Orbital Angular Momentum Mode Purity Research Articles

Orbital angular momentum mode multiplexing communication in multimode fibers

Vortex beams with orbital angular momentum (OAM) have infinite mode orthogonality, which can greatly boost communication capacity through mode-division multiplexing. However, current exploration of OAM beam transmission primarily focuses on free-space or specialty fibers limited by the OAM spiral phase wavefront characteristics. Due to the mode field phase sensitivity, these methods are constrained by turbulence interference and fabrication error tolerance, restricting transmission distance of OAM beam at practical application. Herein, we propose an OAM transmission scheme using commercial multimode fiber (MMF) exploiting the eigenmodes superposition theory. Leveraging linear superposition of HE and EH vector modes with ± (π/2) phase shift over MMF, OAM excitation and transmission can be supported with low interference. Simultaneously, due to the decreased demand for fabrication precision in the gradient core structure, the tolerance for machining errors is enhanced. As a proof-of-concept, we have demonstrated a multidimensional OAM multiplexing communication with 640-channel (4 OAM modes, 80 wavelengths and 2 polarization states) over a 5 km MMF, successfully transmitting QPSK-OFDM signals at a total rate of 15 Tbit/s. The system demonstrated OAM mode purity exceeding 65% and bit error rate (BER) below the forward error correction (FEC) threshold (3.8 × 10−3). Multiplexed OAM signals can be effectively transmitted in MMF over short to medium distances with acceptable mode crosstalk, while remaining compatible with wavelength and polarization dimensions, enabling the construction of multi-dimensional multiplexing local area networks.

Multiplication of orbital angular momentum via multi-plane light conversion.

The multiplication of orbital angular momentum (OAM) modes using optical coordinate transformation is useful for OAM optical networks, but the scalability of this scheme is limited by the ray model. Here, we propose an alternative scheme for the scalable multiplication of OAM modes based on modified multi-plane light conversion (MPLC) that can extend azimuthal and radial indices of OAM modes supported by the multipliers and unlock a new degree of freedom for radial high-order OAM states that has been restricted in the zero order. The multiplication for 20 OAM modes with radial index p = 0 and 10 OAM modes with radial index p = 1 is performed in simulation and experiment. The 3-dB optical bandwidth corresponding to the purity of OAM modes covers the entire C-band experimentally. This novel, to the best of our knowledge, approach to manipulating OAM states provides valuable insights and flexible strategies for high-capacity OAM optical communication and high-dimensional optical quantum information processing.

Co-Modulation of Spin Angular Momentum and High-Order Orbital Angular Momentum Based on Anisotropic Holographic Metasurfaces

In this communication, a new method for modulating spin angular momentum (SAM) and orbital angular momentum (OAM) based on anisotropic holographic metasurfaces is proposed, which can realize the co-modulation of SAM, high order OAM, and beam’s direction simultaneously. The tensor unit adopts an ellipse patch with three adjustable parameters without slotting, and a design and analysis method suitable for this kind of tensor unit is proposed based on the two-dimensional effective impedance database for geometrical size mapping. Based on the tensor unit and proposed methods, the vortex beams with a single SAM state, a single high order OAM mode, and different directions are generated at 22 GHz. Then, the deflected vortex beams with the same SAM state and mixed OAM modes, or the deflected vortex beams with different SAM states and OAM modes are realized using a single feed and a single metasurface integrated for the first time. Simulation and experimental measurement results show that the OAM mode purity can reach more than 80% of the theoretical best value in the main SAM state. The proposed design methods and the device can achieve arbitrary modulation of SAM and OAM, providing a solution for combining and applying the two to multiplexing communications.

Parabolic-Index Ring-Core Fiber Supporting High-Purity Orbital Angular Momentum Modes

We design a graded-index ring-core fiber with a GeO2-doped silica ring core and SiO2 cladding. This fiber structure can inhibit the effect of spin-orbit coupling to mitigate the power transfer among different modes and eventually enhance the orbital angular momentum (OAM) mode purity. By changing the high-index ring core from the step-index to parabolic graded-index profile, the purity of the OAM1,1 mode can be improved from 86.48% to 94.43%, up by 7.95%. The proposed fiber features a flexible structure, which can meet different requirements for mode order, effective mode area, etc. Simulation results illustrate that the parabolic-index ring-core fiber is promising in enhancing the OAM mode purity, which could potentially reduce the channel crosstalk in mode-division-multiplexed optical communication systems.

A broadband orbital angular momentum generator utilizing polarization conversion metasurface at microwave frequencies

In this paper, we achieved first- and second-order orbital angular momentum (OAM) vortex beams by a single-layer polarization conversion metasurface (PCM). Firstly, we present the basic PCM unit, composed of a short line and two pairs of oval split rings along the diagonal direction attached to a substrate layer. The agreement of numerical simulations and experimental measurements proves that the employed structure’s polarization conversion ratio (PCR) is more than 90% from 8.12 to 22.81 GHz under normal incidence (corresponding to the fractional bandwidth of 94.9%). We reveal the physical mechanism of the proposed PCM by an equivalent circuit method in broadband. Furthermore, to construct the reflective PCM array, the reflection phase of 2π can be obtained by adjusting the geometric parameters of the units. The OAM vortex beams with different topological charges (l= −1, l= −2) can be achieved by specific compensation phases. Simultaneously, we calculate the purity of different OAM modes from 10 to 15 GHz, demonstrating that the main-mode dominates broadband. Finally, the proposed reflective array realizes a stable response to the variations in the incidence angle from 0° to 45°. The proposed method provides an opportunity to realize different topological charges of OAM waves by PCM units in broadband.

OAM mode purity improvement based on antenna array

Orbital Angular Momentum (OAM) waves are characterized by helical wave fronts and orthogonality between different modes. Therefore, OAM waves have huge potential in improving wireless communications' channel capacity and radar imaging's resolution. Consequently, the generation and application of OAM waves have attracted a lot of attention. And many methods are proposed to generate OAM waves. Although antenna array is the most popular method of generating OAM waves, OAM waves generated by antenna array have redundant modes. However, all advantages of OAM waves are closely related to infinite OAM modes. Thus, to better apply OAM waves to wireless communications and radar, it is very important to reduce unnecessary OAM modes and improve the OAM mode purity. In order to improve the OAM mode purity, two combined antenna arrays composed of X direction antenna and Y direction antenna array are proposed in this paper. The X direction antenna array and the Y direction antenna array are supplied by the excitations with the same amplitude and fixed phase shift. The overall phase shift of the X direction antenna array is π/2 more or less than that of the Y direction antenna array. The results of formulas and antenna models in CST show that the combined antenna arrays can generate OAM waves with less redundant modes in x component, y component and z component. Besides, the z component carries pure OAM modes.

GeO2-doped ring-core photonic crystal fiber for supporting robust orbital angular momentum modes.

Orbital angular momentum (OAM)-mode-supported photonic crystal fibers (PCFs) have inspired intensive research in modern fiber optics due to the robust propagation and theoretically unlimited signal-carried channels. In this paper, a dual-cladding GeO2-doped ring-core PCF is designed, and a strategy for optimizing OAM mode properties is analyzed by structure parameters and GeO2-doping concentration. Numeric results show that high structural degrees of freedom are available to improve the effective refractive index separation (within the vector modes), chromatic dispersion, effective mode field area, nonlinear coefficient, and OAM mode purity in terms of inner cladding, outer cladding, and ring-core. In particular, the effective refractive index separation and chromatic dispersion can exhibit a high order magnitude of 10-3 and a low value in the broad band from 1.3 µm to 1.7 µm, respectively. In addition to structural optimization, doping high index material into the ring-core is another way to regulate the fiber performance by controlling the doping concentration. A systematic investigation shows that as the doping concentration increases, the effective refractive index separation and mode purity increase obviously, while the dispersion and mode field area gradually decrease. This flexible manipulation offers a method for customizing the optical properties of OAM-supported PCFs in communication and sensor systems.

All-fiber switchable orbital angular momentum mode-locked laser based on TM-FBG

In this paper, a simple all-fiber switchable orbital angular momentum (OAM) mode-locked laser is demonstrated. The laser is mainly composed of a single-mode fiber Bragg grating (FBG), a two-mode fiber Bragg grating (TM-FBG), a two-mode circulator, and a nonlinear polarization rotation system. The coupling properties of the TM-FBG are verified, and an OAM mode-locked laser with switchable topological charges of −1, 0, and 1 is realized. When the pump power is 462 mW, the output powers of the fundamental mode and OAM±1 mode-locked lasers are 9.750 and 2.707 mW, respectively. Their repetition rates are both 10.16 MHz, and the signal-to-noise ratios are 60 and 59 dB. When the pump power is increased to 774 mW, the mode-locked laser can operate in the single-pulse, double-pulse, and triple-pulse states. Their output powers are 5.1, 7.4, and 10.1 mW, respectively. The OAM mode purity higher than 98.9% is experimentally realized.

Generation of arbitrary vector Bessel beams on higher-order Poincaré spheres with an all-dielectric metasurface

Vector Bessel beams may have a spatially varying polarization state combined with a Bessel-type transversal intensity profile. Such beams can carry both spin angular momentum and orbital angular momentum (OAM) and be characterized by points on higher-order Poincar\'e (HOP) spheres. We report a method to produce vector Bessel beams on any point of a HOP sphere of any topological charge by utilizing only a single all-dielectric planar metasurface. Specifically, by a proper choice of the input polarization and distance from the metasurface, the entire HOP sphere surface can be covered. The proposed method is simple and efficient and applies to any $n\mathrm{th}$-order HOP sphere. Furthermore, it yields an extremely high OAM mode purity. Our paper demonstrates a robust approach for generating arbitrary high-quality vector Bessel beams and will likely inspire future applications ranging from optical manipulation to laser beam engineering.

OAM Carrying Vortex Beam Mode Interconversion Using Modular Cascaded Transmitarrays

Orbital angular momentum (OAM) carrying vortex beams has been studied and realized using different methods. However, the designed devices are usually capable of generating fixed OAM modes. Here, the concept of a modular radiating system based on tightly cascaded transmitarrays (TAs) provi- ding a compact and user-accessible modular radiating system suitable for affordable detecting and imaging systems is presented. The TA is based on a novel compact unit cell. The unit cell characteristics are explained and then cascaded for the TAs’ operation. Several prototype TA modules are realized, and their interactions for OAM mode interconversion are investigated. A restricted method for determining the OAM mode purity of the generated beams is employed. The simulated and experimental results are consistent with the theory, verifying the employed strategy.

Low-noise-figure and high-purity 10 vortex modes amplifier based on configurable pump modes.

We have explored an orbital angular momentum (OAM) amplifier of 10 vortex modes under different-order OAM pump modes, i.e. OAM0, OAM1, and OAM2. The all-fiber amplification system consists of an active few-mode erbium-doped fiber (FM-EDF), a mode selective pump (MSP), and a mode selective signal (MSS). These mode selective components are based on fused-taper mode selective couplers (MSC) under different wavelengths fabricated by a passive ring-core fiber (RCF). Under different-order mode pumps, the OAM amplifier experimentally exhibits mode gains (MGs) above 15 dB for 10 vortex modes with the mode purities only 89%, essentially in line with the simulation results. Especially when the signal-mode profiles are better matched to the pump-mode profiles, i.e. the OAM pumps with the same order as signals, the obtained MGs are all over 20.2 dB and the amplified OAM mode purity is up to 97%; the acquired noise figures (NFs) are <4.9 dB and even the minimum NF is 3.2 dB. The results reveal that the OAM amplifier shows low-NF and high-purity characteristics under configurable pump modes in C-band. The amplified high-order OAM mode could be promising for uses in the long-distance mode division multiplexing (MDM) and in mitigation of the upcoming capacity crunch in optical fiber communication.

High-purity orbital angular momentum vortex beam generator using an amplitude-and-phase metasurface.

In this Letter, we propose an approach to generate high-purity orbital angular momentum (OAM) vortex waves using an amplitude-and-phase metasurface (APM). By varying the square split ring opening and orientation angles, the cross-polarized reflection response of the proposed structure can yield full phase and amplitude coverage. Based on the traditional phase-only metasurface (POM), the Chebyshev synthesis method (CSM) is applied to array the metasurface amplitude distribution. Metasurfaces with modes l of 1, 2, 3, and 4 are designed. Compared with the POM, the APM can effectively improve the vortex beam quality and OAM mode purity. The measured results agree well with full-wave simulations. The presented method provides a new, to the best of our knowledge, way to design high-purity OAM generators based on metasurfaces.

Research on the Purity of Orbital Angular Momentum Beam Generated by Imperfect Uniform Circular Array

As a pathbreaking approach to generate orbital angular momentum (OAM) beam, uniform circular array (UCA) has been extensively researched and applied. However, the impact of the imperfect UCA (IUCA) converted from a UCA with the position of antenna elements on the purity of the generated OAM beam is rarely considered. In order to verify the effect of position deviation on the OAM beam, this letter deduces the analysis model of the IUCA and the OAM mode purity model. The OAM mode purity refers to the weight of the intensity of the “expected OAM mode.” Numerical simulation results show that the OAM beam generated by IUCA is a superposition of OAM beams with a series of different modes, and this results in a certain OAM beam received being contaminated. Compared with the radius deviation, the azimuth deviation of antenna elements has more obvious influence on the OAM mode purity. Moreover, low-mode OAM beams are more suitable for long-distance transmission. Properly increasing the antenna array radius can improve the purity of high-order OAM modes.

Analysis of OAM Performance Using Metalenses of Different Resolutions

Recently, metasurfaces have been extensively studied, especially in connection with the construction of orbital angular momentum (OAM)-carrying beams. However, the dependence of OAM performance on the resolution of unit cells remains to be investigated. In this paper, 27 types of metalenses of varying phase resolutions (1-bit, 2-bit, and 3-bit) and with unit cells of varying width resolutions ( $1\lambda _{0}$ , (1/2) $\lambda _{0}$ , and (1/3) $\lambda _{0}$ ) were designed to analyze this dependence via simulation. A thorough analysis of the simulated results, including the radiation pattern, the near-field amplitude, the near-field phase, and the OAM mode purity, revealed that the OAM performance of a metalens improves with an increase in resolution. In addition, even metalenses of 1-bit phase resolution and $1\lambda _{0}$ width resolution were observed to exhibit OAM mode purities higher than 88%. We expect the conclusions of this study to be useful as guidelines during the future design of OAM metalenses or metasurfaces.

Beam steering of orbital angular momentum vortex wave based on planar phased array

Orbital angular momentum (OAM) vortex electromagnetic waves can provide a new degree of freedom for information modulation at a physical level, which has great prospects of applications in the fields of wireless communication and radar imaging. The application of beam scanning techniques of phased array to OAM vortex electromagnetic wave can increase its communication coverage and expand the detection coverage of vortex radars. Firstly, in this paper, the principle of generating the beam steering vortex electromagnetic beam is discussed and the compensated phase formula for generating beam steering OAM beams is given by planar phased array. Secondly, considering the advantages of phased array antennas in beam scanning and OAM reconfigurability, a planar phased array with 8 × 8 antenna elements at 10 GHz is designed and fabricated. The performances of OAM beam steering and mode reconfigurability are verified. Finally, the performance changes of the deflecting OAM vortex beam at different scanning angles are discussed and analyzed. Simulations and measurements both show that there exist pattern distortion problems when steering angle of OAM beam becomes large. In this paper, the variation of the OAM mode purity is also studied when the scanning angle and the OAM mode number change. The results show that the planar phased array antennas can effectively generate the beam steering OAM vortex beams in a certain angle range. Hence, this paper can provide a reference for the OAM vortex electromagnetic wave communication and the vortex radar in the future.

Orbital angular momentum mode detection of the combined vortex beam generated by coherent combining technology.

Coherent beam combining (CBC) technology has distinct advantages in generating high power vortex beam. In this paper, a circularly arranged coherent beam array (CBA) with discrete vortex phases is constructed to generate vortex beams. We demonstrated that the combined vortex beam (CVB) generated by the CBA is a multiplexing vortices optical field, which sidelobe is the coaxial interference pattern of these spiral harmonic components. Using the designed Dammam vortex grating (DVG), the orbital angular momentum (OAM) spectrum of the CVB is detected. Moreover, taking the target OAM mode purity of the CVB as the evaluation function of active phase control system, we realized the closed-loop phase control of the CBA and obtained the phase-locked output of the CVB.

Directional coupler design for orbital angular momentum mode-based photonic integrated circuits.

An orbital angular momentum (OAM) dividable on-chip directional coupler design is proposed. To guide OAM modes of topological charge number l = ±1, a waveguide needs to support TE01 and TE10 modes with degeneracy. When a directional coupler is made with such an OAM mode waveguide, it is additionally required to equalize the horizontal-direction coupling strengths of those two OAM constitutive eigenmodes. Base on the coupled mode theory formulation, we have found that this requirement can hardly be satisfied and devised a modified cross-shaped waveguide structure to solve this problem. An example design of OAM mode directional coupler is demonstrated. The coupling length of the designed device is 670 µm, and our numerical simulation showed negligible degradation of OAM mode purity during the operation of complete optical power transfer between two waveguides. To the best of our knowledge, this is the first proposal of the on-chip OAM mode directional coupler. The proposed design approach can be applied to implement various devices for OAM mode-based photonic integrated circuits.

Evaluation of the Laguerre-Gaussian mode purity produced by three-dimensional-printed microwave spiral phase plates.

Computer-aided design software and additive manufacturing provide flexibility for the direct fabrication of multi-material devices. This design and fabrication versatility has been investigated for the manufacture of dielectric spiral phase plates (SPP) that generate electromagnetic waves with helical wavefronts. Three types of SPPs designed to produce an orbital angular momentum (OAM) mode number l = |1| were additively manufactured using material extrusion and polyjet fabrication methods. The OAM mode characteristics of the transformed helical microwaves as a function of the SPP geometrical features were investigated experimentally in the 12–18 GHz frequency range. The SPPs were further combined with an additively manufactured dielectric lens that provided a marked improvement in OAM mode purity. Finally, multiplexing and de-multiplexing of two OAM modes were demonstrated successfully using an optimum SPP geometry and arrangement.

Orbital Angular Momentum (OAM) Mode-Reconfigurable Discrete Dielectric Lens Operating at 300 GHz

A novel three-dimensional printed discrete dielectric lens (DDL) antenna with orbital angular momentum (OAM) mode reconfigurability operating at 300 GHz is proposed in this article. The antenna consists of a stationary lower DDL and an upper in-plane rotatable DDL, fed by a stationary pyramidal horn. The DDL pair can transform the quasi-Gaussian beam from the feed source into vortex waves, and its OAM mode number, l , can be dynamically reconfigured among 0, ±1, and ±2 by a simple mechanical in-plane rotation of the upper DDL panel with specific angles relative to the stationary lower one. For the DDL design, the height of each dielectric post pixel element is tuned to realize the required transmission phase. The OAM mode-reconfigurability of the proposed terahertz (THz) antenna was validated by both simulations and measurements. The simulated OAM mode purity bandwidth of the design DDL antenna are 40%, 31.3%, and 25.8% for the OAM mode l = 0, +1, and +2, respectively. In addition, the THz line-of-sight link experiments show that acceptable mode isolations among different OAM channels can be achieved by the THz DDL antenna, validating its application for OAM mode diversity.

Three‐dimensional profiling of collimated radio‐frequency orbital angular momentum beams

The use of orbital angular momentum (OAM) modes in radio communication is thought to enhance capacity. This work focuses on using the l = +1 mode transmitted from a 180 mm diameter, 8‐element circular antenna array. The transmitted OAM beam was collimated by using a spherical mirror and the intensity and phase were investigated. A xyz scanning stage was used to profile the propagating OAM beam in three dimensions, resulting in a detailed investigation into the effects of collimation on the OAM beam. The proposed system was shown to reduce the beam divergence from 36.6° to 1.2°, without affecting the OAM mode purity of the beam for a frequency range of 4–6 GHz. This investigation showed a step towards realising practical control over the divergence of OAM‐carrying beams.