Generation of orbital angular momentum hologram using a modified U-net

Orbital angular momentum(OAM)holography has become a promising technique in information encryption,data storage and opto-electronic computing,owing to the infinite topological charge of one single OAM mode and the orthogonality of different OAM modes.In this paper,we propose a novel OAM hologram generation method based on a densely connected U-net(DCU),where the densely connected convolution blocks(DCB)replace the convolution blocks of the U-net.Importantly,the reconstruction process of the OAM hologram is integrated into DCU as its output layer,so as to eliminate the requirement to prepare training data for the OAM hologram,which is required by conventional neural networks through an iterative algorithm.The experimental and simulation results show that the OAM hologram can rapidly be generated with the well-trained DCU,and the reconstructed image's quality from the generated OAM hologram is significantly improved in comparison with those from the Gerchberg-Saxton generation method,the Gerchberg-Saxton based generation method and the U-net method.In addition,a 10-bit OAM multiplexing hologram scheme is numerically demonstrated to have a high capacity with OAM hologram.

Design of Photonic Crystal Fiber Capable of Carrying Multiple Orbital Angular Momentum Modes Transmission

For the traditional photonic crystal fibers with circular air holes, rectangular air holes are added to the fiber cladding. The periodic arrangement of the inner rectangular air holes allows the fiber structure to better match the annular mode field distribution of the vortex beam. The fiber structure was analyzed and calculated by COMSOL Multiphysics 5.4 finite element software, and the characteristics of fiber were analyzed, such as the dispersion, confinement loss, effective mode area and nonlinear coefficient. The results reveal that the photonic crystal fiber structure capable of carrying 50 orbital angular momentum (OAM) modes at the wavelength of 1.15 to 2.0 μm (850 nm). The effective refractive index difference Δneff between vector modes can reach 1 × 10-3, and larger difference can effectively separate the vector modes and improve the transmission performance of OAM modes. Moreover, the fiber has good performance, such as flat dispersion distribution of the low-order modes, low confinement loss below 10-9 dB·m-1, large effective mode field area and small nonlinear coefficient in the 850 nm wavelength range. Therefore, this fiber structure can be applied to the high-capacity communication system of fiber multiplexing OAM. In addition, the good characteristics of this fiber structure are of great significance for the transmission of vortex beam in fiber.

Diffraction deep neural network based orbital angular momentum mode recognition scheme in oceanic turbulence

Orbital angular momentum(OAM)has the characteristics of mutual orthogonality between modes,and has been applied to underwater wireless optical communication(UWOC)systems to increase the channel capacity.In this work,we propose a diffractive deep neural network(DDNN)based OAM mode recognition scheme,where the DDNN is trained to capture the features of the intensity distribution of the OAM modes and output the corresponding azimuthal indices and radial indices.The results show that the proposed scheme can recognize the azimuthal indices and radial indices of the OAM modes accurately and quickly.In addition,the proposed scheme can resist weak oceanic turbulence(OT),and exhibit excellent ability to recognize OAM modes in a strong OT environment.The DDNN-based OAM mode recognition scheme has potential applications in UWOC systems.

OAM mode purity improvement based on antenna array

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

Scheme for generating 1 nm X-ray beams carrying orbital angular momentum at the SXFEL

Optical vortices have the main features of helical wavefronts and spiral phase structures,and carry orbital angular momentum.This special structure of visible light has been produced and studied for various applications.These notable characteristics of photons were also tested in the extreme-ultraviolet and X-ray regimes.In this article,we simulate the use of a simple afterburner configuration by directly adding helical undulators after the SASE undulators with the Shanghai Soft X-ray FEL to generate high intensity X-ray vortices with wavelengths^1 nm.Compared to other methods,this approach is easier to implement,cost-effective,and more efficient.

A new method of calculating the orbital angular momentum spectra of Laguerre-Gaussian beams in channels with atmospheric turbulence

Studying orbital angular momentum（OAM） spectra is important for analyzing crosstalk in free-space optical（FSO）communication systems. This work offers a new method of simplifying the expressions for the OAM spectra of Laguerre-Gaussian（LG） beams under both weak/medium and strong atmospheric turbulences. We propose fixing the radius to the extreme point of the intensity distribution, review the expression for the OAM spectrum under weak/medium turbulence,derive the OAM spectrum expression for an LG beam under strong turbulence, and simplify both of them to concise forms.Then, we investigate the accuracy of the simplified expressions through simulations. We find that the simplified expressions permit accurate calculation of the OAM spectrum for large transmitted OAM numbers under any type of turbulence. Finally,we use the simplified expressions to analytically address the broadening of the OAM spectrum caused by atmospheric turbulence. This work should contribute to the concise theoretical derivation of analytical expressions for OAM channel matrices for FSO-OAM communications and the analytical study of the laws governing OAM spectra.

Radiation pattern analyses of circular aperture antenna to generate radio orbital angular momentum

Circular aperture antenna recently has been regarded as a nature source to generate high power radio orbital angular momentum （OAM） in millimeter （mm） wave; however, the radiation pattern was not investigated. Theoretical derivation of radiation pattern of circular aperture OAM antenna is conducted to evaluate the performance. Extensive simulations verify the validity of the theoretical result. Furthermore, performance of such antenna excited by orthogonal TE and TM modes is compared, which shows the potential application for TEgl mode to create pure OAM g-1 mode in a practical system, providing guidance for generation of twisted radio waves in mm-wave bands.

Generation of a large orbital angular momentum beam via an optical fiber winding around a curved path and its application

Many papers have been published on the study of orbital angular momentum(OAM)of the laser modes based on the Laguerre-Gaussian(LG)beam and helical phase plate or rotating cylindrical lens,and the principal ray of the LG beam remains straight line.These ways are difficult to get a large OAM modes.In this paper,we propose a novel method to prepare a large OAM modes when the light propagates through a optical fiber winding around a curved path,and establish a theoretical framework based on the principal ray path changing.Firstly,we investigate three kinds of winding structure.Secondly,based on the analytical solutions and numerical calculations,we can find that the spiral body can achieve a large OAM temporal pulse.At the same time,based on the zero OAM diffraction diagram,we can obtain an improvement of resolving power beyond Rayleigh’s criterion.Finally,applying a large OAM diffraction pattern to realize 12-bitencodes,we can obtain a high-security optical information transfer system.

A QUANTUM MULTIPLE ACCESS COMMUNICATIONS SCHEME USING ORBITAL ANGULAR MOMENTUM

We propose a quantum multiple access communications scheme using Orbital Angular Momentum (OAM) sector states in the paper. In the scheme, each user has an individual modified Poincare Bloch sphere and encodes his information with his own corresponding sector OAM states. A prepared entangled photon pairs are separated at transmitter and receiver. At the transmitter, each user encodes his information with the sector OAM states on the photons and the superposition of the different sector OAM states is carried by the photons. Then the photons are transmitted through quantum noiseless channel to the receiver. At the receiver, each user could retrieve his information by coincidently measuring the transmitted photons with the receiver side photons which are modulated by a special prepared measurement basis. The theoretical analysis and the numerical simulations show that each user could get his information from the superposition state without error. It seems that this scheme provides a novel method for quantum multiple users communications.

The Development Trends and Research Fronts in Orbital Angular Momentum Technology:A Bibliometric Analysis

Orbital angular momentum(OAM)technology,refers to Laguerre-Gaussian(LG)beams,twisted beams,vector/vortex beams,acoustic vortex beams and fractional vortex beams.It is an emerging and promising technology to improve the communication capacity,spectral efficiency,and anti-jamming capability due to its helical phase fronts and infinite orthogonal states.Although the OAM research began in the 1990s,the developing trends,current status,issues and characteristics through a systematic observation have not yet been performed.This paper presents a knowledge-based evolution of OAM research published in the Web of Science(WoS)from 2011 to 2021 using bibliometric analysis in Citepspace.The results demonstrate that the bandwidth,efficiency,gain,divergence,phase quantization,bulky and complex feeding structures,misalignment,distortion,interferences atmospheric turbulence and diffraction were the key issues found in the OAM technology.The main research hotspots and categories,influential authors,leading journals,best institutions of OAM show a strong bias in favor of their functions and technology developments.The research on OAM was mainly performed by the counties that have developed the 5G and now moving towards 6G communications like China,USA and South Korea.This study would serve as an inclusive guide on the future research trends and status especially for the OAM researchers.

Beam alignments based on the spectrum decomposition of orbital angular momentums for acoustic-vortex communications

Given the enhanced channel capacity of wave chirality,acoustic communications based on the orbital angular momentum(OAM)of acoustic-vortex(AV)beams are of significant interest for underwater data transmissions.However,the stringent beam alignment is required for the coaxial arrangement of transceiver arrays to ensure the accuracy and reliability of OAM decoding.To avoid the required multiple measurements of the traditional orthogonality based algorithm,the beam alignment algorithm based on the OAM spectrum decomposition is proposed for AV communications by using simplified ring-arrays.Numerical studies of the single-OAM and OAM-multiplexed AV beams show that the error of the OAM spectrum increases with the translation distance and the deflection angle of the transceiver arrays.To achieve an ideal arrangement,two methods of the single-array translation alignment and the dual-array deflection alignment are developed based on the least standard deviation of the OAM spectrum(SD-OAM).By decreasing the SD-OAM towards zero using transceiver arrays of 16 transmitters and 16 receivers,accurate beam alignments are accomplished by multiple adjustments in three dimensions.The proposed method is also demonstrated by experimental measurements of the OAM dispersion and the SD-OAM for misaligned beams.The results demonstrate the feasibility of the rapid beam alignment based on the OAM spectrum decomposition by using simplified transceiver ring-arrays,and suggest more application potentials for acoustic communications.

Optimization-based mode selection scheme for OAM millimeter wave system in the off-axis misalignment case

Millimeter-wave transmission combined with Orbital Angular Momentum(OAM)has the advantage of reducing the loss of beam power and increasing the system capacity.However,to fulfill this advantage,the antennas at the transmitter and receiver must be parallel and coaxial;otherwise,the accuracy of mode detection at the receiver can be seriously influenced.In this paper,we design an OAM millimeter-wave communication system for overcoming the above limitation.Specifically,the first contribution is that the power distribution between different OAM modes and the capacity of the system with different mode sets are analytically derived for performance analysis.The second contribution lies in that a novel mode selection scheme is proposed to reduce the total interference between different modes.Numerical results show that system performance is less affected by the offset when the mode set with smaller modes or larger intervals is selected.

Flexible orbital angular momentum mode switching in multimode fibre using an optical neural network chip

Mode-division multiplexing technology has been proposed as a crucial technique for enhancing communication capacity and alleviating growing communication demands.Optical switching,which is an essential component of optical communication systems,enables information exchange between channels.However,existing optical switching solutions are inadequate for addressing flexible information exchange among the mode channels.In this study,we introduced a flexible mode switching system in a multimode fibre based on an optical neural network chip.This system utilised the flexibility of on-chip optical neural networks along with an all-fibre orbital angular momentum(OAM)mode multiplexer-demultiplexer to achieve mode switching among the three OAM modes within a multimode fibre.The system adopted an improved gradient descent algorithm to achieve training for arbitrary 3×3 exchange matrices and ensured maximum crosstalk of less than-18.7 dB,thus enabling arbitrary inter-mode channel information exchange.The proposed optical-neural-network-based mode-switching system was experimentally validated by successfully transmitting different modulation formats across various modes.This innovative solution holds promise for providing effective optical switching in practical multimode communication networks.

Aperture efficiency and mode constituent analysis for OAM vortex beam generated by digital metasurface

A systematic study of the aperture efficiency and mode constituent for orbital angular momentum(OAM) vortex beam generated by digital metasurface is presented. The aperture efficiency and OAM spectrum are computed for different bit numbers. It is found that the aperture efficiency declines for digital metasurface due to the phase quantization error,especially for 1-bit device. Fortunately, the OAM spectrum is barely affected by phase quantization and the designated main mode keeps dominant for different bit numbers, indicating that high purity OAM vortex beam can be generated by digital metasurface. Besides, the influence of topological charge l is also investigated. For a fixed metasurface, the radiation performance deteriorates sharply with the growing of l and the parasitic OAM mode becomes dominant at certain angle.At last, a prototype of 1-bit metasurface was simulated, fabricated and measured in anechoic chamber. The simulation and experiment results verify the correctness of the numerical analysis.

Broadband beam steering for misaligned multi-mode OAM communication systems

Orbital angular momentum(OAM)at radio frequency(RF)has attracted more and more attention as a novel approach of multiplexing a set of orthogonal OAM modes on the same frequency channel to achieve high spectral efficiency(SE).However,the precondition for maintaining the orthogonality among different OAM modes is perfect alignment of the transmit and receive uniform circular arrays(UCAs),which is difficult to be satisfied in practical wireless communication scenarios.Therefore,to achieve available multi-mode OAM broadband wireless communication,we first investigate the effect of oblique angles on the transmission performance of the multi-mode OAM broadband system in the non-parallel misalignment case.Then,we compare the UCA-based RF analog and baseband digital transceiver structures and corresponding beam steering schemes.Mathematical analysis and numerical simulations validate that the SE of the misaligned multi-mode OAM broadband system is quite low,while analog and digital beam steering(DBS)both can significantly improve the SE of the system.However,DBS can obtain higher SE than analog beam steering especially when the bandwidth and the number of array elements are large,which validates that the baseband digital transceiver with DBS is more suitable for multi-mode OAM broadband wireless communication systems in practice.

Efficient separating orbital angular momentum mode with radial varying phase

It is shown that orbital angular momentum(OAM) is a promising new resource in future classical and quantum communications. However, the separation of OAM modes is still a big challenge. In this paper, we propose a simple and efficient separation method with a radial varying phase. In the method, specific radial varying phases are designed and modulated for different OAM modes. The resultant beam is focused to the spots with different horizontal and vertical positions after a convex lens, when the coordinate transformation, including two optical elements with coordinate transformation phase and correct phase, operates on the received beam.The horizontal position of the spot is determined by the vortex phases, and the vertical position of the spot is dependent on the radial varying phases. The simulation and experimental results show that the proposed method is feasible both for separation of two OAM modes and separation of three OAM modes. The proposed separation method is available in principle for any neighboring OAM modes because the radial varying phase is controlled. Additionally, no extra instruments are introduced, and there is no diffraction and narrowing process limitation for the separation.

High-accuracy mode recognition method in orbital angular momentum optical communication system

Vortex optical communication has been a hot research field in recent years. A key step is mode recognition in the orbital angular momentum(OAM) free-space optical(FSO) communication system. In this article, we propose an OAM mode recognition method based on image recognition technology, which uses the interferogram between the vortex beam and the Gaussian beam to identify the OAM mode. In order to resist the influence of atmospheric turbulence on the recognition accuracy, we added a Gaussian smoothing filter into the recognition process. Moreover, we used random phase screens to generate interferogram sets at distances of 1 km and 2 km. The verification result shows that the proposed scheme produces high identification accuracy for the distorted optical field. The average accuracy can reach 100% and 87.78% under the conditions of medium-and strong-turbulence levels, respectively. It is anticipated that these results might be helpful for improving the reliability of the OAM-FSO communication system in the future.

All-fiber linear polarization and orbital angular momentum modes amplifier based on few-mode erbium-doped fiber and long period fiber grating

A few-mode erbium-doped fiber（FM-EDF） is fabricated using modified chemical vapor deposition in combination with liquid solution. The core and cladding diameters of the fiber are approximately 19.44 and 124.12 μm,respectively. The refractive index difference is 0.98%, numerical aperture（NA） is 0.17, and normalized cut-off frequency at 1550 nm is 6.81. Therefore, it is a five-mode fiber, and can be used as a higher-order mode gain medium. Furthermore, a long period fiber grating（LPFG） is fabricated, which can convert LP〈01 mode to LP11 mode, and its conversion efficiency is up to 99%. The first-order orbital angular momentum（OAM） is also generated by combining the LPFG and polarization controller（PC）. Then, an all-fiber amplification system based on the FM-EDF and LPFG, for LP11 mode and first-order OAM beams, is built up. Its on-off gain of the LP11 mode beam is 37.2 d B at 1521.2 nm. The variation, whose transverse mode field intensity of first-order OAM is increased with the increase of pumping power, is obvious. These show that both the LP11 mode and first-order OAM beams are amplified in the all-fiber amplification system. This is a novel all-fiber amplification scheme,which can be used in the optical communication fields.

Design of a single-mode directly modulated orbital angular momentum laser

We propose a design of single-mode orbital angular momentum(OAM) beam laser with high direct-modulation bandwidth. It is a microcylinder/microring cavity interacted with two types of second-order gratings: the complex top grating containing the real part and the imaginary part modulations and the side grating. The side grating etched on the periphery of the microcylinder/microring cavity can select a whispering gallery mode with a specific azimuthal mode number, while the complex top grating can scatter the lasing mode with travelling-wave pattern vertically. With the cooperation of the gratings, the laser works with a single mode and emits radially polarized OAM beams. With an asymmetrical pad metal on the top of the cavity, the OAM on-chip laser can firstly be directly modulated with electrical pumping. Due to the small active volume, the laser with low threshold current is predicted to have a high direct modulation bandwidth about 29 GHz with the bias current of ten times the threshold from the simulation. The semiconductor OAM laser can be rather easily realized at different wavelengths such as the O band, C band, and L band.

Geometric transformations of optical orbital angular momentum spatial modes

With the aid of the bosonic mode conversions in two different coordinate frames, we show that （1） the coordinate eigenstate is exactly the EPR entangled state representation, and （2） the Laguerre-Ganssian （LG） mode is exactly the wave function of the common eigenvector of the orbital angular momentum and the total photon number operator. Moreover, by using the conversion of the bosonic modes, the Wigner representation of the LG mode can be obtained directly. It provides an alternative to the method of Simon and Agarwal.