Research advances of orbital angular momentum based optical communication technology
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摘要:
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。
Abstract:Orbital angular momentum (OAM) carried by the vortex beam provides a new dimension resource in the spatial domain of light waves, which attracting more and more researching attentions. Since the vortex beams with different OAM mode values are orthogonal to each other, the OAM mode is introduced into the field of traditional optical communication, and two new application mechanisms are derived: OAM shift keying (OAM-SK) and OAM division multiplexing (OAM-DM), which provides a potential solution for future high-speed, high-capacity and high-spectrum efficiency optical communication technologies. Based on the basic concepts and theories of OAM beam types and their generation methods, this paper will give a brief overview of typical research cases related to the application mechanisms of these two communication systems. Three key technologies have been discussed, including OAM beam multiplexing technology, OAM beam demodulation technology, and turbulence suppression technology of OAM-based optical communication. Finally, the future developing trends and prospects of OAM-based optical communication technology are analyzed and forecasted.
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Overview: In recent years, expanding capacity of communication systems has become an urgent problem in the communication field, and the exploration of more communication resource dimensions has become an inevitable trend in building high-speed communication technologies. Momentum is a fundamental quantity in physics. Besides linear momentum, structural beam can also carry angular momentum, including spin angular momentum and orbital angular momentum (OAM). OAM is widely studied in classical mechanics and quantum mechanics. It should be noted that the OAM carried by the vortex beam provides a new dimension resource for the spatial domain of the light wave. Using the infinity of OAM mode values and the orthogonality between OAM mode values, OAM-based optical communication technology has changed the previous situation that optical communication is limited to dimensional resources. There are two mechanisms in current OAM-based optical communication. The first is to map the digital signal to different OAM beams and each OAM mode represents one data bit according to the diversity of the OAM modes, which is called OAM shift keying (OAM-SK). The second is to use the OAM beam as the carrier of the modulated signal and utilize the orthogonality between different OAM modes to achieve channel multiplexing so as to multiplying the channel capacity, which is called OAM division multiplexing (OAM-DM). These two communication mechanisms have brought traditional optical communication technology to a new level. In order to achieve high-quality communication performance, they are still urgent problems to make the OAM beams’ generator more integrated, and design more efficient OAM multiplexing and demodulation modules. Here, this paper introduces the basic theory of OAM, and summarizes the types of OAM beams and their generating schemes. At the same time, the typical research schemes of two application mechanisms of OAM-SK and OAM-DM in recent years are summarized, and the key technologies such as OAM multiplexing technology, demodulation technology and atmospheric turbulence suppression technology involved in them are also described in details.
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图 3 (a) 利用螺旋相位板(SPP) (a1),螺旋相位全息图(a2),叉形光栅(a3)产生OAM光束示意;(b)利用模式转换器将HG光束转化为LG光束,(b1) 2阶HG模的分解及LG模式的合成[19],(b2) π/2模式转换器及π模式转换器[19];(c) q板对圆偏振光的作用[89];(d) L形天线组成的超表面产生OAM光束[92];(e)微环谐振器产生OAM光束[106]
Figure 3. (a) OAM beams generated by spiral phase plate(SPP) (a1), spiral phase hologram (a2), forked hologram(a3); (b) HG beam to LG beam with mode converter, (b1) decomposition of 2nd order HG mode and the composition of a LG mode[19], (b2) π/2 and π mode converters[18]; (c) Illustration of q plate's function to input circularly polarized plane-wave light[89]; (d) OAM generated by metasurface composed of L-shape antennas[92]; (e) Illustration of the microring resonator to generate OAM beams[106]
图 4 (a) 基于OAM-SK的FSO通信方案,(a1)系统实验装置,(a2) OAM模式解调结果(部分)[20];(b)基于OAM模式叠加态的编译码通信方案[22];(c) 143 km传输距离的OAM-SK编译码通信方案[23];(d)高速通信中OAM模式数据编码和OAM跳模通信概念[24];(e)基于OAM空间阵列的通信方案[25];(f) OAM模式和振幅分别进行调制的OAM-ASK方案[26];(g)采用计算全息图实现高效的OAM-SK通信[27];(h)基于贝塞尔光束OAM-SK方案[28];(i)基于POV的OAM-SK方案[29]
Figure 4. (a) FSO communication scheme based on OAM-SK, (a1) system experimental device, (a2) demodulation results of OAM modes (partial)[20]; (b) Coding/decoding communication scheme based on superposition of OAM modes[22]; (c) OAM-SK communication scheme with a transmission distance of 143 km[23]; (d) Concept of data encoding and OAM channel hopping in high-speed communication [24]; (e) Communication scheme based on space array of OAM modes[25]; (f) OAM-ASK scheme based on OAM mode and amplitude modulation respectively[26]; (g) Efficient OAM-SK communication based on computational hologram[27]; (h) OAM-SK system scheme based on Bessel beam[28]; (i) OAM-SK system based on POV [29]
图 5 (a) 2束OAM光束复用传输的实验方案[30];(b)利用偏振与空间维度资源的OAM-DM方案,(b1)利用偏振维度的OAM-DM方案,(b2)利用物理空间维度的OAM光束叠加方案,(b3) OAM光束间数据交换方案[34];(c)利用偏振维度与波长维度的OAM-DM方案[35-36];(d)利用DOVG进行复用和解调的OAM-DM实验方案[40]
Figure 5. (a) Experimental scheme for multiplexed transmission of 2 OAM beams[30]; (b) OAM-DM scheme using polarization and spatial dimension resources, (b1) OAM-DM scheme using polarization dimension, (b2) OAM beam superposition scheme using physical space dimension, (b3) Data exchange between OAM beams[34]; (c) OAM-DM scheme using polarization dimension and wavelength dimension[35-36]; (d) Experimental device of multiplexing and demodulation using DOVG[40]
图 6 (a) 采用分束器产生复用OAM光束方案;(b)基于光子集成技术产生复用OAM光束的方案[113];(c)利用DOVG产生复用OAM光束方案[40];(d)利用改进后的Lin算法生成能量均等的复用OAM光束[27];(e) OAM组播原理示意[52];(f)贝塞尔模式组播的原理示意[52];(g) OAM光束非相干叠加与相干叠加的光强分布[117]
Figure 6. Scheme of generating multiplexed OAM beams using (a) BS; (b) Photon integration techniques[113]; (c) DOVG[40]; (d) Modified Lin algorithm[27]; (e) Schematic diagram of OAM multicasting[52]; (f) Schematic diagram of Bessel modes multicasting[11]; (g) Intensity distributions of incoherent superposition and coherent superposition of OAM beams[117]
图 7 (a) 基于SPP的OAM解调方案[74];(b) DOVG示意图[118];(c)采用DOVG实现OAM光束解调的系统[40];(d)基于二维DOVG和计算全息图的两种OAM解调方案的性能对比[117];(e)改进的马赫-曾德尔干涉仪示意图[121];(f)采用多孔衍射实现OAM光束解调原理图;(g)基于光学几何变换的OAM解调原理图[129];(h)对数几何变换的原理及基于螺旋变换的OAM模式分离方案[131];(i)产生LG光束并转化为HG模式进行检测的实验装置图[135],(i1)产生LG光束的相位全息图,(i2) π/2模式转换器
Figure 7. (a) OAM beam demodulation based on SPP; (b) Schematic of DOVG; (c) System for realizing OAM beam demodulation using DOVG; (d) Comparison results of OAM beam demodulations by DOVG and computer generated hologram [117]; (e) Schematic of the modified Mach-Zehnder(MZI) interferometer[121]; (f) OAM beam demodulation based on porous diffraction; (g) OAM demodulation based on optical geometric transformation [129]; (h) Principle of logarithmic geometric transformation and OAM mode separation scheme based on spiral transformation [131]; (i) Experimental device for generating an LG beam and converting it to HG mode for detection, (i1) phase hologram for generating LG beam, (i2) π/2 converter[135]
图 8 (a) 基于改进马赫-曾德尔干涉仪与光学几何变换联合的高密度OAM解调方案系统框图[133];(b1)未引入马赫-曾德尔干涉仪复合OAM光束光强分布及解调效果,(b2)、(b3)引入马赫-曾德尔干涉仪后在AB两个输出端口测得的光强分布及解调结果[133];(c)基于改进马赫-曾德尔干涉仪与复合相位光栅联合的OAM光束解调方案系统框图[134];(d)基于改进马赫-曾德尔干涉仪和复合相位光栅的联合方案对25种OAM模式的解调效果[34]
Figure 8. (a) System diagram of high-density OAM demodulation scheme based on improved Mach-Zehnder interferometer combined with optical geometry transformation [133]; (b1) Intensity distribution and demodulation effect of composite OAM beam without Mach-Zehnder interferometer [133], (b2)、(b3) Intensity distribution and demodulation results measured at two output ports of A、B after introduction of the Mach-Zehnder interferometer; (c) System diagram of OAM beam demodulation scheme based on improved Mach-Zehnder interferometer combined with composite phase grating[134]; (d) Demodulation effect of 25 OAM modes based on improved joint scheme of Mach-Zehnder interferometer and composite phase grating[134]
图 9 (a) CNN经典架构——AlexNet架构。黑色方框表示用于提取图像分布特征的卷积核,输入图像在经过若干层的卷积层(包含激活及池化操作)之后,全连接层的softmax分类器分别计算输入属于每个类的概率,进而实现OAM模式分类[139];(b)基于CNN,KNN,NBC和ANN的自适应解调器的解调性能对比[140];(c)基于CNN与Turbo编译码联合方案的16-ary OAM-SK-FSO通信系统[142];(d)采用多视野池化层改进的CNN结构[143];(e)基于CNN的相干解调OAM-SK系统[26]
Figure 9. (a) Classical architecture of CNN——AlexNet. The black squares represent the convolution kernel used to extract image distribution features, after the input image passes through several layers of convolution layer (including activation and pooling operation), softmax classifier of the fully connection layer calculates the probability of the input belonging to each class respectively, and then realizes OAM mode classification [139]; (b) Demodulation performance comparison of adaptive demodulator based on CNN, KNN, NBC and ANN [140]; (c) Turbo-coded 16-ary OAM shift keying FSO communication system combining the CNN-based adaptive demodulator[142]; (d) Improved CNN structure with a view-pooling layer and schematic diagram of the view-pooling layer[143]; (e) Diagram of the coherently demodulated OAM-SK system based on CNN[26]
图 10 (a) 基于CNN的OAM传输系统方案[145],(a1) OAM传输系统框图,(a2) 6层CNN架构;(b)基于CNN的(b1)多种OAM-SK架构与(b2)性能对比[146]
Figure 10. (a) CNN-based OAM transmission system[145], (a1) OAM transmission system block diagram, (a2) 6-layer CNN architecture; (b) CNN-based (b1) multiple OAM-SK architectures and (b2) performance comparison[146]
图 11 (a) LGp=0, l=0与LGp=1, l=1复用的FSO通信系统实验装置[147];(b)非零径向模式LG光束自由空间接收示意及接收性能[148];(c)基于径向模式的OAM编码系统方案[149],(c1)系统框图,(c2)径向模式解调结果(部分),(c3)字符映射关系与接收能量分布
Figure 11. (a) Experimental setup of a FSO communication system multiplexing LGp=0, l=0 and LGp=1, l=1[147]; (b) Concept and the reception performance of free-space optical link transmitting LG beams with nonzero radial index [148]; (c) OAM coding system scheme based on radial modes, (c1) system block diagram, (c2) radial mode demodulation results (partial), (c3) character mapping relationship and received energy distribution[149]
图 12 基于AO的湍流效应抑制方案。(a)利用高斯探针光束进行波前传感的AO方案[163];(b)双向OAM-FSO链路湍流抑制方案[164];(c)单波长高斯探针光束进行波前传感的AO方案[165];(d)基于GS算法的无WFS的AO方案[168];(e)基于高斯探针光束与GS算法联合的AO方案[169];(f)湍流仿真过程及GS算法校正性能(部分)[170];(g)基于SPGD算法的无WFS方案[172];(h)基于人工神经网络的AO方案[173]
Figure 12. Turbulence suppression scheme based on AO. (a) AO scheme for wavefront sensing using Gaussian probe beam[163]; (b) Turbulence suppression scheme of a bidirectional OAM-FSO link [164]; (c) AO scheme for wavefront sensing with single-wavelength Gaussian probe beam[165]; (d) AO scheme without WFS based on GS algorithm[168]; (e) AO scheme based on Gaussian probe beam combined witrh GS algorithm[169]; (f) Turbulence simulation process and GS algorithm correction performance (partial)[170]; (g) A WFS-free scheme based on SPGD algorithm [172]; (h) AO scheme based on artificial neural network[173]
图 13 基于信号处理的湍流效应抑制技术。(a)基于MIMO均衡算法的OAM通信系统实验及信号均衡结果[174];(b)基于MIMO均衡算法与空间分集技术联合方案的OAM通信系统及BER性能表现[175];(c) LDPC编译码结构[19]及OAM通信系统BER性能表现[177]
Figure 13. Turbulence suppression technology based on signal processing. (a) Experiment of OAM communication systems and results of signal equalizations based on MIMO equalization[174]; (b) BER performance of OAM communication system based on MIMO equalization algorithm and spatial diversity technology[175]; (c) LDPC structure [19] and BER performance of OAM communication system[177]
表 1 不同OAM光束产生方案的性能比较
Table 1. Performance comparison of different schemes of generating OAM beams
SPP SLM DMD Mode conversion Q-plate Metasurface Optical fiber integration Photonic integration Cost Low High Normal Low High Low Normal Normal Speed Normal Fast Fast Normal Fast Normal Fast Fast Conversion effciency Normal High Low Relatively high Relatively high Relatively high High High OAM mode Single Single/multiplex Single/multiplex Single Single Single Single/multiplex Single/multiplex Processing difficulty High High High Low Low High High High System complexity Low Low Low High Low Low Low Low 表 2 不同OAM光束解调方案的性能比较
Table 2. Performance comparison of different schemes of demodulation OAM beams
SPP Diffractive optical element Phase hologram Interference/diffraction Geometric transformation Mode conversion Machine learning Cost Low Normal Low Low High Low Low Speed Normal Fast Fast Normal Normal Normal Fast Demodulation precision Normal Normal Normal Low High Relatively high High OAM mode Single Single /multiplex Single /multiplex Single Single /multiplex Single Single /multiplex Processing difficulty High High High Low High Low Low System complexity Low Low Low Low High High Low -
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