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This paper presents the design, simulation, and experimental validation of a Perfect Optical Vortex (POV) sorter using a novel approach based on a binary Dammann Grating. POV beams, crucial for applications in communication and quantum systems, have traditionally posed challenges in sorting due to crosstalk and fabrication limitations. In this study, a hybrid lens system, combining a modified radial lens and a lateral lens, was proposed and optimized using parameters $\alpha$ and $\beta$ to achieve precise separation between POV modes. The design was simulated, allowing for the customization of the lens based on beam size and system constraints. To further enhance sorting efficiency, a Dammann Grating was employed to encode the phase profile, facilitating easy fabrication and integration into existing optical setups. Simulations and experiments were conducted, demonstrating the system's ability to map each topological charge to a unique radial location with high efficiency. Results indicated minimal crosstalk when the separation between orders exceeded 3, showcasing the potential for this system to be utilized in communication platforms. This research introduces a scalable, efficient, and compact solution for demultiplexing POV beams, paving the way for their seamless integration into optical communication systems.

We propose a high-security hyperchaotic orthogonal frequency division multiplexing (OFDM) encryption scheme based on the orbital angular momentum system, and our research focuses on the security and high-order modulation of the orbital angular momentum (OAM)system. In order to effectively prevent violent decryption by eavesdroppers, we adopt a high security Zhan's hyperchaotic model. In the experiments, the high security performance of the proposed encryption scheme is verified by the OAM optical transmission platform, the proposed encryption scheme has a large key space of 10191, and it is demonstrated that the high-order spatial optical modulation has a small impact on the bit error rate (BER) of the OAM system within the forward error correction (FEC) threshold. The experimental results show that the proposed encryption scheme significantly improves the security performance of OAM spatial optical transmission, and is a promising candidate for the next-generation high-security OAM high-order multiplexed transmission system.

Benefiting from the unprecedented superiority of phase change material on manipulating electromagnetic wave, germanium antimony telluride-based optical devices attract a lot of attention. Here, based on Pancharatnam-Berry phase, reflective metasurfaces are presented by using phase change material-germanium antimony telluride (Ge3Sb2Te6). They can manipulate reflection mode of terahertz wave and realize some functions. In order to verify this, three examples are numerically demonstrated, and they are gradient metasurface, vortex beam generator, and focusing lens. First of all, Ge3Sb2Te6 blocks are used to construct reflective wavefront of gradient metasurface, and then realize switching between anomalous reflection and mirror reflection. Secondly, vortex beam generators are designed with |l| = 1 and |l| = 2, and mode number of orbital angular momentum is reconstructed through the switching of Ge3Sb2Te6 between crystalline state and amorphous state. Finally, a reflective lens is presented, whose focus appears or disappears under different circular polarizations. By adjusting phase state of Ge3Sb2Te6, dynamic adjustment of focusing intensity is realized. Our design may have potential applications in the fields of terahertz switching and communication.

Metasurfaces with dynamically switchable functions have received extensive attention, and are in high demand in various applications. In this work, dynamically switchable metasurfaces are proposed based on phase change material-vanadium dioxide. They consist of vanadium dioxide blocks, silicon dioxide spacer, and gold film. By changing the size of vanadium dioxide block, vanadium dioxide in the metallic state forms a gradient metasurface. A specially arranged gradient metasurface is designed to achieve a 360° phase modulation capability. Normally incident plane wave is reflected to the right side at the angle of 36.9° at 2.5 THz. When vanadium dioxide switches from the metallic state to the insulating state, the switching of anomalous reflection and specular reflection is realized. In addition, when vanadium dioxide is metal, a vortex beam generator is implemented to generate orbital angular momentum beams with l = 1 and l = 2. The number of orbital angular momentum mode can be switched from l = 0 to l = 1 or l = 2 by changing the state of vanadium dioxide. Since the structure of meta-atom is a square block, the design is simple and easy to integrate. At the same time, the switchable function is accurately implemented without side lobe. The proposed design has potential applications in terahertz switching and communication.

The generation and topological configuration of double-layered polarization singularity array (PSA) having paired C-points of opposite topological index through superposing three Laguerre-Gaussian (LG) component beams with orthogonal linear polarization states are demonstrated. The generated PSA beam exhibits diversity in topological geometry and intensity pattern. The topological rule that the zero net topological charge and helicity conservation hold for the entire PSA is also confirmed. Based on our experiment setup consisting of a computer-controlled spatial light modulator (SLM), the PSA morphology can be flexibly customized and dynamically regulated by modulating the parameters of constituent beams. Our scheme provides a convenient device for manipulating polarization singularities and can facilitate the potential applications of singular optics.

Orbital angular momentum of light has attracted great attention in recent years. Optical ferris wheel, composed of the superposition of different orbital-angular-momentum modes, further broadens the application of orbital angular momentum of light in many fields. Here, we experimentally demonstrate a simple way to produce optical ferris wheel with controllable orbital-angular-momentum modes using a mask of spiral arrays. We show that optical ferris wheel can be rotated by adjusting the spiral arrays, and the rotation angle is studied analytically and experimentally. Our technique shows the advantages of wide operation bandwidth and flexible manipulation and the simplicity of our method may find potential application in optical manipulation.

In this article, we propose a non-orbital angular momentum (OAM) method for constructing the spiral twists of spin density (SD) vectors in three-dimensional (3D) optical fields. This is realized by generating strings of SD singularities in a longitudinal plane of a strongly focused HG10 mode beam with circular polarization. We demonstrate that in this plane, the SD singularities manifest themselves as the Gouy phase difference. Through observing the Gouy phase difference, the strings of the SD singularities and their topological reactions are examined. It is found that the spiral twists of SD vectors are constructed in the two-dimensional (2D) space between two strings and their twisting behaviors can be adjusted by the topological reactions of the strings. This finding shows that the spiral twists can be formed in a 2D space rather than the 1D space in previous studies, which provides a more flexible way to utilize this rotational degree of freedom. Our result also clarifies that although the spiral twists of SD vectors can be a result of spin-orbital angular momentum interaction, the OAM is not a necessary condition for the generation of these twists. Our theoretical proposal for structuring SD vectors may have applications in 3D optical manipulation.

A free-space communication scheme is proposed based on orbital angular momentum (OAM) states and pulse position modulation (PPM). It can not only improve the channel capacity, but also be suitable for single-photon detection to realize the communication in the case of ultra-weak light. A parallel coding method is proposed to realize high-capacity and low bit error rate (BER) communication under high background. Theoretical simulation and experimental results show that the scheme can effectively suppress the background noise. The BER is as low as 1.27 × 10$^{-5}$ under the condition of 16 OAM states, 16 single-photon detectors (SPDs) coincidence, and the background noise of 10$^{7}$ counts per second. The BER performance is an order of magnitude better than the traditional PPM, and the channel capacity is increased by 12 times. This work provides an effective solution for deep space high-speed and low BER communication.

A photonic wheel describes a special structure in which the electric vector spins (with time) in a plane containing the propagation direction, while around a C-point (a point with circular polarization) the polarization Möbius strip can be formed topologically in three-dimensional (3D) space. These two interesting but very different structures are both unique to 3D structured fields, and in this article they are studied together and also their connections are analyzed. By highly confining the trigonometric beams, the three-dimensional (3D) fields with a wide area of photonic wheels are constructed, and the characteristics of the photonic wheels in different regions are analyzed. The expression for the C-lines of the photonic wheels is derived, which provides a simple way to get the exact position of the C-lines and also is a basis for observing polarization Möbius strips. Along a C-line, the behaviors of the polarization Möbius strips, including two typical (generic) Möbius strips with opposite indices, and two special topological events seemingly violating the topological law are examined. Our results show that the trick and the paradox in the two special events actually are the manifestations of the trace-dependent property and the observer-dependent property of the polarization Möbius strips respectively. The finding also suggests that even in a short segment of a C-line rich topological events related to polarization Möbius strips can be observed. Our research provides a way to observe the photonic wheels, the polarization Möbius strips and their connection, also supplies a theoretical foundation for special structures in 3D fields.

In this article, we propose, design, and numerically assess a novel parametric refractive index profile for optical fiber, which we refer to as reciprocal-inverse graded index fiber (R-IGIF). The R-IGIF features a shape parameter $\alpha$ that controls the refractive index behavior (i.e. concave vs convex). We performed a systematic scanning of parameter' effects; using finite element method in Comsol Multiphysics, in order to design R-IGIFs that handle robust orbital angular momentum (OAM) channels (i.e. low radial modes). The designed fibers possess high inter-channels separation ($\Delta n_{eff} \geq 1\times$10−4) that outperforms standard step index fiber and inverse parabolic graded index fiber; hence, reduces channels crosstalk. Numerical simulations show that the supported OAM modes are of high purity ($\geq\! 99.9\%$), enabling low-level intrinsic crosstalk ($\leq\! -30\,dB$). Further investigations considered metrics over the C and L ITU-T bands including the cut-offs wavelengths, chromatic dispersion, differential group delay, effective area, and nonlinearity coefficient. Results show potential capability to achieve stable data channels transmission. Further assessments have been carried out under practical condition (i.e. bending) by evaluating the $\Delta n_{eff}$, the intra-mode walk-offs, and the confinement loss. The findings show great resilience to bending effects. Therefore, based on these deep physical investigations of modes carrying data, the R-IGIF could find application in next generation space division multiplexing SDM-Networks.

A flexible first-order OAM modes generation in ring-core fibers (RCFs) based on a mechanically-induced long-period fiber grating (MLPFG) is proposed. By directly fusion splicing a standard single-mode fiber (SMF) and RCFs, the anti-resonant reflection guidance mechanism (ARRGM) is introduced in the RCF, leading to the guided-core modes, so-called anti-resonant modes (AR modes), which have a Gaussian field distribution and are confined in low-refractive index central core of RCFs. A mechanical long-period fiber grating is used to couple the AR modes into the OAM modes propagated in the high-index ring core of the RCF. Results show that the generated OAM modes are the combination of two orthogonal linearly polarized (LP) modes with ±π/2 phase shift in the ring core. The mode conversion efficiency of OAM+1 and OAM−1 can reach 95% and 94%, respectively, indicating promising applications in sensing and communication of the RCFs.

The cross phase (CP) opens up a new horizon for the generation and measurement of vortex beams, and the conversion between Hermite-Gaussian (HG) and Laguerre-Gaussian (LG) modes can be done with a CP. However, the conversion plane is in the far plane, and the width cannot be adjusted. In this article, we present a more convenient and practical mode converter between HG and LG modes based on CP combined with a lens. The feasibility of the converter for the conversion between two modes is theoretically analyzed. The conversion can be accomplished in the determined plane and the width of converted beam can be controlled by adjusting the parameter of converter. Proof-of-concept experiment is carried out to verify theoretical analysis and the experimental results are agree well with theoretical simulations. The transfer of orbital angular momentum (OAM) between two modes after passing through the converter is also analyzed. From the perspective of OAM conservation, the required parameter conditions for the converter are discussed. In principle, such a converter as proposed here might even be applied to microwave, millimeter wave, and terahertz OAM fields.

The Laguerre-Gaussian (LG) and Hermite-Gaussian (HG) modes can be interconverted with an astigmatism phase. In this paper, based on the single beam, the interconversion of LG and HG beams array with the astigmatism phase is analyzed theoretically and the spatial mapping relationship is derived. In the experiment, 1 × 3, 2 × 2, 3 × 3 LG beams arrays are generated and converted into HG beams array via employing a spatial light modulator loaded with 0°, 90°, 45°, −45° astigmatism phases. The experimental results are in great agreement with the theoretical analyses. Moreover, from the number and direction of the dark fringe of converted HG beams array, the orbital angular momentum information of LG beams array can be obtained. This work theoretically perfects the theoretical model of the interconversion between LG and HG beams based on an astigmatism phase, and also provides a theoretical support for measuring the LG beams array.

A photonic wheel describes a special structure in which the electric vector spins (with time) in a plane containing the propagation direction, while around a C-point (a point with circular polarization) the polarization Möbius strip can be formed topologically in three-dimensional (3D) space. These two interesting but very different structures are both unique to 3D structured fields, and in this article they are studied together and also their connections are analyzed. By highly confining the trigonometric beams, the three-dimensional (3D) fields with a wide area of photonic wheels are constructed, and the characteristics of the photonic wheels in different regions are analyzed. The expression for the C-lines of the photonic wheels is derived, which provides a simple way to get the exact position of the C-lines and also is a basis for observing polarization Möbius strips. Along a C-line, the behaviors of the polarization Möbius strips, including two typical (generic) Möbius strips with opposite indices, and two special topological events seemingly violating the topological law are examined. Our results show that the trick and the paradox in the two special events actually are the manifestations of the trace-dependent property and the observer-dependent property of the polarization Möbius strips respectively. The finding also suggests that even in a short segment of a C-line rich topological events related to polarization Möbius strips can be observed. Our research provides a way to observe the photonic wheels, the polarization Möbius strips and their connection, also supplies a theoretical foundation for special structures in 3D fields. [ABSTRACT FROM AUTHOR]

In this article, we propose, design, and numerically assess a novel parametric refractive index profile for optical fiber, which we refer to as reciprocal-inverse graded index fiber (R-IGIF). The R-IGIF features a shape parameter $\alpha$ that controls the refractive index behavior (i.e. concave vs convex). We performed a systematic scanning of parameter' effects; using finite element method in Comsol Multiphysics, in order to design R-IGIFs that handle robust orbital angular momentum (OAM) channels (i.e. low radial modes). The designed fibers possess high inter-channels separation ($\Delta n_{eff} \geq 1\times$ 10−4) that outperforms standard step index fiber and inverse parabolic graded index fiber; hence, reduces channels crosstalk. Numerical simulations show that the supported OAM modes are of high purity ($\geq\! 99.9\%$), enabling low-level intrinsic crosstalk ($\leq\! -30\,dB$). Further investigations considered metrics over the C and L ITU-T bands including the cut-offs wavelengths, chromatic dispersion, differential group delay, effective area, and nonlinearity coefficient. Results show potential capability to achieve stable data channels transmission. Further assessments have been carried out under practical condition (i.e. bending) by evaluating the $\Delta n_{eff}$ , the intra-mode walk-offs, and the confinement loss. The findings show great resilience to bending effects. Therefore, based on these deep physical investigations of modes carrying data, the R-IGIF could find application in next generation space division multiplexing SDM-Networks. [ABSTRACT FROM AUTHOR]

The Laguerre-Gaussian (LG) and Hermite-Gaussian (HG) modes can be interconverted with an astigmatism phase. In this paper, based on the single beam, the interconversion of LG and HG beams array with the astigmatism phase is analyzed theoretically and the spatial mapping relationship is derived. In the experiment, 1 × 3, 2 × 2, 3 × 3 LG beams arrays are generated and converted into HG beams array via employing a spatial light modulator loaded with 0°, 90°, 45°, −45° astigmatism phases. The experimental results are in great agreement with the theoretical analyses. Moreover, from the number and direction of the dark fringe of converted HG beams array, the orbital angular momentum information of LG beams array can be obtained. This work theoretically perfects the theoretical model of the interconversion between LG and HG beams based on an astigmatism phase, and also provides a theoretical support for measuring the LG beams array. [ABSTRACT FROM AUTHOR]

Orbital angular momentum of light has attracted great attention in recent years. Optical ferris wheel, composed of the superposition of different orbital-angular-momentum modes, further broadens the application of orbital angular momentum of light in many fields. Here, we experimentally demonstrate a simple way to produce optical ferris wheel with controllable orbital-angular-momentum modes using a mask of spiral arrays. We show that optical ferris wheel can be rotated by adjusting the spiral arrays, and the rotation angle is studied analytically and experimentally. Our technique shows the advantages of wide operation bandwidth and flexible manipulation and the simplicity of our method may find potential application in optical manipulation. [ABSTRACT FROM AUTHOR]

In this article, we propose a non-orbital angular momentum (OAM) method for constructing the spiral twists of spin density (SD) vectors in three-dimensional (3D) optical fields. This is realized by generating strings of SD singularities in a longitudinal plane of a strongly focused HG10 mode beam with circular polarization. We demonstrate that in this plane, the SD singularities manifest themselves as the Gouy phase difference. Through observing the Gouy phase difference, the strings of the SD singularities and their topological reactions are examined. It is found that the spiral twists of SD vectors are constructed in the two-dimensional (2D) space between two strings and their twisting behaviors can be adjusted by the topological reactions of the strings. This finding shows that the spiral twists can be formed in a 2D space rather than the 1D space in previous studies, which provides a more flexible way to utilize this rotational degree of freedom. Our result also clarifies that although the spiral twists of SD vectors can be a result of spin-orbital angular momentum interaction, the OAM is not a necessary condition for the generation of these twists. Our theoretical proposal for structuring SD vectors may have applications in 3D optical manipulation. [ABSTRACT FROM AUTHOR]

A free-space communication scheme is proposed based on orbital angular momentum (OAM) states and pulse position modulation (PPM). It can not only improve the channel capacity, but also be suitable for single-photon detection to realize the communication in the case of ultra-weak light. A parallel coding method is proposed to realize high-capacity and low bit error rate (BER) communication under high background. Theoretical simulation and experimental results show that the scheme can effectively suppress the background noise. The BER is as low as 1.27 × 10 $^{-5}$ under the condition of 16 OAM states, 16 single-photon detectors (SPDs) coincidence, and the background noise of 10 $^{7}$ counts per second. The BER performance is an order of magnitude better than the traditional PPM, and the channel capacity is increased by 12 times. This work provides an effective solution for deep space high-speed and low BER communication. [ABSTRACT FROM AUTHOR]

We propose and numerically verify a design approach to a grating coupler (GC) for in-plane generation and propagation of quasi-TE vortex modes with azimuthal order $\pm$1 within photonic integrated circuits (PICs). In the considered GC design example, silicon nitride waveguides with silica substrate and cladding are used. The shallowly etched grating is illuminated by the incident optical beam from a standard single-mode fiber. We optimize the GC design parameters to maximize the purity of orbital angular momentum (OAM) mode first for the case of a theoretical ideally fabricated grating, and then consider a more practical case of GC design, when the etching resolution is limited. The numerical modeling results show possibility of the vortex mode generation with >96% purity of the target OAM state $\pm$1 at the wavelength 1550 nm. The proposed design, compatible with the standard nanofabrication process, provides a simple, compact and robust solution for various potential PIC-based applications of OAM beams.

Due to countless orthogonal eigenstates, light beams with orbital angular momentum(OAM) have a large potential information capacity. Recently, deep learning has been extensively applied in recognition of OAM mode. However, previous deep learning methods require a constant distance between laser and receiver. The accuracy will drop quickly if the distance of testing set deviates from the training set. Previous deep learning methods also have difficulty distinguishing OAM modes with positive and negative topological charges. In order to further exploit the huge potential of the countless dimension of state space, we proposed multidimensional information assisted deep learning flexible recognition (MIADLFR) method to make use of both intensity and angular spectrum information for the first time to achieve recognition of OAM modes unlimited by the sign of TC and distance with high accuracy. Also, MIADLFR can reduce the computational complexity significantly and requires much smaller training set.

The explicit expression of orbital angular momentum (OAM) spiral spectrum of Laguerre-Gaussian (LG) beam propagating in anisotropic plasma turbulence is derived based on Rytov theory. The relationship between the detection probability of OAM and the crosstalk with the anisotropy parameters, topological charge, radial index, wavelength, beam width, receiving aperture, inner and outer scales are discussed in detail. In order to quantify the effect of plasma turbulence, we compared it with atmospheric turbulence based on restriction function. To optimize OAM detection probability, we also compared the effects of circular aperture method (CAM) with focusing mirror method (FMM) on the propagation characteristics of the spiral spectrum. The results show that even in the strong turbulence region of atmospheric turbulence, the radial dimension of LG beam in plasma turbulence is nearly twice that of atmospheric turbulence and the modes crosstalk has almost reached its upper bound. Besides, by comparing the two optimization methods, we found the ability to reduce crosstalk is related to the choice of beam width and receiving aperture. Therefore, we give a method to optimally select between the CAM and the FMM. We found the corresponding relationship and gave an analytical formula. The FMM has a greater ability to reduce crosstalk than the CAM when the beam width ≥4 cm and the aperture ≥3 cm. We quantified the effect of plasma turbulence and provide a guideline for the selection of beam width and receiving aperture under these two optimization methods. The research results may be helpful in the field of optical communication.

A novel method is proposed to select vortex beams carrying a specific orbital angular momentum (OAM) mode in turbulence heterodyne coherent mitigation (THCM) link. It is worth mentioning that intelligent phase matching (IPM) of the OAM beams based on the convolutional neural network (CNN) is the remarkable feature. Namely, CNN is particularly trained as the OAM modes classifier by the light intensity distribution patterns of different modes. The classifier actually acts as a mode detector to distinguish OAM modes by the map between the light intensity distribution and OAM mode, and then output mode information (MI). Specially, the phase matching technology is demonstrated to realize selection of specific OAM mode, where exploiting MI to select a specific phase mask is a characteristic of IPM. Subsequently, the phase mask is attached to the Gaussian beam to obtain the OAM beam carrying a special mode. Numerical results show a high IPM accuracy of 99% under medium strength atmospheric turbulence (AT).

Assuming weakly guiding approximation, we examine orbital angular momentum (OAM) mode mixing on account of ellipticity in a fiber and derive a complete set of analytic expressions for spatial crosstalk, using scalar perturbation theory that incorporates fully the existing degeneracy between a spatial OAM mode and its degenerate partner characterized by a topological charge of opposite sign. These expressions, consequently, include an explicit formula for calculating the 2π walk-off length over which an input OAM mode converts into its degenerate partner, and back into itself. We further explore the applicability of the derived expressions in the presence of spin-orbit interaction. The expressions constitute a useful mathematical tool in the analysis and design of fibers for spatially-multiplexed mode transmissions. Their utility is demonstrated with application to a few mode and a multimode step-index fiber.

The explicit expression of orbital angular momentum (OAM) spiral spectrum of Laguerre-Gaussian (LG) beam propagating in anisotropic plasma turbulence is derived based on Rytov theory. The relationship between the detection probability of OAM and the crosstalk with the anisotropy parameters, topological charge, radial index, wavelength, beam width, receiving aperture, inner and outer scales are discussed in detail. In order to quantify the effect of plasma turbulence, we compared it with atmospheric turbulence based on restriction function. To optimize OAM detection probability, we also compared the effects of circular aperture method (CAM) with focusing mirror method (FMM) on the propagation characteristics of the spiral spectrum. The results show that even in the strong turbulence region of atmospheric turbulence, the radial dimension of LG beam in plasma turbulence is nearly twice that of atmospheric turbulence and the modes crosstalk has almost reached its upper bound. Besides, by comparing the two optimization methods, we found the ability to reduce crosstalk is related to the choice of beam width and receiving aperture. Therefore, we give a method to optimally select between the CAM and the FMM. We found the corresponding relationship and gave an analytical formula. The FMM has a greater ability to reduce crosstalk than the CAM when the beam width ≥4 cm and the aperture ≥3 cm. We quantified the effect of plasma turbulence and provide a guideline for the selection of beam width and receiving aperture under these two optimization methods. The research results may be helpful in the field of optical communication. [ABSTRACT FROM AUTHOR]

We propose and numerically verify a design approach to a grating coupler (GC) for in-plane generation and propagation of quasi-TE vortex modes with azimuthal order $\pm$ 1 within photonic integrated circuits (PICs). In the considered GC design example, silicon nitride waveguides with silica substrate and cladding are used. The shallowly etched grating is illuminated by the incident optical beam from a standard single-mode fiber. We optimize the GC design parameters to maximize the purity of orbital angular momentum (OAM) mode first for the case of a theoretical ideally fabricated grating, and then consider a more practical case of GC design, when the etching resolution is limited. The numerical modeling results show possibility of the vortex mode generation with >96% purity of the target OAM state $\pm$ 1 at the wavelength 1550 nm. The proposed design, compatible with the standard nanofabrication process, provides a simple, compact and robust solution for various potential PIC-based applications of OAM beams. [ABSTRACT FROM AUTHOR]

Within the framework of perturbation theory, we explore in detail the mixing of orbital angular momentum (OAM) modes due to a fiber bend in a step-index multimode fiber. Using a scalar wave equation, we develop a complete set of analytic expressions for modemixing, including those for the 2π walk-off length, which is the distance traveled within the bent fiber before an OAM mode transforms into its negative topological charge counterpart, and back into itself. The derived results provide insight into the nature of the bend effects, clearly revealing the mathematical dependence on the bend radius and the topological charge. We numerically simulate the theoretical results with applications to a few-mode fiber and a multimode fiber, and calculate bend-induced modal crosstalk with implications for mode-multiplexed systems. The presented perturbation technique is general enough to be applicable to other perturbations like ellipticity and easily extendable to other fibers with step-index-like profile as in the ring fiber.

We have demonstrated an all-fiber high-order circularly polarized (CP) orbital angular momentum (OAM) modes (de)multiplexing system based on second-order mode selective couplers (SMSCs). Such SMSCs combining standard single mode fiber and ring core fiber (RCF) are fabricated by a fusing and pulling technique, in which the RCF supports second-order vector modes (HE31 and EH11). During a pulling process, the HE31 and EH11 modes could be excited along the coupling region through evanescent wave interaction. By adjusting the polarization states, the excited second-order modes are transformed into two sets of second-order CP OAM (CP-OAM±2) modes and then transmit along the RCF. The second-order CP-OAM±2 modes, carrying a 2.95-Gb/s orthogonal frequency division multiplexing signals filled with the 16-level quadrature amplitude modulation signals, are successfully (de)multiplexed and propagated over a 3.7-km RCF. The experimental investigations show that the developed system to multiplex and transmit CP-OAM±2 can achieve forward error correction threshold that the received optical power is larger than -13 dBm. The obtained results show that the OAM modes can be generated, (de)multiplexed, and effectively transmitted in the RCF with favorable performance by the designed SMSCs.

The effects of asymmetry atmospheric eddies on the beam spreading and wander of Bessel-Gaussian (BG) beams in anisotropic turbulence are studied. Results show that turbulence anisotropy leads to the declines in both beam spreading and wander of BG beams in the atmosphere. A BG beam with larger beam shape parameter η has a narrower spot size in a short-distance range and a wider spot size in a long-distance range. The effective Rayleigh ranges of BG beams for every turbulent situation decrease with the beam shape parameter η. When η is larger than 10, atmospheric turbulence has a negligible effect on effective Rayleigh range. The effective Rayleigh range and beam spreading increase and beam wander decreases as topological charge increase. Beam spreading is mainly affected by turbulence inner-scale, and outer-scale evidently affects beam wander in the atmosphere.

When an eavesdropper performs an optical beam-splitting attack in a free-space optical communications channel, it is referred to as an optical wiretap channel, which is an extension of Wyner's wiretap channel model. Even though physical-layer security can be compromised, it is possible to exploit the noisy and degraded channel conditions experienced by the eavesdropper to obtain positive secrecy capacity even when a shared secret key is not used. In our previous work, we found that employing Bessel-Gaussian beams can help to improve physical-layer security and provide higher secrecy capacity over that of Laguerre-Gaussian beams in a turbulent free-space optical communications channel. In this companion paper, we conducted an experiment exclusively with Bessel-Gaussian beams onto which we encoded a pseudorandom binary sequence to emulate data transmission over this optical wiretap channel. Bit-error rate curves for the intended receiver and the eavesdropper were calculated from which estimates of secrecy capacity were derived. We found that the bit-error rate curves for the eavesdropper were consistently worse than those of the intended receiver under several turbulence conditions and that further evidence of an error floor even when the eavesdropper uses an optical amplifier is promising for secure communications.

Assuming weakly guiding approximation, we examine orbital angular momentum (OAM) mode mixing on account of ellipticity in a fiber and derive a complete set of analytic expressions for spatial crosstalk, using scalar perturbation theory that incorporates fully the existing degeneracy between a spatial OAM mode and its degenerate partner characterized by a topological charge of opposite sign. These expressions, consequently, include an explicit formula for calculating the $2\pi$ walk-off length over which an input OAM mode converts into its degenerate partner, and back into itself. We further explore the applicability of the derived expressions in the presence of spin-orbit interaction. The expressions constitute a useful mathematical tool in the analysis and design of fibers for spatially-multiplexed mode transmissions. Their utility is demonstrated with application to a few mode and a multimode step-index fiber. [ABSTRACT FROM AUTHOR]

A novel method is proposed to select vortex beams carrying a specific orbital angular momentum (OAM) mode in turbulence heterodyne coherent mitigation (THCM) link. It is worth mentioning that intelligent phase matching (IPM) of the OAM beams based on the convolutional neural network (CNN) is the remarkable feature. Namely, CNN is particularly trained as the OAM modes classifier by the light intensity distribution patterns of different modes. The classifier actually acts as a mode detector to distinguish OAM modes by the map between the light intensity distribution and OAM mode, and then output mode information (MI). Specially, the phase matching technology is demonstrated to realize selection of specific OAM mode, where exploiting MI to select a specific phase mask is a characteristic of IPM. Subsequently, the phase mask is attached to the Gaussian beam to obtain the OAM beam carrying a special mode. Numerical results show a high IPM accuracy of 99% under medium strength atmospheric turbulence (AT). [ABSTRACT FROM AUTHOR]

We corroborate that a synergy effect may emerge when the dually superposed orbital angular momentum (OAM) states with two adjacent OAM numbers pass through a weak fluctuation regime of the atmospheric turbulence. We show that this novel effect closely depends on the transition probabilities between this two OAM eigenstates for superposition. While such probabilities become larger, the synergy effect becomes more obvious. Our examples show that the synergy effect enables OAM states to better resist phase fluctuation caused by the atmospheric turbulence.

We present the investigation on chromatic dispersion characteristics of three orbital angular momentum (OAM) states in 5.58 km ring-core fiber (RCF) link. Steady transmissions of different OAM mode orders spanning from -2 to +2 via the 5.58 km RCF are demonstrated, ensuring the stability of dispersion measuring system. For OAM0,1+, OAM+1,1+, and OAM+2,1+, the chromatic dispersion values of 13.8 ps/(nm·km), 20.1 ps/(nm km), and 32.9 ps/(nm· km) are obtained, respectively. Experiments are also carried out on the measurements of dispersion-induced power fading of the radio-frequency (RF) signal in the OAM transmission link, and the results match well with the values calculated from the measured chromatic dispersion.

We study the rotation properties of spiraling elliptic beams in the anisotropy media. Based on the Collins formula, we obtain the exact analytical solution of the spiraling elliptic beams carrying the orbital angular momentum (OAM) to the paraxial wave equation. It is found that the rotation property is closely relevant to the OAM, which can be controlled by the linear anisotropy of media. For an anisotropic media such as the uniaxial crystal, the rotation velocity of spiraling elliptic beams at the output can be controlled only by changing the direction of the optic axis of the uniaxial crystal. Depending on the anisotropy parameter, two rotation modes are predicted for the spiraling elliptic beam. For small anisotropy parameter, the rotation direction of the spiraling elliptic beam is inverted at a certain propagation distance. When the anisotropy parameter is large enough, the invertion of the rotation direction disappears. The results are potentially useful in controlling the optical beams.

We have proposed an effective method of generating the first-order orbital angular momentum (OAM) modes in ring fibers. The first-order OAM modes (L = ±1) can be represented as the linear superposition of the HE21even mode and the HE21odd mode. Through exerting pressure on the ring fiber by a leverage device, the fundamental mode HE11 can be coupled into higher order mode HE21in broadband wavelength ranging from 1530 to 1600 nm. By adjusting the fiber polarization controller and the pressure value exerted on the fiber, both the positive (L = +1) and negative (L = - 1) OAM modes can be generated separately. Moreover, we have demonstrated that the coupling efficiency from the fundamental mode to the first-order OAM modes at wavelength of 1550 nm can reach 67%.

Within the framework of perturbation theory, we explore in detail the mixing of orbital angular momentum (OAM) modes due to a fiber bend in a step-index multimode fiber. Using a scalar wave equation, we develop a complete set of analytic expressions for mode-mixing, including those for the $2\pi$ walk-off length, which is the distance traveled within the bent fiber before an OAM mode transforms into its negative topological charge counterpart, and back into itself. The derived results provide insight into the nature of the bend effects, clearly revealing the mathematical dependence on the bend radius and the topological charge. We numerically simulate the theoretical results with applications to a few-mode fiber and a multimode fiber, and calculate bend-induced modal crosstalk with implications for mode-multiplexed systems. The presented perturbation technique is general enough to be applicable to other perturbations like ellipticity and easily extendable to other fibers with step-index-like profile as in the ring fiber. [ABSTRACT FROM AUTHOR]

We present a flexible method for measuring the orbital angular momentum (OAM) of a linearly polarized beam by means of a metasurface composed of a V-shaped gold plasmonic antenna. When a linearly polarized OAM light illuminates the metasurface, the ordinary beam with the same polarization and topological charge transmits forward, whereas another two extraordinary beams with the same topological charge but with cross polarization are tilted with respect to the ordinary beam. The topological charge of the OAM beam can be detected by the near-field interferogram of the two extraordinary beams. The metasurface is effective for detecting OAM transparently in a broadband wavelength and offers the capability of integration with fiber and on-chip devices.

We present two kinds of interferences on metallic thin films with orbital angular momentum (OAM) beams, namely, double-slit interference and square-slit interference. When an OAM beam is normally incident upon the metallic thin film with two slits, the fringes on the metallic thin film generated by the interference of two surface-plasmon-polariton waves will twist, and the twist amount depends on the topological charge of the incoming OAM beam. When a square slit is employed, a quasi-square optical lattice will be unveiled, and the arrangement of lattice can be modulated by the OAM beam. The fringes and spots are both at the subwavelength level, which show great potential for superresolution microscopy, OAM detection, and spot-array generation.

Orbital angular momentum (OAM) states of photons span a high-dimensional Hilbert space, and by using it, higher information capacity and enhanced security can be achieved for both classical communication and quantum key distribution. In this work, we propose a feasible basis-independent encoding scheme for OAM states and show its application in high-dimensional quantum secure direct communication, which enables direct transmission of meaningful messages, in addition to random secret key establishing. Imperfections of optical components may reduce the efficiency of the encoding scheme and, thus, the channel capacity. We discuss the optimal input distribution under realistic conditions to achieve maximal mutual information and an alternative solution using a different set of mutually unbiased bases in the case of a prime-dimensional quantum system.

The generation, propagation, and detection of high-quality and coherently superimposed optical vortices, carrying two or more orbital angular momentum (OAM) states, is experimentally demonstrated using an optical arrangement based on spatial light modulators. We compare our results with numerical simulations and show that, in the context of turbulence-free wireless optical communication (indoor or satellite), individual OAM state identification at the receiver of an OAM-modulated system can be achieved with good precision, to accommodate for high-dimensional OAM modulation architectures. We apply our results to the simulation of a communication system using low-density parity-check-coded modulation that considers optimal signal constellation design in a channel that includes OAM crosstalk induced by realistic (imperfect) detection.

We propose a metal device containing circular slits to generate a terahertz (THz) orbital angular momentum beam with numerical simulations. The estimation of the polarization extinction ratio is above 20 dB over the bandwidth ranging from 0.3 to 3 THz, and a mode purity of TC = 1 or -1 is close to 1 over a wide bandwidth range, except for the area near the deteriorated valley. We analyze the OAM spectrum and find that the main noise comes from an OAM mode of TC = -3 or 3. When multiple concentric circular slits are employed, a larger transmittance is obtained without the sacrifice of mode purity. The design of such a device is simple with a size of micrometer order, revealing an option to generate a broadband THz vortex beam.

The performance of M-ary orbital angular momentum-shift keying (OAMSK) modulation-based underwater wireless optical communication (UWOC) system is investigated over oceanic turbulence with Laguerre–Gauss beam considered. On the basis of Rytov approximation, the detection probability and power distribution among the received signals are derived. The conditional probability and symbol error rate (SER) are, then, achieved by the maximum likelihood estimation. With Blahut–Arimoto algorithm, the channel capacity for this UWOC system is obtained. The results show that the optimal transmitted OAM mode set S is mainly restricted by the interfering energy and decaying effective energy. With the optimal mode interval achieved, the SER performances for different modulation orders M and the minimum required transmitting power to reach the SER of Pe = 10−9 are sensitive to the variation in oceanic turbulence conditions. Additionally, the value of capacity will shift to a higher value with the increasing M, and a suitable M should be selected to balance the transmitting power consumption and capacity gain. This work is beneficial to M-ary OAMSK modulation-based UWOC system design. [ABSTRACT FROM AUTHOR]

We corroborate that a synergy effect may emerge when the dually superposed orbital angular momentum (OAM) states with two adjacent OAM numbers pass through a weak fluctuation regime of the atmospheric turbulence. We show that this novel effect closely depends on the transition probabilities between this two OAM eigenstates for superposition. While such probabilities become larger, the synergy effect becomes more obvious. Our examples show that the synergy effect enables OAM states to better resist phase fluctuation caused by the atmospheric turbulence. [ABSTRACT FROM PUBLISHER]

We present the investigation on chromatic dispersion characteristics of three orbital angular momentum (OAM) states in 5.58 km ring-core fiber (RCF) link. Steady transmissions of different OAM mode orders spanning from −2 to +2 via the 5.58 km RCF are demonstrated, ensuring the stability of dispersion measuring system. For OAM 0,1^{\;+\;}, OAM +1,1^{\;+\;}, and OAM +2,1^{\;+\;}, the chromatic dispersion values of 13.8 ps/(nm ⋅ km), 20.1 ps/(nm ⋅ km), and 32.9 ps/(nm $\cdot$ km) are obtained, respectively. Experiments are also carried out on the measurements of dispersion-induced power fading of the radio-frequency (RF) signal in the OAM transmission link, and the results match well with the values calculated from the measured chromatic dispersion. [ABSTRACT FROM PUBLISHER]

We have proposed an effective method of generating the first-order orbital angular momentum (OAM) modes in ring fibers. The first-order OAM modes (\rmL = \pm 1 ) can be represented as the linear superposition of the \rmHE{21}^{{\rm{even}}} mode and the \rmHE{21}^{{\rm{odd}}} mode. Through exerting pressure on the ring fiber by a leverage device, the fundamental mode \rmH\rmE{11} can be coupled into higher order mode \rmH\rmE{21}in broadband wavelength ranging from 1530 to 1600 nm. By adjusting the fiber polarization controller and the pressure value exerted on the fiber, both the positive ( \rmL = + 1) and negative ( \rmL = - 1) OAM modes can be generated separately. Moreover, we have demonstrated that the coupling efficiency from the fundamental mode to the first-order OAM modes at wavelength of 1550 nm can reach 67%. [ABSTRACT FROM PUBLISHER]