High-order Orbital Angular Momentum Research Articles

Accurate Mode-Coupling Characterization of Low-Crosstalk Ring-Core Fibers Using Integral Calculation Based Swept-Wavelength Interferometry Measurement

In this paper, to accurately characterize the low inter-mode coupling of the weakly-coupled few mode fibers (FMFs), we propose a modified inter-mode coupling characterization method based on swept-wavelength interferometry measurement, in which an integral calculation approach is used to eliminate significant sources of error that may lead to underestimation of the power coupling coefficient. Using the proposed characterization method, a low-crosstalk ring-core fiber (RCF) with low mode dependent loss (MDL) and with single span length up to 100 km is experimentally measured to have low power coupling coefficients between high-order orbital angular momentum (OAM) mode groups of below -30 dB/km over C band. The measured low coupling coefficients based on the proposed method are verified by the direct system power measurements, proving the feasibility and reliability of the proposed inter-mode coupling characterization method.

Weak turbulence effects on different beams carrying orbital angular momentum.

The study of beams carrying orbital angular momentum (OAM) has been of interest for its use in free-space optical communications (FSOC), directed energy applications, and remote sensing (RS). For FSOC and RS, it is necessary to measure the wavefront of the beam to recover transmitted or environmental information, respectively. In this computational study, common OAM beams such as the Laguerre-Gaussian (LG), Bessel-Gaussian (BG), and Bessel beams are propagated through atmospheric turbulence and compared to their Gaussian beam counterpart. The turbulence is simulated using multiple phase screens within the framework of a split-step method. Beam metrics used to quantify beam propagation will include the spatial coherence radius, OAM spectrum, on-axis intensity, spot size, divergence, and on-axis scintillation. Atmospheric turbulence along the path is limited to the weak scintillation limit, where beam parameters can be predicted analytically using the Rytov approximation. The results show that BG beams and multiplexed BG beams retain more OAM information than their LG and Bessel beam counterparts. The LG beam on-axis intensity and on-axis scintillation are seen to be independent of OAM mode. The scintillation of the LG beam is less than a BG, Bessel, and Gaussian beam across low- and high-order OAM modes. Insight into these results is discussed through studying the beam divergence, while the initial spot sizes of the Gaussian, LG, and BG beams are limited to the same spatial extent.

Generation and Measurement of a Bessel Vortex Beam Carrying Multiple Orbital-Angular-Momentum Modes through a Reflective Metasurface in the rf Domain

We discuss the generation and measurement of a Bessel vortex beam carrying multiple orbital-angular-momentum (OAM) modes in the radio-frequency domain. Considering that a high-order Bessel beam inherently has high-order OAM modes, in this paper, we propose to superpose these high-order Bessel vortex beams together, which can combine the advantages of the orthogonality of the vortex beam's multiple OAM modes and the Bessel beam's nondiffraction property. A theoretical formula and the basic design method are presented. A reflective metasurface is designed as the beam launcher to generate a deflected Bessel vortex beam that carries two OAM modes of $\ensuremath{\ell}=1$ and $\ensuremath{\ell}=2$ simultaneously. The corresponding reflective metasurface is designed, fabricated, and measured. The full-wave electromagnetic simulations and experimental measurements both verify the effectiveness of the design. The proposed method could further promote the application of a Bessel vortex beam in related near-field scenarios such as near-field wireless communication and radar detection and imaging.

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.

High-order orbital angular momentum generation in a helically twisted pig-nose-shaped core microstructured optical fibers.

We propose a helically twisted pig-nose-shaped core microstructured optical fiber (HPC-MOF) for orbital angular momentum (OAM) mode generation. It comprises seven air-hole rings hexagonally arranged with two air holes and one air-hole ring replaced, forming two cores in a line 3 µm from the fiber center and one ring-shaped core. The fiber is helically twisted along the rotation axis. In this fiber, supermodes in inner dual-core can be coupled to high-order modes in outer ring-core, yielding OAM ring-shaped modes at different certain wavelengths, and various OAM modes at different twist rates were investigated in this paper. We demonstrate the distinct coupling differences of symmetric and antisymmetric supermodes in inner dual-core when the supermode coupled to ring-core mode. A modal matching rule is presented to characterize the coupling differences, which is suitable for describing supermode coupling characteristics in HPC-MOFs. Compared to conventional methods, these properties indicate that the fiber can generate higher-order OAM modes and more easily integrate into all-fiber optical communication systems, with potential in OAM generators, light-controlling devices, and integrated optics applications.

Numerical investigation of spiral photonic crystal fiber (S-PCF) with supporting high order OAM modes propagation for space division multiplexing applications

This article proposes a spiral designed photonic crystal fiber (S-PCF) that supports 14 orbital angular momentum (OAM) modes for signal transmission and possess very low nonlinear coefficient and flat dispersion and it could assess for space division multiplexing applications. The core material of the proposed fiber is silicon because it shows excellent transmittance, low thermal response, chemically inert etc. The dispersion variations of 20–110 ps nm−1 km−1 for all OAM-modes are exhibited at 1.00 μm wavelength. In addition, overall dispersion distinction among all modes is under 10 ps nm−1 km−1. Besides, $$TM_{0,1}$$ and $$TE_{0,1}$$ modes are better in dispersion for its flat and low dispersion. Moreover, the nonlinear coefficient is very low that is 2.10 W−1 km−1 for $$EH_{5,1}$$ at 2.50 µm wavelength and the investigated confinement loss is always below than $$10^{ - 5}$$ dB m−1. In addition these basic optical parameters, few other properties such as numerical aperture and the effective-area are also computed. Considering all noble features, this proposed S-PCF is encouraging to be utilized in extra-capacity, large bandwidth and high-volume for optical fiber communication systems.

UCA-Based OAM Non-Orthogonal Multi-Mode Multiplexing

Electromagnetic waves carrying orbital angular momentum (OAM) can improve the spectral efficiency of communication systems by multiplexing a number of OAM modes. However, since being not able to perform water-filling power allocation and adaptive modulation for each sub-channel like closed-loop multiple-input multiple-output (MIMO) systems, the traditional orthogonal multi-mode OAM multiplexing system usually has poor bit error rate (BER) performance due to the inherent large divergence angle of high-order orthogonal OAM beams. Therefore, in this article we propose a non-orthogonal OAM (NO-OAM) multi-mode multiplexing scheme based on uniform circular array (UCA), which regulates the divergence angles of all non-orthogonal OAM beams to be the same, circumventing the problem that large beam divergence of high-order orthogonal OAM modes results in low received signal-to-noise ratio (SNR) at the receive UCA with a fixed aperture. Mathematical analysis and numerical simulations show that in contrast to the existing uniform concentric circular array (UCCA) beam adjustment scheme, the proposed NO-OAM scheme has slightly better beam adjustment effect but with only one UCA. Moreover, in contrast to the traditional orthogonal OAM multi-mode transmission, the proposed NO-OAM multi-mode multiplexing scheme has asymptotically equivalent channel capacity and much lower BER.

Hollow Ring-Core Photonic Crystal Fiber With >500 OAM Modes Over 360-nm Communications Bandwidth

We propose and design a hollow As2S3 ring-core photonic crystal fiber (PCF) with 514 radially fundamental orbital angular momentum (OAM) modes over 360 nm communications bandwidth across all the O, E, S, C, and L bands. The designed PCF with 40 $\mu \text{m}$ -radius air core and 150 nm-width As2S3 ring can support eigenmodes up to HE130,1 and EH128,1. The numerical analysis shows that the designed ring PCF has large effective refractive index contrast, and can transmit up to 874 OAM modes near 1.55 $\mu \text{m}$ . Simulation results show that in the C and L bands, the PCF with a hollow-core radius of 40 $\mu \text{m}$ and a ring width of 0.15 $\mu \text{m}$ can retain an $2.5\times 10 ^{-3}$ effective refractive index difference between the two highest order OAM modes, which achieves effective mode separation, thereby achieving stable OAM mode transmission. The $n_{eff}$ difference between the even and odd fiber eigenmodes and the intra-mode walk-off are also carefully studied under different bending radii. The results show that higher-order OAM modes has better tolerance to the fiber bending, compared with the lower-order modes. The fiber has the potential to support ultra-high capacity OAM mode division multiplexing in the optical fiber communication systems.

Helical Long Period Fiber Grating Inscribed in Elliptical Core Polarization-Maintaining Fiber

A high-quality helical long period fiber grating (HLPFG) in elliptical core polarization-maintaining fiber (PMF) was experimentally demonstrated by using the hydrogen-oxygen flame heating technique. Such a single HLPFG with a length of 6.5 mm could be used to generate orbital angular momentum (OAM) modes, i.e., OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">±2</sub> . The secret of the successful generation of the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">±2</sub> mode is that the elliptical core PMF has two-fold symmetry in the cross-section. The resonance dip of the coupling attenuation and wavelength was strongly dependent on the length and helical pitch of the HLPFG. The HLPFG also exhibited a higher strain sensitivity of 30.7 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> . The proposed method opens a new way to generate a higher-order OAM mode by using multiple-fold symmetry fiber.

Wavelength Tunable OAM Mode Converters Based on Chiral Long-Period Gratings

We have demonstrated direct generation of high-order orbital angular momentum (OAM) modes by chiral long-period gratings (CLPGs) written in a few mode fiber (FMF) via a CO2 laser. The high-order core modes (LP11, LP21, and LP31 modes) were realized by the CLPGs through high order diffraction modes with efficiencies of more than 99.69%. The -1st, −2nd, and −3rd order OAM modes can be generated directly by the CLPGs with various grating periods. The proposed method opens a new path to implement all fiber high-order OAM mode generation. The OAM mode converters can be adopted as wavelength tunable mode converters by adjusting twist rate of the CLPGs, which could be widely used in optical communication and fiber sensing system based on FMFs.

Extended aperture sample reception method for high-order orbital angular momentum vortex beam mode number measurement.

The traditional phase gradient method can effectively measure the orbital angular momentum (OAM) number of the vortex beam. However, its spatial sampling phase aperture span is strictly restricted within only π radian. In this paper, we presented a more flexible extended aperture sampling reception (EASR) method for the radio frequency (RF) applications, which can break through this restriction condition. It could make the reception and measurement methods about the OAM mode number become more complete and versatile. By converting the higher-order OAM mode to a lower-order OAM mode, the spatial phase aperture span between the adjacent receiving sampling points can realize extensions. We have conducted a comprehensive theoretical analysis and summarized the general guidelines of this EASR method in the main body of the paper. Subsequently, we perform the related numerical simulation calculations to receive and measure the OAM mode number of a high-order mode vortex beam using the EASR method. Simulation results and theoretical analysis are in good agreement.

Nonlinearity-Aware Adaptive Bit and Power Loading DMT Transmission Over Low-Crosstalk Ring-Core Fiber With Mode Group Multiplexing

In this article, based on a specially designed ring-core fiber (RCF) with low inter-mode group (MG) crosstalk, we experimentally demonstrate orbital-angular-momentum (OAM) MG-multiplexing (MGM) transmission with intensity modulation and direct detection (IM/DD) utilizing discrete multi-tone (DMT) modulation. Two adjacent high-order OAM MGs |l| = 2 and |l| = 3 with low inter-MG crosstalk of -24.3 dB over 1-km RCF transmission system are employed for data transmission simultaneously. In addition, a low-complexity frequency-domain polynomial nonlinear equalizer (FD-PNLE) combining with adaptive bit and power loading are implemented to mitigate the modulation/detection related nonlinearities as well as modal dispersion induced high-frequency power fading. Experimental results show that compared with linear equalization, the capacities of MGs |l| = 2 and |l| = 3 using FD-PNLE can be significantly improved by 13% and 11.1% with adaptive bit and power loading DMT modulation over 1-km low-crosstalk RCF at a received optical power (ROP) of -13 dBm, respectively. Based on the nonlinearity-aware adaptive bit and power loading DMT signal, successful 186.4-Gbit/s MGM transmission over 1-km low inter-crosstalk RCF is realized only with 20-GHz electrical signal bandwidth. Therefore, the low-crosstalk RCF-based MGM scheme combined with advanced modulation format shows great potential in large-capacity low-cost short-reach optical interconnects.

On-chip enhanced electro-optic Kerr effect for manipulation of optical orbital angular momentum modes

The mode division multiplexing (MDM) technique using higher order orbital angular momentum (OAM) carrying modes through a channelized bandwidth provides enhanced capacity communication systems. Furthermore, encrypted channels via OAM based high-dimensional quantum key distribution (QKD) improve the transmission rate and security. In such applications, mode-selective manipulation of the spatially distributed (here, OAM) modes is a significant function to implement MDM and QKD networks. This paper proposes a novel versatile-designed chip which is configured in a Ycut periodically poled lithium niobate (PPLN) photonic wire. This integrated optical device acts as spatial mode converter for data modulated on higher order OAM = ±2ћ modes. The conversion is based on an enhanced electro-optic Kerr effect via phase-mismatched cascaded polarization coupling interactions of decomposed guided modes in two successive PPLN sections. The high-purity (91%) and low-voltage (<14.6 V) of the proposed device enable its operation in compatibility with applicable commercial modulators in OAM-based MDM and QKD systems.

Optomechanical detection of light with orbital angular momentum.

We present the design of an optomechanical device that allows sensitive transduction of the orbital angular momentum of light. An optically induced twist imparted on the device is detected using a photonic crystal cavity optomechanical system. This device allows the measurement of the orbital angular momentum of light when photons are absorbed by the mechanical element or the detection of the presence of photons when they are scattered into new orbital angular momentum states by a sub-wavelength grating patterned on the device. Such a system allows the detection of optical pulses with an l = 1 orbital angular momentum field that have an average photon number of 3.9 × 103 at a 5 MHz repetition rate, assuming that detector noise is not limiting measurement sensitivity. This scheme can be extended to higher order orbital angular momentum states.

Acoustic vortices with high-order orbital angular momentum by a continuously tunable metasurface

Metasurface-based acoustic vortex generators formed by fixed microstructures can only transfer the first-order orbital angular momentum (OAM) at a single frequency in a waveguide. Here, the multiple order acoustic vortices are realized through a helical metasurface design that is geometrically compact, broadband, and tunable. The proposed metasurface consists of matched helices rotating into the corresponding helical channels, allowing us to modulate the reflected phase shifts of the individual units. The units are arranged in a compact circular configuration and then passively transform an incident plane wavefront into the desired spiral wavefront with a controllable topological charge. The reported theoretical, numerical, and experimental results evidence that multiple orders of OAM modes can be continuously generated by using only a single metasurface. The tunable vortex design is relevant to the pragmatic applications of metasurfaces in real-time acoustic communications, spanners, and tweezers.

Gyro-devices – natural sources of high-power high-order angular momentum millimeter-wave beams

The Orbital Angular Momentum (OAM) carried by light beams with helical phasefront (vortex beams) has been widely employed in many applications such as optical tweezers, optical drives of micro-machines, atom trapping, and optical communication. OAM provides an additional dimension (diversity) to multiplexing techniques, which can be utilized in addition to conventional multiplexing methods to achieve higher data rates in wireless communication. OAM beams have been thoroughly studied and used in the optical regime but in the mm-wave and THz-wave region, they are still under investigation. In these frequency bands, there are difficulties associated with beam-splitting and beam-combining processes as well as with the use of spiral phase plates and other methods for OAM generation, since the wavelength is much larger compared to those at optical frequencies, leading to higher diffraction losses. The present paper describes the natural generation of high-power OAM modes by gyro-type vacuum electron devices with cylindrical interaction circuit and axial output of the generated rotating higher-order transverse electric mode TEm,n, where m &gt; 1 and n are the azimuthal and radial mode index, respectively. The ratio between the total angular momentum (TAM) JN and total energy WN of N photons is given by m/ω, where ω is the angular frequency of the operating mode, which in a gyrotron oscillator is close to the TEm,n-mode cutoff frequency in the cavity. Therefore, m/ω = Rc/c, where Rc is the caustic radius and c the velocity of light in vacuum. This means that the OAM is proportional to the caustic radius and at a given frequency the same for all modes with the same azimuthal index m. Right-hand rotation (co-rotation with the electrons) corresponds to a positive value of m and left-hand rotation to negative m. The corresponding OAM mode number (topological charge) is l = m – 1. Circularly polarized TE1n modes only possess a Spin Angular Momentum (SAM: s = ±1). TE0n modes have neither SAM nor OAM. This is the result of the photonic (quasi-optical) approach to derive the TAM of modes generated in gyrotrons. The same result follows from the electromagnetic (EM) wave approach for the TAM within a given waveguide volume per total energy of the EM wave in the same volume. Such high-power output beams with very pure higher-order OAM, generated by gyrotron oscillators or amplifiers (broadband) could be used for multiplexing in long-range wireless communications. The corresponding mode and helical wavefront sensitive detectors for selective OAM-mode sorting are available and described in the present paper.

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.

Two-Octave Supercontinuum Generation of High-Order OAM Modes in Air-Core As₂S₃ Ring Fiber

In this work, we design and simulate an air-core As 2 S 3 ring fiber for high-order orbital angular momentum (OAM) supercontinuum generation. We show that the chromatic dispersion of the ring fiber can be substantially tailored by proper optimization of the air-core radius. Two-octave supercontinuum carrying OAM 17,1 mode, spanning from 1560 to 6250 nm, is obtained by pumping a 50-fs 100-kW secant hyperbolic pulse centered at the wavelength of 3800 nm into the designed fiber with 50-μm air-core radius and 1-μm ring width. We further engineer the chromatic dispersion of some other OAM modes and perform simulations of supercontinuum spectra using different kilowatt-level peak power, which indicates that the fiber we design represents a promising avenue for supercontinuum generation of all the OAM l,1 modes (|l| ≤17). The proposed fiber is suitable for the transmission of OAM beams in infrared wavelength range and it could promote the development and application of high-order OAM beams.

Orbital angular momentum mode of cylindrical spiral wave-guide

The common feature of traditional methods of preparing orbital angular momentum (OAM) light beams propagating along the &lt;i&gt;z&lt;/i&gt; axis is that the wave-front phase is changed and the chief ray of beam is basically unchanged. But it is difficult to obtain a high &lt;inline-formula&gt;&lt;tex-math id="M5"&gt;\begin{document}$m\hbar $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20190997_M5.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20190997_M5.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; OAM. To solve the above problem, we establish a theoretical framework based on the change of the chief ray of beam instead of the change of wave-front phase. The differential geometry theory is used to verify the theoretical assumption that the light transmitted by the cylindrical spiral wave-guide can carry high &lt;inline-formula&gt;&lt;tex-math id="M6"&gt;\begin{document}$m\hbar $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20190997_M6.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20190997_M6.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; OAM. To measure the OAM optical fiber output, we use the diffraction method to detect the phase of vortex, that is, we can use a microscope to observe the phase distribution of optical fiber end face. We consider the output of linearly polarized light along the tangent direction of the fiber to observe its diffraction pattern. The transmission of optical fiber around the cylinder is the main light. The diameter of optical fiber is constant, and the light wave transmitting into the optical fiber is Bessel beam. For the linear fiber output, we need to consider only the linear fiber Bessel beam. The output cross section of the wave surface in the fiber is approximately that of plane wave. When &lt;inline-formula&gt;&lt;tex-math id="M7"&gt;\begin{document}$\theta &gt; {\theta _0}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20190997_M7.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20190997_M7.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, we use the flow coordinates &lt;inline-formula&gt;&lt;tex-math id="M8"&gt;\begin{document}$(\alpha,\beta, \gamma)$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20190997_M8.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20190997_M8.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; to calculate the diffraction pattern of the cross section of the optical fiber when light travels in the optical fiber around the cylinder, which shows the characteristics of vortex. The optical field distribution carries a high-order OAM mode. When &lt;inline-formula&gt;&lt;tex-math id="M9"&gt;\begin{document}$\theta = {\theta _0}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20190997_M9.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20190997_M9.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, cylindrical orbital optical fibers transit to linear orbital optical fibers. We calculate the diffraction pattern of the cross section of the optical fibers propagating in a straight line. It is an Airy spot, namely a circular aperture diffraction spot. The optical field distribution has no higher-order OAM mode. When the order of the output beam is small, the output shows certain uniformity and symmetry, when the order of the output beam increases gradually, the output beam shows some inhomogeneity and asymmetry.

Air-Core Ring Fiber With &gt;1000 Radially Fundamental OAM Modes Across O, E, S, C, and L Bands

In this work, an air-core ring fiber is designed with a record high 1322 orbital angular momentum (OAM) modes at 1550 nm while maintaining radially single-mode condition. Moreover, it can support over 1004 OAM modes across all O, E, S, C, and L bands, exploiting to our knowledge the highest number of OAM modes ever supported in the optical fiber within a wide wavelength range. Simulations show that, across the C and L bands, the fiber with 55-μm air-core radius and 0.45-μm ring width can preserve 3.3 × 10 -3 effective refraction index difference between the two highest-order OAM modes HE 340,1 and EH 271,1 . This enables efficient mode separation, and thus achieving stable OAM modes transmission. The effective refractive index differences between the even and odd fiber eigenmodes are also analyzed in the elliptical and bent fibers. We note that higher-order OAM modes are more tolerant to the fiber ellipticity and bending. This ring fiber design has the potential to increase the spectral efficiency and the overall capacity in fiber-based communications system.