OAM Modes Research Articles

Terahertz Ring-Core Fiber for First-Order Radial OAM Modes Transmission Without Phase Distortion

Terahertz Ring-Core Fiber for First-Order Radial OAM Modes Transmission Without Phase Distortion

Coherence vortices by binary pinholes

Abstract Singularity in a two-point complex coherence function, known as coherence vortices, represents zero visibility with a helical phase structure. In this paper, we introduce a novel technique to generate the coherence vortices of different topological charges by incoherent source transmittance with exotic structured binary pinholes. The binary pinhole structures have been realized by lithography, followed by wet etching methods. We control the transmittance from the incoherent source plane using these exotic apertures, which finally results in a coherence vortex spectrum that features multiple and pure orbital angular momentum modes. The generation of the coherence vortices is achieved within the two-point complex spatial coherence function. The spatial coherence function exhibits the helical phase profile in its phase part, and its absolute part shows a doughnut-shaped structure. A theoretical basis is developed and validated with simulation, and experimental results. The coherence vortex spectra with OAM modes superposed with opposite topological charges, known as photonic gears, are also generated with the proposed theory.

Simple and Adjustable LP₀₁–LP₁₁–OAM Mode Conversion Method in One Single-Mode Fiber With Half-Dome Lens

Simple and Adjustable LP₀₁–LP₁₁–OAM Mode Conversion Method in One Single-Mode Fiber With Half-Dome Lens

Flexible Generation of OAM mode Reconfigurable Beams and Spinning OAM Beams Based on Active Induced Magnetism Huygens’ Metasurface

Flexible Generation of OAM mode Reconfigurable Beams and Spinning OAM Beams Based on Active Induced Magnetism Huygens’ Metasurface

Design of low loss THz dual guided photonic crystal fiber with supporting of 68 OAM modes and 8 LP modes

Design of low loss THz dual guided photonic crystal fiber with supporting of 68 OAM modes and 8 LP modes

Investigating orbital angular momentum modes in multimode interference (MMI) waveguides and revealing their mode conversion property

In this work, the propagation of OAM modes in multimode interference (MMI) waveguides, as the basic elements in many integrated optical devices, is studied to utilize their benefits in integrated OAM applications. OAM modes shape the OAM-maintaining image at the specific length of an MMI waveguide. As the most effective parameters on the properties of the generated image, waveguide’s width (W), topological charge (ℓ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\ell$$\\end{document}) and waist radius (WR) of the input OAM modes are investigated. Power overlap integral (POI) is used to evaluate the quality of images. The investigations show that the calculated POI is enhanced by increasing in WR of the input mode (from 86.91% for WR = 1.5 µm to 98.92% for WR = 3.5 µm; where W = 15 µm and ℓ=±1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\ell = \\pm 1$$\\end{document}). Furthermore, the increase in the waveguide’s width leads to decrease the quality of the self-imaged mode (from 98.90% for W = 15 µm to 85.15% for W = 50 µm; where WR = 3 µm and ℓ=±1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\ell = \\pm 1$$\\end{document}). It is also demonstrated that mode conversion between even order of OAM modes with opposite topological charges can occur at OAM-maintaining length of the MMI waveguides, which is the most outstanding achievement of this survey for optical communication systems.

Design and investigation of a terahertz hollow-core fiber with double-layer ring core for OAM mode transmission

A terahertz hollow-core fiber having a double-layer ring core is presented and investigated to support OAM mode transmission. These two annular cores will work as two separate OAM transmission spaces that are not interfered with and can support more OAM mode transmissions. A numerical model has been developed to assess the transmission performances of EH and HE modes in the 0.3∼0.4 THz frequency band. Using only a single material to design the suggested fiber, 22 (in) + 58 (out) OAM modes can be stably transmitted at 0.36 THz. Neighboring EH and HE modes in the identical OAM family can obtain effective refractive index differences of more than 10−4. Furthermore, the designed fiber demonstrates excellent transmission properties, with the highest mode purities in the inner and outer rings being 92.22% and 93.78%, respectively. In the inner ring the effective mode field area is between 5.37 × 106 and 1.79 × 107 μm2, while in the outer ring, it is between 8.95 × 106 and 1.58 × 107 μm2. The HE1,1 mode has the smallest change in dispersion, with values of 2.76 and 0.87 ps THz−1 cm−1 in two high refractive index transmission ring cores, respectively. Additionally, the isolation (ISO) parameter is also analyzed. These mode characteristics indicate that the fiber structure has significant potential for applications in high-capacity terahertz communication systems.

Helical Long-Period Fiber Grating-Based Band- Selectable and Bandwidth-Enhanced Flat-Top Filter and Its Application to Wideband OAM Mode Converter

Helical Long-Period Fiber Grating-Based Band- Selectable and Bandwidth-Enhanced Flat-Top Filter and Its Application to Wideband OAM Mode Converter

Recognizing OAM mode of high-order radial vortex beam superposition states employing convolutional neural network

Recognizing OAM mode of high-order radial vortex beam superposition states employing convolutional neural network

Characterizing the fractional coherence vortices through the area of the intensity cross-correlation function

We have generated the fractional coherence vortices using the speckle patterns obtained from the scattering of the fractional vortex beams. In this study, we found the topological charge of the fractional vortex beam with a resolution of 0.01 using the area of the coherence function of scattered fractional optical vortex beams. We have also provided accuracy for the measurement of topological charges of fractional vortex beams using the studies of the area of the coherence function. Our experimental results are well matched with the theoretical results. These fractional coherence functions can be used to generate a security key for data authentication and data encryption. In addition, fractional vortex beams have multiple OAM modes and can be used to address the explosive growth in free-space optical communication.

Effective Realization of Higher Order Circular Polarized OAM Mode Circular Patch Antenna

Effective Realization of Higher Order Circular Polarized OAM Mode Circular Patch Antenna

Amplification of Multi-Order OAM Modes With High Gain and Low Differential Modal Gain

Amplification of Multi-Order OAM Modes With High Gain and Low Differential Modal Gain

ExtraTrees-hidden Markov model based equalizer for mode division multiplexing ring-core fiber communication with non-orthogonal MultiCAP modulation

This paper presents an orbital angular momentum mode division multiplexing (OAM-MDM) ring-core fiber transmission method based on non-orthogonal multiband carrierless amplitude and phase (NMCAP) modulation with an extremely randomized trees-hidden Markov model (ExtraTrees-HMM)-based equalizer. The ExtraTrees-HMM equalizer uses the statistical characteristics of the received distorted signals to model the nonlinear channel of the system to classify these distorted signals into corresponding constellation classes. Experiments were conducted using a 216 Gbit/s OAM-MDM NMCAP modulation optical fiber communication system with 2 km ring-core fiber transmission and the results show that compared with a conventional Volterra nonlinear equalizer (VNE), the proposed ExtraTrees-HMM equalizer could improve the receiver sensitivity by 1 dB for OAM mode l = + 2, and 0.6 dB for OAM mode l = + 3. In addition, the computational complexity of the proposed equalizer was reduced by 43.94% compared with the VNE. In brief, the ExtraTrees-HMM is a promising equalization candidate for ultra-high-capacity inter-data-center interconnections.

Beam steering using delays generated from an optical OAM mode shifting recirculating loop

This paper investigates the performance of generated delays from an orbital angular momentum mode shifting recirculating delay loop (OAM-SL) in simulation using Kirchhoff-Fresnel diffraction (KFD) and the delays applicability for RF signal beam steering applications. OAM-SL is a novel structure that has been recently reported to produce multiple delayed signal replicas on independent OAM carriers using a single compact system. The objective of this research is to explore the impact of several limiting factors on the quality of OAM-SL delays for beam steering applications, such as where there is (i) displacement of the OAM mode shifter, (ii) displacement of the center of the OAM mode demultiplexer at the loop output, and (iii) the limited resolution. The methodology includes: (i) simulating the beam propagation using KFD, (ii) producing the output as a coherent summation of the recirculating beams, and (iii) characterizing the SCR by dividing the power of the recirculating beam (ρ) on its OAM mode by the power accumulated from the other recirculating beams. Our findings show that delays generated from a system with lin=−30, lshift=+1, and a resolution of 1024 pixels can steer a 100 MHz signal at a steering angle of 11.5° in a four antenna system with 0.1° error if loop alignment is perfect. Also, for steering the 100 MHz signal in an eight antenna system, we analyze the system assuming the system loss can be compensated, and a similar result of 0.1° error is obtained. Additionally, we report the beam steering performance at 23.6° and 36.8°.

Improving orbital angular momentum mode transmission with imperfect ring–core fiber

Orbital angular momentum (OAM), inherent to light beams characterized by helical phase fronts, represents an alternative spatial degree of freedom for photons. In recent years, OAM-based communication has gained significant attention as a potential solution to increase the capacity and spectral efficiency of optical fiber communication systems. In optical fibers, OAM modes can be generated and manipulated by utilizing specialized optical components and techniques. One approach involves using specially designed optical fibers, such as ring–core fibers, which have the capability to support multiple OAM modes. In the manufacturing process, the ring fiber is hardly made perfectly, especially the doped-cores. The introduced defect breaks the symmetry of the ring–core, which has a serious impact on OAM mode transmission. Using a material with a slightly higher refractive index than the cladding to wrap the ring–core is effective in reducing impairment of defects. After numerical simulation, the proposed method improves the walk-off length by a factor of 2–40.

Efficient synchronous retrieval of OAM modes and AT strength using multi-task neural networks.

Encoding information using OAM beams as carriers greatly alleviates the capacity crisis in communication systems. When transmitted through the atmospheric channel, OAM beams are influenced by the random fluctuations in the refractive index caused by atmospheric turbulence, resulting in phase distortion and intensity dispersion of the beams, leading to severe signal interference. Due to the high randomness of atmospheric turbulence, it is essential for OAM mode recognition methods to have good stability to ensure communication quality. We establish an equivalence relationship between the continuous dynamics system and the network unit RUEM, ensuring its stability through theoretical derivation and numerical experiments. We propose a multitask neural network model, OATNN, embedded with RUEM to achieve efficient simultaneous recognition of turbulence intensity in atmospheric turbulence environments and OAM modes in free-space optical communication systems. Numerical experimental results show that under four turbulence intensity levels, the network achieves a recognition accuracy of 99.37%, and for ten modes, the recognition accuracy is 99.05%. Additionally, we explore the performance of this network in a 2000m channel transmission scenario.

Broadband OAM Mode Generator Based on Helical Long-Period Grating Inscribed by Automated CO-Laser

Broadband OAM Mode Generator Based on Helical Long-Period Grating Inscribed by Automated CO-Laser<sub />

OAM waves with fewer redundant OAM modes generated by two circularly polarized antenna arrays

Two circularly polarized antenna arrays, which reduce two redundant OAM modes, are proposed in this paper. One antenna array is composed of 8 right-handed circularly polarized (RHCP) antennas and radiates OAM mode l in x component, OAM mode l in y component and OAM mode (l+1) in z component. The other antenna array has 8 left-handed circularly polarized (LHCP) antennas and transmits OAM mode l in x component, OAM mode l in y component and OAM mode (l−1) in z component. The RHCP and LHCP antenna arrays produce OAM waves with fewer redundant OAM modes compared to the traditional single-direction antenna array. The feed excitations of the two circularly polarized antenna arrays have the same feeding system, which includes power dividers and phase shifters. The reflection coefficient parameters, radiation patterns and phase distributions are measured to display the radiated characteristics of the two proposed methods. Both the simulation and the experimental results demonstrate the effectiveness of the proposed methods.

Transition Between Quadratic and Quartic Solitons With OAM Modes in Passive Driven Fiber Resonator

Transition Between Quadratic and Quartic Solitons With OAM Modes in Passive Driven Fiber Resonator

Superposed Laguerre‐Gaussian Beams‐Based Orbital Angular Momentum Holography

AbstractOrbital angular momentum (OAM) holography implements OAM of light as an information carrier to reconstruct holographic images, due to a physically unbounded set of orthogonal OAM modes. Here, superposed Laguerre‐Gaussian (LG) beams are employed as incident beams to achieve OAM holography. Notably, this scheme utilizes exact complex‐amplitude modulation, as opposed to mere phase‐only modulation, ensuring the maintenance of amplitude information. In the absence of a matching field, the output images retain the properties of incident beams with a varying number of bright petals in each sampling pixel. On the other hand, when specific incident beams are illuminated, the output images can be reconstructed with multiple matching fields that lead to different intensities. It is essential to recognize that the bright spot in each sampling pixel of the output image achieves its peak intensity only when the matching field is the conjugate of the incident light field. This work offers a novel perspective on reconstructing holographic images through various matching fields and modulating the brightness of distinct holographic images in OAM‐multiplexing holography. Additionally, a theoretically infinite number of bright petals in each sampling pixel can also serve as information carriers in holography.