Spatial Mode Division Multiplexing for High Speed Optical Coherent Detection Systems

Spatial mode-division multiplexing is emerging as a potential solution to further increasing optical fiber capacity and spectral efficiency. We report a dual-mode, dual-polarization transmission method based on mode-selective excitation and detection over a two-mode fiber. In particular, we present 107 Gbit/s coherent optical OFDM (CO-OFDM) transmission over a 4.5 km two-mode fiber using LP01 and LP11 modes in which mode separation is performed optically.     

High-Speed Multimode Fiber Transmission by Using Mode-Field Matched Center-Launching Technique

We report that the center-launching technique can be improved to selectively excite the fundamental mode of multimode fiber (MMF). This ldquomode-field matchedrdquo center-launching technique enables us to excite only the fundamental mode in the MMF and, consequently, avoid the inherent limitations imposed by the differential mode delay. We realize this mode-field matched center-launching technique simply by fusion-splicing a single-mode fiber (SMF) pigtailed transmitter to the MMF. The splicing condition is optimized to expand the core of SMF slightly so that it can match the mode field distribution of the fundamental mode of MMF. The results show that, by using this launching technique, we can achieve the transmission characteristics similar to SMF and drastically increase the bandwidth-distance product of MMF. For demonstrations, we have successfully transmitted 10- and 40-Gb/s signals over 12.2 and 3.7 km of MMF, respectively, without using any dispersion compensation techniques. We have also evaluated the robustness of the MMF link implemented by using the proposed launching technique against the mechanical perturbations such as the lateral offset between fiber connectors, fiber bending, and fiber shaking.     

基于多模光纤偏振烧孔效应的双波长掺铒光纤激光器

基于多模光纤(MMF)引入的偏振烧孔(PHB)效应,研制了一种环形双波长掺铒光纤(EDF)激光器.单模光纤(SMF)-MMF-SMF组成的结构使MMF不同偏振方向的反射模在波长上分开,利用PHB效应实现双波长的输出,输出波长间隔可通过改变MMF的长度改变.实验对比了1.6 m和3.0 m长的MMF输出波谱特性,结果表明...     

Principal Modes in Graded-Index Multimode Fiber in Presence of Spatial- and Polarization-Mode Coupling

Power-coupling models are inherently unable to describe certain mode coupling effects in multimode fiber (MMF) when using coherent sources at high bit rates, such as polarization dependence of the impulse response. We develop a field-coupling model for propagation in graded-index MMF, analogous to the principal-states model for polarization-mode dispersion in single-mode fiber. Our model allows computation of the fiber impulse response, given a launched electric-field profile and polarization. In order to model both spatial- and polarization-mode coupling, we divide a MMF into numerous short sections, each having random curvature and random angular orientation. The model can be described using only a few parameters, including fiber length, number of sections, and curvature variance. For each random realization of a MMF, we compute a propagation matrix, the principal modes (PMs), and corresponding group delays (GDs). When the curvature variance and fiber length are small (low-coupling regime), the GDs are close to their uncoupled values, and scale linearly with fiber length, while the PMs remain highly polarized. In this regime, our model reproduces the polarization dependence of the impulse response that is observed in silica MMF. When the curvature variance and fiber length are sufficiently large (high-coupling regime), the GD spread is reduced, and the GDs scale with the square root of the fiber length, while the PMs become depolarized. In this regime, our model is consistent with the reduced GD spread observed in plastic MMF.     

新一代多模光纤

按照以太网协议建设的光纤局域网的传输速率正迅速升级到1Gbit/s和10Gbit/s这时光纤提出了更高的要求。IEEE正在制定10Gbit/s以太网标准,10Gbit/s以太网标准将考虑采用新一代的多模光纤。文章产 叙述新一代多模光纤的发展前景,然后综述了新一代多模光纤概念、测量新一代多模光纤带宽的新方法和制造新一代多模光纤的技术。     

Coherent Transmission Direct Detection MIMO Over Short-Range Optical Interconnects and Passive Optical Networks

Building on a recently introduced quadratic multiple-input multiple-output (Q-MIMO) channel model making use of quadratic forms instead of linear channel matrices, we explore diverse signaling formats over the Q-MIMO channel in the frequency flat regime, highlighting the unique characteristics of optical multimode fiber (MMF) systems versus their wireless counterparts. In particular, we treat multiple-input single-output multimode fiber systems, comparing them with single-input single-output MMF systems and obtaining insight into the possibility of efficiently coupling the output of an MMF into a single-mode fiber. We further treat vector amplitude modulation over MMF MIMO and study the receiver zero-forcing (ZF) technique for MMF MIMO transmission, deriving its performance and comparing with a recently introduced ZF beamforming technique based on precoding at the transmitter, which is shown to be superior to receiver ZF.      

Exploiting diversity in multimode fiber communications links via multisegment detectors and equalization

we propose diversity combination via optical multisegment detectors and electrical equalization techniques to mitigate the effects of intermodal dispersion in multimode fiber (MMF). With no equalization in MMF links, intermodal dispersion produces intersymbol interference (ISI) in the received optical signal that severely limits the achievable data transmission rates and fiber link lengths. Our quasi-simulated performance results (obtained with measured impulse responses) demonstrate that multisegment detection and equalization provide a low-cost and efficient solution to combat ISI and hence, to enhance the performance of MMF communications links.     

All-fiber add-drop wavelength-division multiplexer based onintermodal coupling

We demonstrate a novel all-fiber add-drop multiplexer by using a tilted Bragg grating written on a two-mode fiber and a mode-selective coupler. The device can be made potentially with low loss and low cost, useful for add-drop of a small number of channels in wavelength-division-multiplexing (WDM) communication systems     

All-fiber wavelength-tunable acoustooptic switches based on intermodal coupling in fibers

In this paper, we demonstrate a novel all-fiber wavelength-tunable acoustooptic switch utilizing intermodal coupling in a two-mode fiber (TMF). Its all-fiber configuration consisting of a fiber acoustooptic tunable filter and a mode-selective coupler results in the low loss (<2 dB) operation. The operating bandwidth >50 nm, the switching time of 40 /spl mu/sec, and the crosstalk of 20 dB were achieved. By controlling the design parameters of the two-mode fiber, the 3-dB bandwidth of the switched signal was varied from 2.5 nm to >35 nm. A novel all-fiber dynamic optical add-drop multiplexer is also demonstrated using two acoustooptic switches in series.     

Design of wavelength-switching erbium-doped fiber lasers with a multimode fiber Bragg grating using spatial-mode excitation and selection techniques

A multimode fiber Bragg grating (MMFBG)-based wavelength-switching erbium-doped fiber laser and its application in displacement sensing is proposed and demonstrated. According to the spatial-mode distribution in the multimode fiber (MMF), the modes reflected by the MMFBG at different wavelengths build a wavelength-dependent mode field on the transverse plane of the MMF. By varying spatial-launching position of the single-mode fiber against the MMFBG to achieve different mode group excitation and selection, single-wavelength lasing at 31 discrete wavelengths has been realized over a wide wavelength range.     

单模-多模光纤产生系列Bottle beam

提出一种利用多模光纤的多模干涉效应在自由空间中获得多个局域空心光（Bottle beam）的新方法。单模-多模光纤结构是一段多模光纤无偏心地连接到一段单模光纤上,光由单模光纤传输到多模光纤激发产生一系列的LP0,n 模,由于多模干涉效应在多模光纤中相互叠加,当入射到自由空间后形成了多个Bottle beam。文中对光束传输过程进行理论分析并利用Matlab 进行仿真实验, 结果表明在自由空间中可以获得系列Bottle beam。当多模光纤纤芯直径分别为45 m,60 m 和90 m 时所选择的光场段内的Bottle beam 的尺寸大小基本相同（约400 m20 m）,而第一个空间暗域沿轴向两侧相对光强差值分别为0.62,0.41 和0.11,可见当多模光纤的纤芯直径越大时所得到的Bottle beam 暗域的轴向两侧光强越相近,因此也越有利于囚禁微粒。     

多模光纤模式群分集复用系统

多模光纤(MMF)已成为"最后一千米"短距离通信的理想介质,但多模光纤的模间色散限制了其传输能力.模式群分集复用(MGDM)是种光路的多输入多输出(MIMO)技术,该技术利用多模光纤中不同的模式群作为相互独立的信道来传输多路信号,极大地提升了多模光纤的传输能力.设计实现了一个基于75 m梯度型多模光纤(GIMMF)直接检测的两路MGDM通信系统,不同模式群的激发利用选择模式激发实现,利用偏置连接器和多模耦合器实现模式群的复用及解复用,并利用该系统实现了模拟视频信号和非归零码(NRZ)数字信号的传输.     

单根多模光纤数字扫描成像方法

针对现有多模光纤数字扫描成像方法,形成扫描聚焦光斑计算量大、速度慢的问题,提出了一种基于自适应并行坐标（Adaptive Parallel Coordinate,APC）算法的单根多模光纤数字扫描成像方法,通过控制多模光纤输入端的光场,在单根多模光纤出射端实现光斑的聚焦和扫描。建立了单根多模光纤数字扫描成像数学模型;采用自适应并行坐标算法对空间光调制器（Spatial Light Modulator,SLM）加载相位进行优化,有效缩短了扫描聚焦光斑的形成时间,提高了成像速度。实际生成3030个聚焦光斑,对50 m50 m范围的分辨率板进行扫描成像。实验证明,该方法实现了单根多模光纤对距离光纤端面60 m处平面内的目标成像,分辨率达到2.46 m。     

New two-mode bistability in an asymmetric absorptive multisectionlaser diode: theoretical analysis

A novel two-mode bistable laser diode (TMBLD) with an asymmetric cavity configuration is proposed and analyzed. It consists of a gain region, a saturable absorber region, and a set of mode-selective mirrors (or filters). These elements are arranged such that the saturable absorber acts as an intracavity loss for one mode and a bleachable absorber external to the cavity of the other mode. Both modes share the same gain medium and compete for the same gain. Bleaching of the absorber leads to a switching of the modes and possible bistability. This two-mode bistability results from the complementary processes of mode competition in the gain section and the mode-dependent effective cross section of the saturable absorber. A rate equation analysis of the device indicates that a switching time less than 200 ps and a modulation rate over 3 Gb/s should be obtainable     

基于倾斜光纤光栅的温度不敏感振动传感器

提出了一种基于倾斜光纤光栅与多模光纤相结合的温度不敏感振动传感器,其振动传感头是在倾斜光纤光栅与单模光纤之间加入一小段多模光纤所组成。倾斜光纤光栅的反射光谱有布拉格模和包层模两部分组成,其中多模光纤的作用是将倾斜光纤光栅反射包层模耦合到单模光纤的基模。倾斜光纤光栅包层模对外界振动很敏感,通过传感器的包层模平均输出功率完成对外界振动物理量测量。由于采用强度解调的方式,可以大大降低传感器装置的复杂性。实验表明：当传感器温度从20 ℃上升到70 ℃时,传感器的输出平均光功率均方根误差为0.01 μW,其反射光谱平均输出功率影响很小,故可以避免外界温度对测量结果的影响。     

MEMS可调谐平顶窄带光学滤波器

设计了一种基于MEMS技术的可调谐光学滤波器,它通过光栅将输入的宽带光信号色散展开,以一个MEMS扭镜选择将对应滤波器通带的光信号反射至输出端,从而实现光学滤波和波长调谐功能。滤波器的输入端采用单模光纤,输出端采用多模或者少模光纤,可以实现窄带且平顶的通带特性。经过参数优化,仿真分析得结果显示,采用多模/少摸光纤输出的两种滤波器,其0.5 dB和25 dB带宽分别为0.95 nm/0.29 nm和1.39 nm/0.69 nm,分别满足100 GHz和50 GHz信道间隔的DWDM系统要求。由于输出端采用多模或者少摸光纤,从该滤波器输出的光信号不能继续在单模光纤中传输,只能由光探测器接收,因此该滤波器一般应用于全光网络节点中的下载端口。     

Hybrid AM-VSB/256-QAM/Internet transport systems in the campus

We propose and demonstrate simultaneous transport of 1556 nm downstream CATV analog video, 256-quadrature amplitude modulation (QAM) digital TV and Internet access signals, as well as 1310 nm upstream Internet access signals over the existing multimode fiber (MMF) in the campus. This technique makes possible analog and digital video transmission, gigabit Ethernet as well as Internet access to the classrooms through the existing MMF. Our proposed system has the distinctive advantages of simplicity and lower cost, and is thus eminently suitable for fiber to the classrooms application.     

Self-Alignment of Optical Devices With Fiber for Low-Cost Optical Interconnect Modules

We propose a novel self-alignment process of optical devices with optical fiber. A vertical-cavity surface-emitting laser (VCSEL) was automatically coupled with a multimode fiber (MMF) through the surface tension of a liquid adhesive within 1.5 s. Misalignment between the center of the VCSEL and the fiber was measured to be 15 mum, which is acceptable for coupling the VCSEL with the MMF. High-speed pulse modulation of the self-aligned VCSEL up to 5 Gb/s, as well as at 1 Gb/s, was demonstrated. The average optical output power was as high as -5.9 dBm at 1 Gb/s.     

On the performance of the electrical equalization technique in MMF links for 10-gigabit ethernet

In this paper, the electrical equalization technique utilized in multimode fiber (MMF) links for 10-Gigabit Ethernet is reported. This paper presents and compares three kinds of equalizers: linear equalizer, decision feedback equalizer (DFE), and a nonlinear finite-impulse-response DFE (NL-FIR-DFE) based on the analysis of nonlinear operations of direct modulation and detection in MMF links. Computer simulations reveal that NL-FIR-DFE exhibits superior performance in comparison with normal DFE. The equalization performance for an MMF channel with bandwidth 500 MHz-km as well as 160 MHz-km is demonstrated. It is demonstrated that with direct modulation of cost-effective vertical-cavity surface-emitting laser (VCSEL) or Fabry-Perot laser, the transmission distance for installed MMF with a bandwidth of 500 MHz-km at 10 Gb/s can be extended to 300 m with an appropriate offset restricted mode launch condition by utilizing normal DFE and to more than 650 m for NL-FIR-DFE. Moreover, it is shown that the transmission distance for an MMF with a bandwidth of 160 MHz-km can reach 300 m with more than seven times bandwidth improvement by using NL-FIR-DFE.