Order Orbital Angular Momentum Modes Research Articles

Epsilon-near-zero photonic crystal fibers for a large mode separation of orbital angular momentum modes

An orbital angular momentum (OAM) mode is a good candidate as an orthogonal basis mode set for mode division multiplexing (MDM) owing to its unique phase properties. Photonic crystal fibers support large numbers of OAM modes with notable performances. However, the effective index difference between high order OAM modes is not large enough. In this work, we propose two types of epsilon-near-zero (ENZ) materials-embedded circular photonic crystal fibers for high mode separation. Compared with previously developed photonic crystal fibers, the designed fibers exhibit a high refractive index contrast owing to the inserted ENZ materials. The designed fibers support up to 42 OAM modes. Most of these OAM modes provide a large effective index difference (Δneff > 10−3) while maintaining a radial sing mode condition over a wide wavelength range from 1.3 μm to 1.8 μm. The proposed fibers could potentially be exploited in MDM for stable transmission of OAM modes and other OAM-based applications.

CUCA Based Equivalent Fractional Order OAM Mode for Electromagnetic Vortex Imaging

An equivalent fractional order orbital angular momentum (OAM) mode based on concentric uniform circular array (CUCA) is proposed in this paper. It is known that the azimuthal resolution of electromagnetic vortex imaging is closely related to the number of OAM modes. Although the multiple-in-multiple-out (MIMO) scheme gives better azimuthal resolution than the multiple-in-single-out (MISO) scheme, it suffers aliasing problem, which means the scattering points with their azimuthal difference larger than 180 degree can not be separated correctly. One possible way to solve this is adding fractional modes in MIMO scheme, but unfortunately, fractional modes are unstable and can not transmit to far field. In this paper, an equivalent fractional order OAM mode is introduced to deal with the problem to achieve higher azimuthal resolution by MIMO scheme. The transmitted and received modes are set to be two adjacent integer modes to realize an equivalent fractional mode. Since the radiation patterns generated by uniform circular array (UCA) for different modes are not aligned, concentric uniform circular array (CUCA) is applied and optimized to overcome this shortage. Simulation results demonstrate the validity of the proposed method.

Convolutional Neural Network Assisted Optical Orbital Angular Momentum Identification of Vortex Beams

The rapid and accurate identification of large-scale orbital angular momentum (OAM) modes is crucial for expanding the application of vortex beams (VBs). In this paper, an OAM mode recognition method based on convolutional neural networks (CNNs) is proposed and investigated. We construct an 8-layer CNN possesses complex feature extraction capability and train it to own powerful anti-turbulence competence by feeding the intensity patterns of VBs interfered by Gaussian beam. After supervised training of a large sample set, the CNN model takes on excellent network generalization ability and can well detect VBs with the mode range of [−50,50]. The simulation results indicate that under the influence of weak and medium turbulences, the average recognition accuracy exceeds 99%. Even under strong turbulence, the accuracy also reaches 98.54%. Meanwhile, the identification time is only 1.55ms per OAM mode with Intel(R) Xeon(R) Gold 6148 CPU. Moreover, the influence of different Gaussian beam waists, VB orders, input training sets, and CNN structures on OAM mode recognition performance, is fully studied. These results demonstrate that our proposed method can achieve higher accuracy and higher order OAM mode detection at a fast speed, which contributes a more effective method for the recognition of VBs.

On-Chip Guiding of Higher-Order Orbital Angular Momentum Modes

Higher-order orbital angular momentum (OAM) mode guiding in a waveguide which is suitable for on-chip integration has been investigated. Based on the relation between the Laguerre-Gaussian mode and the Hermite-Gaussian mode, it has been shown that two degenerate guided modes of π/2l-rotation symmetry can support the l-th order OAM mode. In order to mimic the rotational symmetry, we have proposed the waveguide structure of a cross-shaped core and designed a waveguide that can support OAM modes of ±1 and ±2 topological charges simultaneously at a wavelength of 1550 nm. Purity of the OAM modes guided in the designed waveguide has been assessed by numerically calculating their topological charges from the field distribution, which were close to the theoretical values. We also investigated the guiding of OAM modes of ±3 and ±4 topological charges in our proposed waveguide structure, which revealed the possibility of the separate guiding of those OAM modes with relatively lower purity.

Performance of circular photonic crystal fiber transmitting orbital angular momentum modes under macro-bending

Circular photonic crystal fibers (C-PCFs) have been proved to be a good candidate for orbital angular momentum (OAM) modes supporting fiber, because they possess many excellent features, such as good quality of OAM modes, large separation of the effective indices, low confinement loss, flat dispersion and a large mode field effective area. This paper gives evidence that the C-PCF also shows good performance in the perspective of bending strength. We establish a model to research the macro-bending characteristics of the C-PCF using conformal transformation. Through the analyses, we find that phases of all OAM modes are not sensitive to bending and higher order OAM modes show better bending resistance. Besides that, the 9 mm radius of curvature can be taken as the bending tolerance of the C-PCF, considering mode field characteristics, loss, intra-mode walk-off and inter-mode crosstalk. Finally, the comparison is presented between the C-PCF and ring-core fiber (RCF) with similar OAM modes, and we can conclude that the C-PCF shows an obviously better performance, immune to bending, than that of the RCF.

An OAM Patch Antenna Design and Its Array for Higher Order OAM Mode Generation

In this letter, a systematic design procedure is proposed on how to implement the characteristic mode analysis to design and optimize an orbital angular momentum (OAM) antenna with circular polarization. The main idea of this work is by combining proper high-order modes to fit the current distribution of the analytical model. It is shown that by using proper high-order modes, a single antenna element could act like an array making the antenna system more compact. As a prototype, a patch antenna working at the 2.4 GHz Wi-Fi bands to generate the right-hand circularly polarized OAM beam with topology charge ${l_{{{\bf OAM}}}} = -1$ is designed, fabricated, and measured to demonstrate the design method. In addition, an array composed by the proposed antenna element is designed and experimented. New physical phenomenon is found that the OAM modes of the antenna element and the array can be combined considering the multiplication theorem. The combination relaxes the restriction of $|{l_{\bf OAM}}| to some extent in comparison with the traditional uniform circular arrays for OAM generation.

Nonlinear frequency doubling characteristics of asymmetric vortices of tunable, broad orbital angular momentum spectrum

We report on a simple experimental scheme to generate and control the orbital angular momentum (OAM) spectrum of the asymmetric vortex beams in a nonlinear frequency conversion process. Using a spiral phase plate (SPP) and adjusting the transverse shift of the SPP with respect to the incident Gaussian beam axis, we have transformed the symmetric (intensity distribution) optical vortex of order l into an asymmetric vortex beam of measured broad spectrum of OAM modes of orders l, l − 1, l − 2, …, 0 (Gaussian mode). While the position of the SPP determines the distribution of the OAM modes, we have also observed that the modal distribution of the vortex beam changes with the shift of the SPP of all orders and finally results in a Gaussian beam (l = 0). Using single-pass frequency doubling of the asymmetric vortices, we have transferred the pump OAM spectra, l, l − 1, l − 2, …, 0, into the broad spectra of higher order OAM modes, 2l, 2l − 1, 2l − 2, …, 0 at green wavelength, owing to OAM conservation in nonlinear processes. We also observed an increase in single-pass conversion efficiency with the increase in asymmetry of the pump vortices producing a higher power vortex beam of mixed OAM modes at a new wavelength than that of the pure OAM mode.

Orbital Angular Momentum (OAM) Generation by Cylinder Dielectric Resonator Antenna for Future Wireless Communications

In this paper, we demonstrate that the solid cylinder dielectric resonator antenna (CDRA) is a high radiation efficient device to generate orbital angular momentum (OAM) waves with different OAM modes at different frequencies. Theoretical formulations of its radiation fields are deduced in the cylindrical coordinate system, which shows that all components of the electric and magnetic fields have the azimuth dependence of e jmφ . A full-wave simulation on a practical size CDRA is then implemented to validate the theoretical derivations. The simulation results show that different separated OAM modes can be obtained at different frequencies. Specifically, the higher order OAM modes are generated at higher electron magnetic modes, which makes CDRA a promising method to overcome the narrow band and multiband issues existing in the design of current OAM devices. Furthermore, a maximum radiation efficiency of 97.1% at 13.3 GHz, and a minimum radiation efficiency of 72.4% at 19.45 GHz are obtained in our simulation, which is a high radiation efficiency feature at the OAM resonant frequencies.

850-nm hybrid fiber/free-space optical communications using orbital angular momentum modes.

Light beams can carry orbital angular momentum (OAM) associated to the helicity of their phasefronts. These OAM modes can be employed to encode information onto a laser beam for transmitting not only in a fiber link but also in a free-space optical (FSO) one. Regarding this latter scenario, FSO communications are considered as an alternative and promising mean complementing the traditional optical communications in many applications where the use of fiber cable is not justified. This next generation FSO communication systems have attracted much interest recently, and the inclusion of beams carrying OAM modes can be seen as an efficient solution to increase the capacity and the security in the link. In this paper, we discuss an experimental demonstration of a proposal for next generation FSO communication system where a light beam carrying different OAM modes and affected by ℳ turbulence is coupled to the multimode fiber link. In addition, we report a better and more robust behavior of higher order OAM modes when the intermodal dispersion is dominant in the fiber after exceeding its maximum range of operation.

Orbital-Angular-Momentum Polarization Mode Dispersion in Optical Fibers

The orbital-angular-momentum (OAM) modes in optical fibers have polarization mode dispersion (PMD) properties similar to those of single-mode fibers (SMFs). The + l and − l order OAM modes supported by the same fiber vector modes undergo random cross coupling and exhibit a frequency-dependent time delay. We name this effect “OAM-PMD” and extend the formalism developed for PMD in SMFs to describe OAM-PMD. The characteristics of the modal beat lengths, birefringence correlation lengths, and the mean value of OAM-PMD are investigated. A fixed-analyzer technique is proposed and demonstrated to characterize this phenomenon in OAM fibers. Two different types of OAM fiber are examined. The measured results are compared with the theoretical calculations.