applied to enhance the directivity of vortex beams for different OAM modes. Analyses indicate that large steering angle would lead to some distortion of the phase-front distribution. Finally, the imaging model after beam optimisation is established and the sparse Bayesian method is exploited to image the target with high resolution. The work can benefit the generation of OAM-carrying beams and its wide applications in radio communication and radar imaging.

Experimental Array for Generating Dual Circularly-Polarized Dual-Mode OAM Radio Beams

Recently, vortex beam carrying orbital angular momentum (OAM) for radio communications has attracted much attention for its potential of transmitting multiple signals simultaneously at the same frequency, which can be used to increase the channel capacity. However, most of the methods for getting multi-mode OAM radio beams are of complicated structure and very high cost. This paper provides an effective solution of generating dual circularly-polarized (CP) dual-mode OAM beams. The antenna consists of four dual-CP elements which are sequentially rotated 90 degrees in the clockwise direction. Different from all previous published research relating to OAM generation by phased arrays, the four elements are fed with the same phase for both left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP). The dual-mode operation for OAM is achieved through the opposite phase differences generated for LHCP and RHCP, when the dual-CP elements are sequentially rotated in the clockwise direction. The measured results coincide well with the simulated ones, which verified the effectiveness of the proposed design.

Sidelobe Suppression and Beam Collimation in the Generation of Vortex Electromagnetic Waves for Radar Imaging

For the electromagnetic (EM) vortex imaging purposes, the sidelobe suppression and the beam collimation method in the generation of orbital angular momentum (OAM) beams is proposed. Based on the concentric ring array, the objective function for the generic algorithm is defined to calculate the signal amplitude for each ring. Comprehensive simulations are conducted to validate the effectiveness of the proposed method. Results show that the main lobes of the radiation pattern of different OAM modes are collimated in the same direction and the sidelobes are all lower than –20 dB. Furthermore, the imaging model of the concentric-ring array is established, and the target image is obtained through numerical simulations. The work can advance the development of the EM vortex imaging technique and novel radar detection technology as well.

Radial Uniform Circular Antenna Array for Dual-Mode OAM Communication

A radial uniform circular array (UCA) is proposed for orbital angular momentum (OAM) generation and dual-mode communication based on a multilayer design. Theoretical derivation is presented for the demonstration of the OAM generation from radial UCAs. The UCA of single mode is realized by cascading an eight-dividing feeding network with equal magnitude and specific phases for each of two neighbor ports and eight microstrip antenna elements. Both the full-wave simulations and measurements of a final fabricated antenna array are carried out. From 5.72 to 5.95 GHz, the proposed antenna possesses good −15-dB bandwidths at the ±1 modes, and a very weak cross coupling (less than −24 dB) exists in this frequency band. The helical phase wavefronts are obtained, and the radiation patterns are presented. Moreover, the dual-mode multiplexing is achieved with isolations of different channels more than 19 dB in measurements.

A Broadband Dual-Polarized Dual-OAM-Mode Antenna Array for OAM Communication

The generation of multimode orbital angular momentum (OAM) carrying beams has attracted more and more attention. A broadband dual-polarized dual-OAM-mode uniform circular array is proposed in this letter. The proposed antenna array, which consists of a broadband dual-polarized bow-tie dipole array and a broadband phase-shifting feeding network, can be used to generate OAM mode −1 and OAM mode 1 beams from 2.1 to 2.7 GHz (a bandwidth of 25%) for each of two polarizations. Four orthogonal channels can be provided by the proposed antenna array. A 2.5-m broadband OAM link is built. The measured crosstalk between the mode matched channels and the mode mismatched channels is less than −12 dB at 2.1, 2.4, and 2.7 GHz. Four different data streams are transmitted simultaneously by the proposed array with a bit error rate less than 4.2×10-3 at 2.1, 2.4, and 2.7 GHz.

Three-Dimensional Simultaneous Arbitrary-Way Orbital Angular Momentum Generator Based on Transformation Optics.

In wireless communications, people utilize the technology of diversity against multipath fading, so as to improve the reliability of communication equipment. One of the long-standing problems in diversity antennas is the limited number of diversity in a certain space. In this paper, we provide a solution to this issue by a three-dimensional (3D) simultaneous arbitrary-way orbital angular momentum (OAM) generator (3D SAWOG) based on transformation optics. The proposed 3D SAWOG consists of a metamaterial block and a group of transformation cylinders, by which arbitrary-way planar wavefronts can be converted to helical wavefronts with various topological charges simultaneously. The 2D four-way OAM generator and the 3D SAWOG are analyzed, designed, and simulated. The simulation results validate the performance of a 3D SAWOG successfully, indicating that the proposed model possess a high mode purity and expansibility. The SAWOG can be used as a novel diversity antenna array due to the orthogonal property among different modes, which could provide more degrees of freedom than traditional dual-polarization antennas, further improving the reliability of the communication systems.

Ultrathin Complementary Metasurface for Orbital Angular Momentum Generation at Microwave Frequencies

Electromagnetic (EM) waves with helical wavefront carry orbital angular momentum (OAM), which is associated with the azimuthal phase of the complex electric field. OAM is a new degree of freedom in EM waves and is promising for channel multiplexing in communication system. Although the OAM-carrying EM wave attracts more and more attention, the method of OAM generation at microwave frequencies still faces challenges, such as efficiency and simulation time. In this work, by using the circuit theory and equivalence principle, we build two simplified models, one for a single scatter and one for the whole metasurface to predict their EM responses. Both of the models significantly simplify the design procedure and reduce the simulation time. In this paper, we propose an ultrathin complementary metasurface that converts a left-handed (right-handed) circularly polarized plane wave without OAM to a right-handed (left-handed) circularly polarized wave with OAM of arbitrary orders and a high transmission efficiency can be achieved.

Orbital Angular Momentum Generation With Ultra-Compact Bragg-Assisted Silicon Microrings

A silicon microring resonator with an embedded Bragg grating for orbital angular momentum (OAM) generation is designed and experimentally demonstrated. The device consists of a 3.9 ${\mu }\text{m}$ -radius ring resonator with cylindrical silica holes along the central path and exploits the radially polarized component of the propagating electric field in the resonator’s strip waveguide, enabling both the bandwidth broadening and the emission efficiency improvement, while reducing the footprint. The experimental results show that the device achieves a 3dB bandwidth of 134 GHz and an efficiency up to 6.5%. The interference patterns are measured for different OAM orders in which the number of spirals are clearly visible proving that the device can support different topological charges. Results are in good agreement with simulations.

Merging Geometric Phase and Plasmon Retardation Phase in Continuously Shaped Metasurfaces for Arbitrary Orbital Angular Momentum Generation

Traditional technologies to generate and manipulate the orbital angular momentum (OAM) suffer from bulky size and do not lend themselves to nanophotonic systems. An ultrathin metasurface based on abrupt phase shift has recently been proposed as an alternative method. Nevertheless, gradient phase was generally approximated by multiple meta-atom/molecules with discrete levels of abrupt phase shift, which not only increases the design and fabrication complexity but also causes difficulties in obtaining simultaneous electrical and optical functionality. Furthermore, a discontinuous phase profile would introduce phase noise to the scattering fields and deteriorate the purity of the OAM beams. Here, we propose a wavefront engineering mechanism by virtue of the spin–orbit interaction in continuously shaped plasmonic metasurfaces, which offers a new approach to generate OAM modes of high purity. Equally important, a method producing arbitrary OAM topological charge, integral and fractional, is demonstrated by mer...

Plasmonic and Dielectric Metasurfaces: Design, Fabrication and Applications

Two-dimensional metasurfaces are widely focused on for their ability for flexible light manipulation (phase, amplitude, polarization) over sub-wavelength propagation distances. Most of the metasurfaces can be divided into two categories by the material type of unit structure, i.e., plasmonic metasurfaces and dielectric metasurfaces. For plasmonic metasurfaces, they are made on the basis of metallic meta-atoms whose optical responses are driven by the plasmon resonances supported by metallic particles. For dielectric metasurfaces, the unit structure is constructed with high refractive index dielectric resonators, such as silicon, germanium or tellurium, which can support electric and magnetic dipole responses based on Mie resonances. The responses of plasmonic and dielectric metasurfaces are all relevant to the characteristics of unit structure, such as dimensions and materials. One can manipulate the electromagnetic field of light wave scattered by the metasurfaces through designing the dimension parameters of each unit structure in the metasurfaces. In this review article, we give a brief overview of our recent progress in plasmonic and dielectric metasurface-assisted nanophotonic devices and their design, fabrication and applications, including the metasurface-based broadband and the selective generation of orbital angular momentum (OAM) carrying vector beams, N-fold OAM multicasting using a V-shaped antenna array, a metasurface on conventional optical fiber facet for linearly-polarized mode (LP11) generation, graphene split-ring metasurface-assisted terahertz coherent perfect absorption, OAM beam generation using a nanophotonic dielectric metasurface array, as well as Bessel beam generation and OAM multicasting using a dielectric metasurface array. It is believed that metasurface-based nanophotonic devices are one of the devices with the most potential applied in various fields, such as beam steering, spatial light modulator, nanoscale-resolution imaging, sensing, quantum optics devices and even optical communication networks.

Conversion of the optical orbital angular momentum in a plasmon-assisted second-harmonic generation

We experimentally demonstrate the plasmon-assisted second-harmonic generation of an optical orbital angular momentum (OAM) beam. Because of the shape resonance, the plasmons in a periodic array of rectangular metal holes greatly enhance the nonlinear optical conversion of an OAM state. The OAM conservation (i.e., 2l1 = l2 with l1 and l2 being the OAM numbers of the fundamental and second-harmonic waves, respectively) holds well under our experimental configuration. Our results provide a potential way to realize nonlinear optical manipulation of an OAM mode in a nano-photonic device.

Experimental Investigation of LP11Mode to OAM Conversion in Few Mode-Polarization Maintaining Fiber and the Usage for All Fiber OAM Generator

In this paper, the LP11 mode evolution in a few mode-polarization maintaining fiber is experimentally investigated. Selective conversion from the LP11a/LP11b mode to the LP11b/LP11a , OAM+1, and OAM−1 modes are demonstrated with varying wavelengths and fiber lengths. This property may have nice applications in spatial division multiplexing (SDM) systems, and one of them, i.e., the all-fiber compact and robust scalar orbital angular momentum (OAM) generator, which is cascaded by a mode-selective coupler and a few mode-polarization maintaining fiber, is also demonstrated with expected performance for conversion between LP01 mode and OAM+1/OAM−1.

Tunable orbital angular momentum generation in optical fibers.

We present a method in this Letter to generate optical vortices with tunable orbital angular momentum (OAM) in optical fibers. The tunable OAM optical vortex is produced by combining different vector modes HE2,meven (HE2,modd) and TE0,m (TM0,m) when l=1 or combining HEl+1,meven (HEl+1,modd) and EHl-1,modd (EHl-1,meven) when l>1 with a π/2 phase shift. The vortex can be regarded as a result of overlapping two orthogonal optical vortex beams of equal helicity but opposite chirality with a π/2 phase shift. We have experimentally demonstrated the smooth variation of OAM from l=-1 to l=+1 by adjusting a polarizer at the output end of the fiber.

Continuously tunable orbital angular momentum generation using a polarization-maintaining fiber.

We demonstrate the generation of orbital angular momentum (OAM) in a two-mode polarization-maintaining (PM) optical fiber. We combine two linearly polarized modes of PM fiber to generate linearly polarized optical vortex beams with OAM. The average OAM can be finely varied by changing the phase between modes. We have quantitatively measured the resulting OAM to vary between ±1ℏ per photon while varying the relative phase between the LP11e- and LP11o-like fiber modes. The modal purity is 97%.