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.

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.

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.

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.

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.

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.

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.

Tailoring OAM spectrum of high-order harmonic generation driven by two mixed Laguerre–Gaussian beams with nonzero radial nodes

The extreme ultraviolet(XUV)light beam carrying orbital angular momentum(OAM)can be produced via high-order harmonic generation(HHG)due to the interaction of an intense vortex infrared laser and a gas medium.Here we show that the OAM spectrum of vortex HHG can be readily tailored by varying the radial node(from 0 to 2)in the driving laser consisting of two mixed Laguerre-Gaussian(LG)beams.We find that due to the change in spatial profile of HHG,the distribution range of the OAM spectrum can be broadened and its shape can be modified by increasing the radial node.We also show that the OAM mode range becomes much wider and its distribution shape becomes more symmetric when the harmonic order is increased from the plateau to the cutoff when the driving laser has the nonzero radial nodes.Through the map of coherence length and the evolution of harmonic field in the medium,we reveal that the favorable off-axis phase-matching conditions are greatly modified due to the change of intensity and phase distributions of driving laser with the radial node.We anticipate this work to stimulate some interests in generating the XUV vortex beam with tunable OAM spectrum through the gaseous HHG process achieved by manipulating the mode properties of the driving laser beam.

具有可重构特征的轨道角动量天线技术研究进展

轨道角动量(OAM)因其模式具有理论上无穷且正交互不干扰的特点,在扩展信道容量方面展现出良好的优势,为解决日趋紧张的频谱资源提供了一种新型设计自由度。面对复杂多样的无线通信场景,设计具有可重构特征的OAM天线,是实现多模态复用、智能信息感知和人工智能天线的物理层基础。该文首先结合可重构天线实现机理,给出了OAM可重构天线设计的方法及具备的特点;然后,系统性综述了具有可重构特征的OAM天线的研究进展;最后,对未来设计具有可重构特征的OAM天线研究进行了展望。

OAM光通信技术研究进展

涡旋光束携带的轨道角动量(OAM)为光波的空间域提供了新的维度资源,吸引了越来越多研究人员的关注。由于具有不同OAM模式值的涡旋光束相互正交,因此将OAM模式引入传统光通信领域,衍生出两种新的应用机制--OAM键控(OAM-SK)与OAM复用(OAM-DM),这为未来实现高速、大容量及高频谱效率的光通信技术提供了潜在的解决方案。本文将从OAM光束的类别和产生方法等基本概念理论出发,对这两种通信应用机制相关的典型研究案例做简要概述,并重点论述三种关键技术,包括OAM光束复用技术、OAM光束解调技术以及OAM光通信的大气湍流效应抑制技术。最后,对OAM光通信技术的未来发展趋势及其前景进行了分析与展望。

基于改进Xception实现涡旋光束轨道角动量识别

当光束在海洋中传输时,湍流的存在会严重影响光束的质量,导致接收端光场产生扭曲和退化现象。为解决该问题,提出一种基于改进深度可分离网络(IXception)的方法,用于实现通过海洋湍流传输的涡旋光束轨道角动量模态识别。采用分步相位屏的思想,基于功率谱反演法仿真涡旋光束在海洋中的传输过程,并建立入射光场发生的退化、扭曲的散斑场数据集,用数据集来训练IXception识别散斑场中涡旋光束的轨道角动量。IXception延用Xception架构思想,结合了残差结构和倒置残差结构,能够提取高度空间深度特征,减少网络结构参数的冗余,增强泛化能力。研究结果表明,IXception在20 m和80 m湍流中对扭曲光场轨道角动量的识别率达到了99.20%与97.9%。随着传输距离的增加,IXception的识别率会略有降低,但与Xception模型相比,IXception识别性能更好。

多轨道角动量模式谐振环形行波天线设计

为解决目前轨道角动量(OAM)天线难以在同一频率下多模工作的问题,推动OAM在无线通信领域的实际应用,设计、加工并测量了一种基于单层板的谐振环形行波天线,引入4个馈电探针将不同相位差的信号馈入行波天线。经仿真分析和实际测量,该天线可以在3 GHz上谐振并分别产生了携带有OAM模式l=0,l=±1,l=-2的涡旋电磁波束。该天线结构简单并且可以在同一频率下产生多种模式OAM,能够有效增加无线通信的信道容量,具有一定的理论意义和实用价值。

OAM光通信路由器件研究

随着轨道角动量(OAM)光通信技术的研究不断深入,OAM路由器件在提升通信系统的灵活性和实用性方面扮演着重要的角色,近些年,具有特定功能的OAM路由器件的研究得到了飞速发展。基于此,对近些年OAM路由器件的研究成果进行了概述,对其中可实现信道切换、多播、滤波、添加/提取、跳变和模式划分6类OAM路由器件的原理、研究方法及优缺点进行了讨论与总结,同时对OAM路由器件未来的发展趋势进行了展望。

一种基于多模态OAM波束的目标特征智能识别方法

高效率目标探测需要借助探测信号的低相关性空间调制,调制数量大且具有时间独立性。携带轨道角动量(OAM)的涡旋波束具有无穷多种模态且不同模态之间相互正交,借助强色散材料可以实现频率域的多模态OAM波束产生。该文首先对OAM的传播特性进行推导,给出了符合探测需求的多模态OAM波束源特征;在此基础上,研究了不同模态的OAM波束在3种不同应用场景下目标反射回波信号特性,采用卷积神经网络对不同反射场景下的数据特征进行提取,实现了对未知场景的判断及场景内的目标识别,并进行了抗噪性能分析。实验结果表明:理想状态下,网络对目标场景判断的准确率可达97:5%;各反射场景中的两个相邻目标的间隔大于某一阈值时,网络对目标位置的识别准确率均高于80%。但目标识别效果有环境依赖性,当SNR<20 dB时,3种场景内的目标识别准确率均大幅降低。

应用PO-AI方法分析加罩OAM天线的辐射特性

应用物理光学-口面积分(PO-AI)方法,数值分析加载天线罩的轨道角动量(OAM)波天线的辐射特性,评判介质天线罩对各阶模式OAM波的辐射特性、幅相分布以及模式纯度的影响程度。通过与现有文献中天线罩实例的测试值相对比,验证了算法的正确性和有效性;考察了开口波导环形阵列OAM波天线工作于不同的模式阶数时,半球形天线罩所引起的空间场幅-相分布的变化。在此基础上,计算了一个正切卵形、多层介质天线罩情况,验证了PO-AI算法对复杂结构的适应性。从数值结果可见,天线罩对高阶OAM辐射波的幅值分布影响较大。数值计算也证实该算法可以快速预估介质罩存在时OAM波形的畸变程度,有助于实现对天线罩各参数的快速优化。

基于Matlab的水下OAM光通信仿真实验设计

选取拉盖尔-高斯光束作为实验研究对象,采用功率谱反演法产生海洋湍流随机相位屏,构建海洋湍流模型。通过Matlab仿真分析拉盖尔-高斯光束在海洋湍流下传输的可行性,分析了海洋湍流对轨道角动量螺旋谱分布的影响。结果表明:随着C_(n)^(2)和α的逐渐增大,轨道角动量螺旋谱分布的弥散程度也会增大。螺旋谱分布特性的相关结论在构建轨道角动量键控或轨道角动量复用系统时,可为轨道角动量的模态选取提供参考。

Improving millimeter-wave imaging quality using the vortex phase method

This paper investigates a new vortex wave imaging approach to improve the imaging quality of small metal targets of size less than 1.5 mm.Antennas with different spiral phase plates are designed to efficiently transmit vortex beams with orbital angular momentums(OAMs).By analyzing the OAM spectrum of the target,it was discovered that the predominant reflection contains a particular OAM mode that carries abundant azimuthal information.This can be explained by the OAM selectivity of the target and the guidance of the vortex transmitting beam.A simple reflection vortex imaging system was designed to capture the phase information.Measurement results show that the high image contrast reaches 14.9%,which is twice as high as that of the imaging without OAM.Both of simulations and experiments demonstrate that the vortex phase imaging approach proposed in this paper can effectively improve the imaging quality at 80 GHz.This approach is suitable for other millimeter wave imaging systems and is helpful to improve the resolution in anti-terrorism security checks.

海洋湍流下OAM光通信仿真实验设计

水下光通信UWOC具有传输速率高、容量大等特点,轨道角动量OAM具有空间螺旋相位分布特性,为水下光通信提供了空间自由度这一新的信息调制维度。该文建立了水下光通信系统下的海洋湍流随机相位屏模型,设计了海洋湍流下OAM光通信仿真实验,仿真验证了OAM作为信息载体的可行性,并为将其应用于通信系统实验教学与研究提供参考。