Generation Of Orbital Angular Momentum Modes Research Articles

Polarization-insensitive, wideband reflective metasurface generating orbital angular momentum vortex beam

In this paper, a low-profile polarization-insensitive reflective metasurface is proposed to generate high gain and wideband orbital angular momentum (OAM) in X-band (in the frequency range of 8−12GHz). By adjusting a subwavelength pseudo-screw-shaped meta-atom’s size the required phase compensation has been achieved for designing a reflectarray antenna at 10GHz. The theoretical analyses and simulation results confirm that the proposed structure can effectively generate the first-, second-, third-, and fourth-order OAM vortex beams at the operating frequency. To verify the bandwidth of the suggested metasurface, the structure is implemented for first-order OAM mode generation at various frequencies. The outcomes of investigations show the OAM operational bandwidth and 1.5-dB gain bandwidth are attained to be 40 %, and 27.5 %, respectively. Meanwhile, approximately 14.35 % reflection efficiency was procured for the overall design at the operating frequency of 10GHz. The numerical aperture of the reflectarray metasurface is also conceptually defined and calculated for varying positions of the center-feed antenna. The admitted performance and substantial characteristics of the proposed OAM reflectarray antenna may make it a useful candidate for radar imaging and detection applications as well as broadband microwave communications.

Multi-order orbital angular momentum mode generators based on integrated long-period fiber gratings.

We propose integrated long-period fiber gratings (LPFGs) fabricated by a CO2 laser to realize a multi-channel and multi-order orbital angular momentum (OAM) mode generator. The integrated LPFG is inscribed on multiple surfaces of the few-mode fiber (FMF) by rotating the fiber in the opposite direction at an angle θ. By controlling the rotation angle, the number of integrated LPFGs can be set. The selected rotation angle is 43 ∘, which can integrate up to nine LPFGs, i.e., realizing that the number of channels for first-order orbital angular momentum (OAM) mode conversion is nine. The integrated LPFGs fabricated in this method allow a flexible design of channel spacing. In addition, the flexible selection of the integrated grating period achieves the simultaneous generation of multi-channel second-order and third-order OAM mode conversion. The multi-channel and multi-order OAM mode generators have important application in optical communication multiplexing systems and OAM sensing.

Full C- and L-band covered second-order OAM mode generator based on a thinned helical long-period fiber grating

A full C- and L-band covered second-order orbital-angular-momentum (OAM) mode generator has been proposed and experimentally demonstrated, which is realized by using a helical long-period fiber grating (HLPG) but inscribed in a thinned four-mode fiber. By optimizing the design of grating period and fiber diameter of the proposed HLPG, an ultra-broadband rejection filter with a depth of ∼23 dB, a bandwidth of ∼156 nm @-10 dB (ranging from 1522 nm to 1678 nm) and a bandwidth of ∼58 nm @-20 dB (ranging from 1574 nm to 1632 nm), has been successfully obtained as a typical sample. To the best of our knowledge, this is the first demonstration of such ultra-broadband second-order OAM mode generator by using only one fiber component, i.e., the thinned HLPG. In addition, the proposed generator is less polarization-dependent and less temperature-sensitive than those of the conventional HLPGs, which is believed to be considerably helpful to find potential applications of the device itself in wavelength division multiplexing (WDM) and OAM mode division multiplexing (MDM) optical fiber communication systems.

Bi-directional high-order orbital angular momentum mode generation using gratings in high numerical aperture fiber

We propose a novel design and theoretically analyse it for bi-directional orbital angular momentum (OAM) mode generation in high numerical aperture (NA) optical fibers using the combination of a long period grating (LPG), a short-period (Bragg) grating and a three-port optical circulator. The proposed design consists only of standard fiber gratings rather than specially designed fibers, such as ring-shaped or other types of fibers, making our model relatively simple. Full-vector true-mode analysis has been used to study the modal characteristics of the supported modes in the high-NA fiber. High-NA optical fibers can effectively relieve the degeneracy of the linearly polarized modes. A circularly polarized (CP) light is injected into a single mode fiber, the output end of which is axially spliced into a high-NA fiber. An appropriately designed raised-cosine apodized tilted LPG (TLPG) is used for broad bandwidth coupling of the fundamental HE11 mode to the co-propagating HE21 mode. The employed TLPG converts the CP fundamental core mode (HE 11x ± iHE 11y ) to ±1 order OAM modes (HE 21even ± iHE 21odd ). Both the CP mode and the generated OAM pass through the circulator in the direction of port 1 to port 2. A second fiber is connected at the output of port 2 and a tilted fiber Bragg grating (TFBG) is used to couple the forward-propagating HE11 core mode with the counter-propagating HE21, EH11 and HE31 core modes. The TFBG reflected higher order (±1 and ±2) OAM modes are transmitted through port 3. We observe that the TFBG has a mode-conversion efficiency of more than 95 % for each mode conversion. The generated vortex beam topological charges have been verified using the coaxial interference method.

Ultra-broadband multi-channel orbital angular momentum mode generator using a helical long-period grating pair

We demonstrate an ultra-broadband multi-channel orbital angular momentum (OAM) mode generator using a helical long-period grating (HLPG) pair in a few-mode fiber. The numbers of the channel of the OAM mode generator can be flexibly adjusted by designing the cascaded length of the HLPG pair. When the cascaded length is 50 cm, the maximum channel number of OAM mode generator reaches 46, and the mode convert efficiencies of the channels are more than 90 %, which is, to the best of our knowledge, the highest number of channels among OAM mode converters. By reducing grating length and enhancing the refractive index modulation depth, the 3 dB bandwidth of the HLPG is significantly increased, and thus the wavelength range of the HLPG pair can be expanded. The maximum wavelength ranges of the channels with coupling efficiency greater than 90 % are measured to be 379 nm experimentally. By adjusting the spiral direction of the helical refractive index modulation, we can achieve multi-channel ± 1 order OAM mode conversion for each channel. The multi-channel OAM mode generator holds substantial potential for applications in mode division multiplexing systems, wavelength filters, and fiber sensing.

Redefining uniform circular array’s limit by using square patch elements for higher order orbital angular momentum mode generation

This article proposes a technique to design the Uniform Circular Array (UCA) antenna for the generation of higher order of orbital angular momentum (OAM) modes using lesser number of antenna elements. Earlier, the highest possible OAM mode number (lmax) generated using conventional N-element UCA is limited by number of antenna elements N ≥ 2|lmax|. In the proposed design, a square patch element is placed between two antenna elements of conventional 4-element UCA to generate higher order OAM modes (0, ±1, ±2, and ±3 OAM modes) with lesser number of antenna elements. The proposed UCA is compact in size and requires less complex power dividing phase networks in comparison to conventional 8-element UCA to generate similar lmax. The proposed UCA with square patch is designed at 5.2 GHz and lmax of a N-element UCA with square patch is now limited by number of antenna elements as N ≥ |lmax|. The simulated and measured phase patterns for various OAM modes are in good agreement and supported by mathematical analysis. A comparison of the new structure with the conventional Uniform Circular Arrays available in the literature is also tabulated.

Design of Orbital Angular Momentum Antenna Array for Generating High-Order OAM Modes

Orbital angular momentum (OAM) modes can offer high density and high-capacity communication. The traditional phased array antenna can only produce a limited number of OAM beam modes l, usually less than half of the number of array elements (N): −N/2 < lmax < N/2. An OAM antenna array for generating high-order OAM modes is proposed in this letter. The proposed antenna array consists of a ring patch antenna that can generate vortex waves with OAM mode l = 1 or −1 and a phase-shifting feeding network. By adding different feed excitation signals to each element, the generated beam carries a higher-order mode: l = N or −N, breaking the previous limitations. Near-field measurements were conducted on antenna arrays composed of 3, 4, and 5 elements, revealing a high degree of correspondence between their phase distribution and radiation patterns with numerical simulation results. This alignment further substantiates the practical efficacy of this approach in significantly enhancing the generation of high-order OAM modes within antenna arrays. This advancement improves component utilization efficiency, reduces system complexity, and meets the high demands for spectral resources and channel capacity in future communication applications.

Simultaneous Efficient Excitation of Multiple Higher-Order OAM Modes Using a Single Long-Period Fiber Grating

In this paper, a method for the simultaneous generation of multiple higher-order orbital angular momentum (OAM) modes in a few-mode fiber (FMF) using a single long-period fiber grating (LPFG) is proposed and demonstrated. The high diffraction order of the grating is used to achieve the resonant coupling of multiple modes, and the coupling coefficients of different order modes are balanced by a preset-twist method. This solves the two coupling problems of over-coupling for lower-order modes and difficult excitation of higher-order modes faced by multi-order mode generation, and overcomes the challenge of balanced excitation of multiple modes with high coupling efficiency. In the experiment, the simultaneous generation and flexible combination of first-order, second-order and third-order OAM modes are successfully achieved in a six-mode fiber, in which the mode order, resonant wavelength and wavelength interval can be flexibly controlled. By fabricating with grating periods of 2300 μm, 782 μm and 990 μm, respectively, high conversion efficiency of first-order and second-order modes, first-order and third-order modes, second-order and third-order modes can be generated simultaneously. In addition, for the first time, the first-order, second-order and third-order modes are generated simultaneously at a grating period of 1630 μm with conversion efficiencies of 98.61%, 99.11% and 98.89%, respectively. The mode profiles and interference patterns of different order modes show that multiple higher-order OAM modes with insertion loss less than 1 dB and mode purity over 95% are successfully generated.

Full C-band covered and DWDM channelized high channel-count all-fiber orbital-angular-momentum mode generator based on the fiber gratings.

To generate the orbital-angular-momentum (OAM) modes at multiple wavelengths, which exactly fit with the dense-wavelength-division-multiplex (DWDM) channel grids, is important to the DWDM-based OAM mode-division-multiplex (MDM) fiber communication system. In this study, a full C-band covered and DWDM channelized OAM mode generator is firstly proposed and experimentally demonstrated, which is realized especially by using a broadband helical long-period fiber grating (HLPG) combined with a phase-only sampled multichannel fiber Bragg grating (MFBG). As a proof-of-concept example, the DWDM channelized two complementary 51-channel OAM mode generators have been successfully demonstrated, each of which has a channel spacing of 100 GHz (∼0.8 nm), an effective bandwidth of ∼40 nm, a high azimuthal-mode conversion efficiency of 90%, and high uniformities in both inter- and intra-channel spectra as well. To the best of our knowledge, this is the first time for proposal and experimental demonstration of such a high channel-count and DWDM channelized first-order OAM mode (l = 1) generator, which can also be used for multichannel higher-order OAM mode generation as long as the utilized HLPG is capable of generating a broadband higher-order OAM mode. The proposed device has potential applications to DWDM-based OAM fiber communications, OAM comb lasers, OAM holography, and OAM sensors as well.

High-Order OAM Mode Generator Using Multi-Cascaded Long-Period Fiber Gratings

We propose and demonstrate the inscription of multi-cascaded long-period fiber gratings (LPFGs) in a few-mode fiber using a CO2-laser. The multi-cascaded LPFGs containing three gratings were fabricated successfully. The first LPFG was used to realize the mode conversion from LP01 mode to LP11 mode, the second one was used to convert LP11 mode to LP21 mode, and the third one achieved the higher-order mode conversion from LP21 mode to LP31 mode. The second-order and third-order orbital angular momentum (OAM) modes can be generated using the cascaded LPFGs with the second and third gratings, respectively. The multi-cascaded LPFGs have the promising potential to be adopted as mode converters and mode scramblers in the mode division multiplexing system thanks to the flexible mode conversion among different higher-order modes.

A Low-Profile Dielectric Resonator Antenna Array for OAM Waves Generation at 5G NR Bands.

This paper presents the generation of orbital angular momentum (OAM) vortex waves with mode +1 using dielectric resonator antenna (DRA) array. The proposed antenna was designed and fabricated using FR-4 substrate to generate OAM mode +1 at 3.56 GHz (5G new radio band). The proposed antenna consists of 2 × 2 rectangular DRA array, a feeding network, and four cross slots etched on the ground plane. The proposed antenna succeeded in generating OAM waves; this was confirmed by the measured radiation pattern (2D polar form), simulated phase distribution, and intensity distribution. Moreover, mode purity analysis was carried out to verify the generation of OAM mode +1, and the purity obtained was 53.87%. The antenna operates from 3.2 to 3.66 GHz with a maximum gain of 7.3 dBi. Compared with previous designs, this proposed antenna is low-profile and easy to fabricate. In addition, the proposed antenna has a compact structure, wide bandwidth, high gain, and low losses, thus meeting the requirements of 5G NR applications.

OAM mode generation in helically twisted hollow-core antiresonant fiber.

We investigate the orbital angular momentum (OAM) mode-generating process of a long-period onefold chiral fiber grating (L-1-CFG) based on a helically twisted hollow-core antiresonant fiber (HC-ARF). Taking the right-handed L-1-CFG for an example, we theoretically and experimentally prove that the first-order OAM+1 mode can be generated by only inputting a Gaussian beam. We fabricated three right-handed L-1-CFG samples based on the helically twisted HC-ARF with twist rates (α) of -0.42, -0.50, and -0.60 rad/mm, where the twisted HC-ARF with α of -0.42 rad/mm can achieve high OAM+1 mode purity of 94%. Subsequently, we present simulated and experimental transmission spectra at the C-band, and sufficient modulation depths were obtained at wavelengths of 1550 nm and 1561.5 nm in the experiment.

Ultra-Wideband OAM Mode Generator Based on a Helical Grating Written in a Graded-Index Few-Mode Fiber

An ultra-wideband orbital-angular-momentum (OAM) mode generator has been proposed and demonstrated both theoretically and experimentally, which is based on a helical long period fiber grating (HLPG) but written in a graded-index few-mode fiber (GIFMF). Unlike all the HLPG-based broadband OAM generators reported to date, where the utilized HLPG (fabricated only in step-index fibers) all were operated at wavelengths very close to the dispersion-turning point (DTP), the GIFMF-based HLPG proposed in this study is, however, operated at wavelengths far from its DTP, which provides a new and robust method enabling to produce a ultra-wideband OAM mode generator with an ultra-high conversion-efficiency and yet with a great flexibility in controlling the central wavelength of itself. As a typical example, the first-order OAM mode generator with a central wavelength of ∼1510 nm, a bandwidth of ∼235 nm, and a conversion-efficiency larger than 99% (∼20.7 dB) has been successfully obtained. To the best of our knowledge, this is the first report for practical demonstration of the GIFMF-based HLPG.

Helically twisted double-cladding-three-core photonic crystal fiber for generation of orbital angular momentum

Aiming at the shortcomings of helically twisted single-cladding-few-core photonic crystal fibers in generating orbital angular momentum (OAM), the double-cladding and three-core structures with non-uniform inner and outer air holes are introduced into a photonic crystal fiber for the first time and the generation of high-order OAM modes through helical twisting is realized. The fiber is expected to reduce the losses of the generated OAM modes by introducing a specially designed double-cladding structure, while the three cores distributed in a regular triangle around the center are expected to increase the number of generated OAM modes. On the basis of optical transformation theory, the optical fiber is systematically analyzed by the finite element method. It is found that with the twist rate <i>α</i> = 7853.98 rad/m, the generated OAM modes include “OAM<sub>–4,1</sub>, OAM<sub>+9,1</sub>, OAM<sub>+10,1</sub>, OAM<sub>+11,1</sub>, OAM<sub>+13,1</sub>”, where +13 is the highest order in the OAM modes currently generated by using helically twisted fibers. And the losses of OAM modes are all less than 1.64×10<sup>–3</sup> dB/m, which is at least two orders of magnitude lower than the lowest OAM mode loss reported in the existing references (Napiorkowski M, Urbanczyk W S 2018 <i>Opt. Express</i> <b>26</b> 12131), and their purity is greater than 93%. Further studies show that the generation of orbital angular momentum depends on the resonant coupling between the core supermode and the ring-core mode, and the parity of the order of the generated OAM modes is related to the polarization direction of the fiber core supermode and the ring-core mode.

High-efficiency broadband third-order OAM mode converter based on a multi-period preset-twist long-period fiber grating.

All-fiber mode converters for generating orbital angular momentum (OAM) beams have many applications in optical communications, optical sensing and lasers. Currently, it is a great challenge to use a long-period fiber grating (LPFG) to broadband excite high-order OAM modes above the second-order. Here, we demonstrate a preset-twist LPFG fabrication method, which introduces asymmetry in the refractive index modulation area, for efficient generation of third-order modes. Through optimization, the generation of third-order OAM modes with 99.55% conversion efficiency, 0.81 dB insertion loss, and over 99% purity is achieved with only 40 pitch number. In addition, a multi-period preset-twist LPFG is proposed and demonstrated to achieve the excitation of broadband third-order mode with conversion efficiency of more than 99%, insertion loss of less than 1 dB, and mode purity of more than 90%. The 15 dB bandwidth (96.8% conversion efficiency) of the LPFG is 109 nm in the wavelength range from 1475 nm to 1584 nm, and the 20 dB bandwidth (99% conversion efficiency) of the LPFG is 92 nm from 1488 nm to 1580 nm. To the best of our knowledge, this is the first time to generate efficient and broadband third-order mode using a long-period fiber grating.

High capacity terahertz communication systems based on multiple orbital-angular-momentum beams

Structured electromagnetic waves carrying orbital angular momentum (OAM) have been explored in various frequency regimes to enhance the data capacity of communication systems by multiplexing multiple co-propagating orthogonal OAM beams (i.e. mode-division multiplexing (MDM)). Terahertz (THz) communications in free space have gained interest as THz waves tend to have: (a) larger bandwidth and lower beam divergence than millimeter-waves, and (b) lower interaction with matter conditions than optical waves. In this paper, we review recent experimental demonstrations of OAM-based THz MDM communication systems, including (a) THz MDM system with two multiplexed OAM beams; (b) THz OAM multiplexing together with frequency-division-multiplexing and polarization-division-multiplexing; (c) multiplexing a full set of two-dimensional Laguerre–Gaussian () beams; and (d) THz integrated OAM emitter for OAM mode generation and multiplexing. System performance of THz OAM links with the effect of turbulence, divergence, and multipath is also simulated and analyzed.

Generation, Transmission, and Amplification of OAM Modes in the PbSe-Doped Ring-Core Fiber Carrying 3D Printed Spiral Phase Plate

Vortex beams carrying orbital angular momentum (OAM) have increasingly attracted attention in the field of optical communication. However, transmission is still an issue due to transmission loss, especially in optical fibers. In this work, we proposed, designed, and fabricated micro spiral phase plates (SPPs) directly on an end facet of a piece of PbSe-doped ring-core fiber (RCF) through two-photon polymerization, realizing the integration of OAM beam generation, transmission, and amplification. The prepared RCF comprises a double-clad structure with a core-clad refractive index difference of 2.2% and the fluorescence range is 1150 nm–1700 nm. The intensity distribution of the OAM beam and the spiral interference fringes were obtained, which indicated that the OAM mode (|l|=1, 2, 3, 4) was generated and transmitted directly within the fiber. The small-signal amplification of four OAM modes was accomplished at 1550 nm under a pump power of 634 mW. The on–off gain is >13.2 dB for all modes and the differential mode gain (DMG) is <1.7 dB. The SPP-carrying RCF structure demonstrates the integration of generation, transmission, and amplification of higher-order OAM modes in all-fiber systems.

Mode Purity Improvement of High‐Order Vortex Waves

Vortex electromagnetic waves can carry orbital angular momentum (OAM) modes of different order number, which are mutually orthogonal. Communication technologies multiplexing different OAM topological charges have attracted extensive attention in improving spectrum utilization. In this regard, the efficient generation of OAM modes becomes a key issue. However, it is difficult to generate high-order OAM modes by using a microstrip antenna with a simple structure, and the mode purity is usually not high. Methodologically, the OAM wave of a microstrip antenna is mostly achieved by synthesizing degenerate eigenmodes. In this paper, combined with eigenmode analysis, the construction of high-order and high-purity OAM mode microstrip antennas is carried out from the perspectives of radiating element, ground plane, and structural symmetry. For a regular hexagonal patch antenna of the 4th-order OAM mode, the mode purity is improved from about 40% to over 80% by removing the inner conductor of the patch layer, using a circular ground plane, and adding lug patches on the outside. This method of improving mode purity through the analysis of mode significance and characteristic current distribution also has guiding significance for the design of other high-order OAM mode patch antennas.

Stable orbital angular momentum mode generator based on helical long-period fiber grating

We developed a helical long-period fiber grating (HLPG), which was formed by twisting step-index double-clad fiber (SDF) under the irradiation of a CO2 laser, and can generate stable orbital angular momentum (OAM) mode. The mode coupling and transmission characteristics of the SDF-HLPG were investigated both theoretically and experimentally. The results prove that this grating can couple the light wave into the inner cladding mode of the SDF, and directly generate OAM mode. Subsequently, the tolerance of the SDF-HLPG to environmental disturbance was also evaluated. The OAM mode has low sensitivities to mechanical torsion and bending, and has almost no responses to strain and surrounding refractive index. This indicates that the SDF provides a channel for the OAM mode to avoid high external interference. Therefore, this kind of SDF-HLPG has the potential for communication applications.

All-Fiber Fourth-Order OAM Mode Generation Employing a Long Period Fiber Grating Written By Preset Twist

There are great challenges in the generation of higher-order modes for long period fiber grating (LPFG). The cross-coupling coefficient between the fundamental mode and the higher-order azimuthal modes is extremely small due to the difficulty of angular modulation with traditional methods. Therefore, how to generate higher-order orbital angular momentum (OAM) modes with a high efficiency and low insertion loss is an urgent problem to be solved. In this paper, an efficient all-fiber approach to generate fourth-order OAM mode by employing a preset twisted LPFG (PT-LPFG) in a few-mode fiber (FMF) is presented. The difficulty of generating fourth-order mode utilizing traditional single-side exposure carbon dioxide laser manufacturing method and the advantages of the PT-LPFG in enhancing the coupling coefficient are theoretically analyzed in detail. In experiment, the conversion from the fundamental mode to the fourth-order mode is achieved, and the fourth-order interference patterns demonstrated the successful generation of the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">±4</sub> modes. The conversion efficiency of the OAM modes is over 99% and the purity is measured to be more than 90%. To the best of our knowledge, this is the first time to generate the fourth-order OAM mode in an all-fiber system using single long-period fiber grating.