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。     

Free-space communication through turbulence: a comparison of plane-wave and orbital-angular-momentum encodings

Abstract

Free-space communication allows one to use spatial mode encoding, which is susceptible to the effects of diffraction and turbulence. Here, we discuss the optimum communication modes of a system while taking such effects into account. We construct a free-space communication system that encodes information onto the plane-wave (PW) modes of light. We study the performance of this system in the presence of atmospheric turbulence, and compare it with previous results for a system employing orbital-angular-momentum (OAM) encoding. We are able to show that the PW basis is the preferred basis set for communication through atmospheric turbulence for a system with a large Fresnel number product. This study has important implications for high-dimensional quantum key distribution systems.     

Efficient generation and control of different-order orbital angular momentum states for communication links

We present an optical scheme to encode and decode 2 bits of information into different orbital angular momentum (OAM) states of a paraxial optical beam. Our device generates the four light angular momentum states of order ±2 and ±4 by spin-to-orbital angular momentum conversion in a triangular optical loop arrangement. The switching among the four OAM states is obtained by changing the polarization state of the circulating beam by two quarter-wave plates, and the 2 bit information is transferred to the beam OAM exploiting a single q plate. The polarization of the exit beam is left free for an additional 1 bit of information. The switching among the different OAM states can be as fast as a few nanoseconds, if suitable electro-optical cells are used. This may be particularly useful in communication systems based on light OAM.     

Wide‐Range Tunable Fluorescence Lifetime and Ultrabright Luminescence of Eu‐Grafted Plasmonic Core–Shell Nanoparticles for Multiplexing

Wide‐range, well‐separated, and tunable lifetime nanocomposites with ultrabright fluorescence are highly desirable for applications in optical multiplexing such as multiplexed biological detection, data storage, and security printing. Here, a synthesis of tunable fluorescence lifetime nanocomposites is reported featuring europium chelate grafted onto the surface of plasmonic core–shell nanoparticles, and systematically investigated their optical performance. In a single red color emission channel, more than 12 distinct fluorescence lifetime populations with high fluorescence efficiency (up to 73%) are reported. The fluorescence lifetime of Eu‐grafted core–shell nanoparticles exhibits a wider tunable range, possesses larger lifetime interval and is more sensitive to separation distance than that of ordinary Eu‐doping core–shell type. These superior performances are attributed to the unique nanostructure of Eu‐grafed type. In addition, these as‐prepared nanocomposites are used for security printing to demonstrate optical multiplexing applications. The optical multiplexing experiments show an interesting pseudo‐information “a rabbit in a well” and conceal the real message “NKU.”     

Wavelength-multiplexing system for single-mode image transmission

The expanding use of optical communication by means of optical fibers and the situation of drastically increasing amounts of data to be transmitted urge the exploration of novel systems permitting the transmission of large amounts of spatial information by fiber with smaller spatial resolution. An optical encoding and decoding system is suggested for transmitting one- or two-dimensional images by means of a single-mode fiber. The superresolving system is based on wavelength multiplexing of the input spatial information, which is achieved with diffractive optical elements. Preliminary experimental results demonstrate the capabilities of the suggested method for the one- and two-dimensional cases.     

一种新的水声MIMO空时频编码技术

基于贝尔实验室垂直分层空时编码（Vertical Bell Labs Layered Space-Time,VBLAST）的水声多输入多输出（Multiple-Input Multiple-Output,MIMO）通信技术,通过充分开发空间资源,可在不增加系统带宽的条件下成倍提高数据传输速率,是一种极具潜力的高速水下通信技术。但由于其接收信号中同时含有同频干扰及多径干扰,当信道多径扩展严重时,其性能下降严重。为此,提出一种新的空时频编码技术,该技术通过将各层信号调制到彼此正交的子载波上,不但可在接收端完全消除同频干扰,同时还能获得频率分集增益,从而使水声MIMO通信系统的性能获得极大改善。仿真及湖试结果证明了其有效性。     

基于正交频分复用的水下转发器的设计

为解决长距离高速水声数据传输问题,采用接力式水声数据转发传输是一个很好的解决方案。结合正交频分复用技术的特点设计实现了一个数字式的水下转发器系统。系统功耗低、结构简单,可以很好地解决水声通信中存在的多途干扰和窄带宽问题,实现高速、可靠的水下数据转发的目的。湖试中,在6000m的距离内,3kHz带宽下,转发速率为4.32kb/s,误码率低于10-3。湖试结果表明,基于正交频分复用的水下转发器可以达到较高的转发速率和较低的误码率。     

基于VBLAST的水声MIMO高速通信技术研究

为实现高速水下通信,提出了一种基于VBLAST（贝尔实验室垂直分层空时）编码的水声MIMO（多输入多输出）通信技术。该技术通过在发射端和接收端同时采用多个阵元进行多发多收,借助空间复用,可有效解决水下通信系统带宽资源缺乏的难题,使系统在不增加带宽的条件下成倍提高数据传输速率。在水声MIMO通信系统的接收端,提出采用带有二阶数字锁相环、基于自适应空时判决反馈均衡的相干解调技术。该技术可有效克服水声MIMO信道中的多径干扰及同频干扰,且能自适应跟踪信道变化。仿真及湖试结果证明了所提水声MIMO高速通信技术的良好性能。     

An Acoustic Meta-Skin Insulator

Acoustic metamaterials with artificial microstructures are attractive to realize intriguing functions, including efficient waveguiding, which requires large impedance mismatches to realize total side reflection with negligible transmission and absorption. While large impedance mismatch can be readily realized in an air environment, acoustic waveguiding in an underwater environment remains elusive due to insufficient impedance mismatch of state-of-the-art metamaterials. Here, a superhydrophobic acoustic metasurface of microstructured poly(vinylidene fluoride) membrane, referred to as a “meta-skin” insulator, which is able to confine acoustic waves in an all-angle and wide spectrum range due to tremendous impedance mismatch at stable air/water interfaces, viz., the Cassie–Baxter state is demonstrated. By utilizing the meta-skin insulator with broadband and high throughput, orbital-angular-momentum multiplexing at a high spectral efficiency and binary coding along large-angle bending channels for bit-error-free acoustic data transmission in an underwater environment are demonstrated. Very different from optical and/or electrical cable communications, acoustic waves can be simply and effectively coupled into remote meta-skin acoustic fibers from free space, which is technologically significant for long-haul and anti-interference communication. This work can enlighten many fluidic applications based on efficient waveguiding, such as in vivo ultrasound medical treatment and imaging.     

基于MSS-PDS和单阵元PTRM信道均衡的深海远程水声通信方案

为了提高水声通信的性能,提出了一种联合运用M元扩频（MSS）通信和Pattern时延差编码（PDS）水声通信体制,并采用单阵元被动式时间反转镜（PTRM）来实现声信道均衡的深海远程水声通信方案。该方案的M元扩频-Pattern时延差编码通信技术既能胜任远程水声通信,又能提高通信速率;单阵元被动式时间反转镜信道均衡技术既可抑制多途扩展产生的码间干扰,又能提高信噪比。根据深海声道特性分析,该方案将通信固定节点置于声道轴以获取会聚增益,提高通信距离。计算机仿真结果证明,所提出的深海远程水声通信方案具有很好的鲁棒性和可行性。     

基于深度学习的OFDM半相干水声通信方法

针对正交频分复用(Orthogonal Frequency Division Multiplexing, OFDM)水声通信中常用的相干和非相干通信分别面临的对多普勒敏感和频谱效率低的问题,提出一种高阶幅度键控调制的半相干通信技术,将OFDM符号时频帧结构中全部频点采用高阶幅度键控调制方式,并利用信号幅度信息完成半相干信道估计。通过两种基于深度学习的算法优化半相干信道估计这一非线性过程,较非相干通信有效提高了频谱效率,较一定信噪比下的相干通信提高了鲁棒性,降低了误比特率和系统复杂度,并利用元学习算法降低深度学习算法对训练数据的依赖。最后,提取海试信道数据,完成OFDM半相干水声通信系统仿真,验证了所提方法在频谱效率和系统误比特率性能方面较非相干和相干通信的优势,当信道长度改变时,基于元学习的算法依然可以获得较好的性能。     

A kind of multilevel authentication system for multiple-image by modulated real part synthesis and iterative phase multiplexing

A kind of multilevel authentication system for multiple-image based on modulated real part synthesis and iterative phase multiplexing in the Fresnel domain is proposed. In the design process of the low-level authentication system, a series of normalized real part information are iteratively generated by phase retrieval algorithm in the Fresnel domain, and the final private keys for different individual low-level certification images can be fabricated by binary amplitude modulation, superposition, synthesis, and sampling; while in the design process of the high-level authentication system, the final private keys for different individual high-level certification images can be generated by iterative phase information encoding and multiplexing. During the high-level authentication, the meaningful certification image can be reconstructed by the inverse Fresnel transform with the corresponding correct private keys, meanwhile, the correlation coefficient is utilized as judgment criterion; while in the low-level authentication, with the help of correct keys, the noise-like image with meaningless information can be recovered, but a remarkable peak output in the nonlinear correlation coefficient can be generated, which is adopted as the criterion to judge whether the low-level authentication is successful or not. Theoretical analysis and numerical simulations both verify the feasibility of the proposed method.     

Cluster analysis of acoustic emission signals in pitting corrosion of low carbon steel

The pitting corrosion characteristics of low carbon steel specimens are studied by acoustic emission (AE) and electrochemical techniques, in a 3.0 wt.% NaCl solution acidified to pH 2.0. The acoustic emission signals generated by pitting corrosion are classified based on multiple acoustic emission parameters using K‐means clustering algorithm, then each classified signals are analyzed by acoustic emission parameters correlation plot and distribution with time. Furthermore, each acoustic source characteristics is extracted using Gabor wavelet transform (WT) in the time and frequency domain. An error back propagation (BP) artificial neural network (ANN) is trained according to the classified signals, so as to successfully identify the acoustic emission signals from parallel experiments. Experimental results show that the hydrogen bubble activation, oxidized film rupture and pit growth are typical acoustic emission sources in pitting corrosion process, which can be effectively classified by cluster analysis and recognized by back propagation neural network. The data gathered from laboratory tests combined with the real data from acoustic emission on‐line storage tank floor inspection can help to evaluate the bottom corrosion severity and interpreter the corrosion source, further to make the on‐site testing more reliable and reduce the risk.     

Multiplexed Nucleic Acid Hybridization Assays Using Single‐FRET‐Pair Distance‐Tuning

Multiplexed photoluminescence (PL) detection plays an important role in chemical and biological sensing. Here, it is shown that time‐gated (TG) detection of a single terbium‐donor‐based Förster resonance energy transfer (FRET) pair can be used to selectively quantify low nanomolar concentrations of multiple DNAs or microRNAs in a single sample. This study demonstrates the applicability of single‐TG‐FRET‐pair multiplexing for molecular (Tb‐to‐dye) and nanoparticle (Tb‐to‐quantum‐dot) biosensing. Both systems use acceptor‐sensitization and donor‐quenching for quantifying biomolecular recognition and modification of the donor–acceptor distance for tuning the PL decays. TG intensity detection provides extremely low background noise and a quick and simple one‐step assay format. Single‐TG‐FRET‐pair multiplexing can be combined with spectral and spatial resolution, paving the way for biosensing with unprecedented high‐order multiplexing capabilities.     

OFDM水声通信下行非正交多址接入功率分配方法研究

针对基于功率域非正交多址接入（Power Domain Non-orthogonal Multiple Accesses,PD-NOMA）的正交频分复用（Orthogonal Frequency Division Multiplexing,OFDM）水声下行通信系统的功率分配问题,提出了一种基于中断概率的功率分配方法。用户节点在系统初始化阶段根据源节点广播的组网数据包获取水声信道的统计特征,源节点根据水下用户反馈的信道特征参数建立水下用户的中断概率模型,以最小化两用户的中断概率和为目标建立目标函数,在中断概率区域边界上遍历搜索最优的功率分配系数。仿真结果表明,该方法在保证公平性的条件下,有效降低了用户节点的中断概率,提高了系统的频谱利用率和误码性能。     

Electronic Method with Direct Time Encoding for Precision Measurement of Electric Power over a Wide Range of Frequency

The principle of power measurement by thermal means has the advantage of a very large bandwidth and gives the possibility to realize time or frequency encoding in a simple way. The new method described is based on a known operation using special thermal converters with several heaters. The multiple heating, however, is done by a multiplexing operation on single-heater converters. It is shown that, by means of this principle, a power meter can be set up enabling precision measurement in a wide range of frequency. The measurement of real power in polyphase systems is another application of the same principle.     

矢量水听器在水声通信系统中的应用

将矢量水听器应用于水声通信系统,利用矢量水听器的指向性屏蔽强干扰。建立了基于矢量水听器的多载波正交频分复用(OFDM)系统模型,对各向同性噪声场中矢量水听器的增益进行了推导,为抑制相干干扰提供了新的解决方案,并进行了仿真验证。仿真研究表明,该方案能够有效地增强系统的抗各向同性干扰的能力,达到提高信噪比的目的,降低了误码率,增加了系统的稳健性。湖上试验验证了该方法的有效性,表明基于矢量水听器的水声通信系统可为实现高速水声通信提供一种有效的途径。     

Nonparaxial propagation properties of an anomalous hollow beam with orbital angular momentum

The analytical nonparaxial propagation formula of an anomalous hollow beam (AHB) with orbital angular momentum (OAM) in free space is derived based on the generalized Raleigh–Sommerfeld diffraction integral. The nonparaxial properties of AHB with OAM such as intensity, phase and OAM density distributions are studied in detail, using the pertinent nonparaxial propagation formula. The comparison between the paraxial and nonparaxial results is also carried out. The results show that the nonparaxial properties of an AHB with OAM are determined by the initial beam parameters, such as beam waist size and topological charge and propagation distance.     

Implementation of single-beam multiplexing encoding with a dually modulated spatial light modulator

We propose a novel method for signal storage and encryption, called single-beam multiplexing encoding. The single beam is composed of an inside signal beam and an outside reference beam. The signal beam is amplitude modulated, and the reference beam is phase modulated. The dual modulation is implemented by a spatial light modulator (SLM). Multiplexing holography with different reference beams from different directions, called directional multiplexing, is analyzed in detail. With an SLM based on a twisted nematic liquid crystal display, we demonstrate a single-beam directional multiplexing method using a holographic encoding technique, and the retrieved signals are presented. This encoding system is more stable, miniaturized, and flexible. It should be of great interest for applications in signal encryption as well as for high-capacity data storage.     

The Absence and Importance of Operando Techniques for Metal‐Free Catalysts

Operando characterization techniques have played a crucial role in modern technological developments. In contrast to the experimental uncertainties introduced by ex situ techniques, the simultaneous measurement of desired sample characteristics and near‐realistic electrochemical testing provides a representative picture of the underlying physics. From Li‐ion batteries to metal‐based electrocatalysts, the insights offered by real‐time characterization data have enabled more efficient research programs. As an emerging class of catalyst, much of the mechanistic understanding of metal‐free electrocatalysts continues to be elusive in comparison to their metal‐based counterparts. However, there is a clear absence of operando characterization performed on metal‐free catalysts. Through the proper execution of operando techniques, it can be expected that metal‐free catalysts can achieve exceptional technological progress. Here, the motivation of using operando characterization techniques for metal‐free carbon‐based catalyst system is considered, followed by a discussion of the possibilities, difficulties and benefits of their applications.