RoF系统中三种传输方案的研究与分析

对RoF系统中基带传输、中频传输、射频传输三种传输方案进行详细的描述,分析色散作用对其传输的影响,并利用Opt i syst em仿真软件搭建了仿真模型,对三种结构进行仿真。仿真结果表明:基带传输系统受色散影响最小;中频传输系统次之;射频传输系统受色散影响最大。为减弱色散作用的影响,在射频传输方案中采用了单边带调制技术,频谱图和眼图验证了单边带调制具有较强的抗色散性能。     

强色散管理抑制160Gb/s光时分复用传输系统中的带内四波混频效应

在160Gb/s光时分复用(OTDM)系统中,带内四波混频(IFWM)效应是影响系统性能的因素之一。分析了IFWM效应对160Gb/s光时分复用传输系统性能的影响,对采用强色散管理的方法抑制IFWM效应进行了理论分析和实验验证。传输链路采用大色散斜率的标准单模光纤(SSMF)和色散、色散斜率补偿光纤混合的色散管理方式,能够很好地抑制"1"比特处的脉冲幅度变化和"0"比特处产生的寄生脉冲,提升系统的整体性能。在不需要前向纠错和功率代价小于3.6dB的条件下,实现了较长时间(时间大于2h)的无误码(误码率小于10-12)传输。     

色散对ROF系统性能的影响

阐述了色散的基本原理,通过对ROF(radio over fiber)系统的研究指出,随着传输速率的升高,光纤色散会限制系统的传输性能,严重的还会造成信号的失真.在幅度调制方式(DSB调制与SSB调制)、波长数调制(单波长调制与双波长调制)及传输中对信号所造成的啁啾现象的3个不同方面,综合分析了色散对ROF系统影响,得出了SSB调制要优于DSB调制的结论,为改善ROF系统的性能提供了一定的理论基础.     

基于DCF长距离光纤通信系统的色散补偿

通过介绍光纤色散的产生及其对传输系统的影响,引出了色散补偿技术.在多种色散补偿方法中,侧重探讨了色散补偿光纤(DCF)技术.基于DCF在标准单模光纤上传输2245 km的情况下进行了研究,获得了一些对实际色散补偿系统有参考价值的结论.     

波分复用系统中色散系数选择优化设计

对传输容量为4 × 2.5Gb/s、传输距离为300km的波分复用系统进行了仿真实验.在考虑了非线性效应的情况下,分析了不同色散系数对系统性能的影响.仿真结果表明,在路径平均色散为零的波分复用光纤线路中,色散系数选择在20～30ps/(nm·km)左右系统性能最优.     

光纤通信加性噪声中孤子传输非线性问题

光纤中具有轻微的非线性和一定的色散特性,这两种特性分开来说,将会损害光纤通信中系统的性能,但是在一定的条件下,这种小的非线性的影响可以抵消色散的影响,产生孤子.在无耗的光纤介质中,孤子可以传输任意距离而不改变形状.实际的有耗光纤中,为了使用孤子,用光放大器来抵消光纤的损耗,以维持链路上孤子能量的稳定.通常在光纤中非线性和色散效应都比较弱,对光纤的作用表现为两种效应的相加性.放大器的自发辐射噪声(ASE),通过两种不同的机理增加了系统的误码率(BER):(1)通常的加性噪声的影响;(2)孤子速度的抖动.第二种效应导致脉冲到达时间的抖动,这就是Gordon-Haus效应.由于光纤存在损耗,光孤子在光纤里的传输过程中,其幅度随传输距离按指数规律衰减,必须对孤子能量进行放大.一种是分布式光放大器方法;另一种是集总式光放大器方法.分布式EDFA使用低浓度的掺铒光纤作为传输介质.集总式放大就是在光纤线路中每隔一段距离L接入一段集总式光纤放大器(目前通常采用EDFA)来补偿孤子能量的损失.本文研究了孤子传输在存在损耗和放大器自发辐射噪声(ASE)时的孤子传输方程及其解.并对孤子传输的控制作了概述.(PD3)     

归零码和非归零码传输系统模

对单信道40Gbit/s归零码（RZ）和非归零码（NRZ）传输系统进行了实际模拟。考虑了光纤损耗，二阶群速率色散、三阶群速度色散、偏振模色散及放大器噪声对系统影响，采用Q值判别法计算了系统可传输的最大距离，给出了系统眼图。计算结果表明，RZ的最大传输距离大于NRZ码，RZ系统传输性能优于NRZ系统。     

塑料光纤在通信网中的应用

通过对塑料光纤(POF)传输特性的分析,了解和熟悉塑料光纤在损耗、带宽、色散及温度等方面的性能特征。根据塑料光纤在短距离高速通信网传输数据中的的优越性能,设计了以塑料光纤为传输介质的全光网络传输系统和语音电路传输系统。     

光PM-DQPSK调制格式性能研究

对偏振复用-33%归零-差分正交相移键控(PM-33%RZ-DQPSK)和偏振复用-抑制载波归零-差分正交相移键控(PM-CSRZ-DQPSK)调制格式进行了传输性能仿真,在仿真过程中,分别考虑了光纤系统中的偏振模色散(PMD)、色散(CD)和非线性效应(SPM)三种限制因素。仿真结果表明偏振复用系统比非偏振复用系统更易受到PMD、CD和非线性效应的影响。整体来看,PM-CSRZ-DQPSK调制格式的传输性能比PM-33%RZ-DQPSK更为优秀。     

光放大技术的应用

在增大光缆传输系统容量，延伸电中继距离的技术中起重要作用的技术是：低衰减和低色散的单模光纤技术、单纵模激光器或单频光源技术、光放大技术、外部调制技术、高速调制解调技术、波分复用(WDM)技术、频分复用(FDM)技术、相干光通信技术以及光孤子传输技术等。其中光放大技术对光纤通信系统的容量和结构产生了革命性的影响，它使光纤线路可以实现全光传输，     

Impact of nonlinear phase noise on direct-detection DQPSK WDM systems

The performance degradation of differential quadrature phase-shift keying (DQPSK) wavelength-division-multiplexed (WDM) systems due to self-phase modulation (SPM)- and cross-phase modulation (XPM)-induced nonlinear phase noise is evaluated in this letter. The XPM-induced nonlinear phase noise is approximated as Gaussian distribution and summed together with the SPM-induced nonlinear phase noise. We demonstrate that 10-Gb/s systems, whose walkoff length is larger than 40-Gb/s systems', are more sensitive to XPM-induced nonlinear phase noise than 40-Gb/s systems. Furthermore, DQPSK WDM systems show lower tolerance to both SPM- and XPM-induced nonlinear phase noise than differential phase-shift keying WDM systems.     

Long-haul WDM transmission using higher order fiber dispersionmanagement

We propose a fiber dispersion management scheme for large-capacity long-haul wavelength division multiplexing (WDM) transmission systems that considers not only second- but also third-order dispersion characteristics using transmission fibers with opposite dispersion signs. It eliminates the waveform distortion of WDM signals that originates from the existence of third-order dispersion, which is a constraint placed on WDM capacity in conventional dispersion management, while reducing the interchannel interaction caused by the interplay of fiber nonlinearity and second-order dispersion. Design concept of the scheme is discussed to show the feasibility of using actual fiber parameters. An experimental investigation on transmission performance regarding the signal pulse format, nonreturn-to-zero (NRZ) and return-to-zero (RZ), and interchannel interaction caused by four-wave mixing (FWM) and cross-phase modulation (XPM) is described for optimizing WDM system performance. It is experimentally shown that RZ pulse transmission is possible without significant spectral broadening over a wide wavelength range in dispersion managed fiber spans. Using these results together with a wideband optical amplifier gain-bandwidth management technique, yields long-distance WDM transmission with the capacity of 25×10 Gb/s over 9288 km     

波分复用系统中的高阶色散管理技术

光纤色散和非线性是影响密集波分复用(DWDM)系统性能的主要因素。高阶色散管理技术，可以有效地减小四波混频(FWM)和信号波形失真。介绍了该技术的基本原理及不同色散分布方法对系统性能的影响。     

Performance Limitations of Subcarrier Multiplexed WDM Signal Transmissions Using QAM Modulation

We theoretically investigate the performance limitations of subcarrier multiplexed (SCM) wavelength-division-multiplexing (WDM) systems using optical double-sideband (DSB) modulated, 16-quadrature amplitude-modulated (QAM) signals. The performance limitations are investigated using crosstalk power and SCM channel spacing for various transmission conditions, including impairment factors such as dispersion and fiber nonlinearities for a single wavelength channel first. The effects of WDM channel spacing on SCM systems with multiwavelength channels are also evaluated via the calculated bit error rate (BER) performance, based on the performance limitations found in the single-wavelength simulation. This enables us to provide guidelines for the design of SCM/WDM systems for fiber-to-the-home (FTTH) network in WDM–passive optical network (PON) architecture, based on the performance limitations.      

Design of subpicosecond dispersion-flattened fibers

Dispersion flattened (DF) fibers are required for wide-band WDM systems. The DF fibers designed in the past have dispersion in the range of 2.0-3.0 ps/km-nm. In this letter, we define a generalized refractive index profile that can be characterized by few controlling parameters. An optimum refractive index profile is obtained by minimizing the maximum dispersion over the wavelength range of 1300-1600 nm with respect to profile parameters. The designed fiber gives dispersion less than 1.0 ps/km-nm over 1350-1590 nm wavelength range. Sensitivity of the dispersion performance to the profile parameters is also discussed.     

Volterra series approach for optimizing fiber-optic communicationssystem designs

A new methodology for designing long-haul fiber-optic communication systems is presented. We derive the overall Volterra series transfer function of the system including linear dispersion, fiber nonlinearities, amplified spontaneous emission (ASE) noise from the fiber amplifiers, and the square-law nature of the direct detection (DD) system. Since analytical expressions for the probability of error are difficult to derive for the complex systems being used, we derive analytical expressions for an upper bound on probability of error for integrate-and-threshold detection at the receiver. Using this bound as a performance criterion, we determine the optimal dispersion parameters of each fiber segment required to minimize the effects of linear dispersion, fiber nonlinearities and ASE noise from the amplifiers. We study the dependence of optimal dispersion parameters on the average power levels in the fiber by varying the peak input power levels and the amplifier gains. Analytical expressions give us the freedom to choose system parameters in a practical manner, while providing optimum system performance. Using a simple system as an example, we demonstrate the power of the Volterra series approach to design optimal optical communication systems. The analysis and the design procedure presented in this work can be extended to the design of more complex wavelength division multiplexed (WDM) systems     

Closed-form dynamic range expression of dual-electrode Mach-Zehnder modulator in radio-over-fiber WDM system

A dual-electrode Mach-Zehnder modulator for modulating a single channel or multiple channels in wavelength-division multiplexing (WDM) radio-over-fiber systems has been studied and analyzed to determine the effect of bias shifting on the dynamic range (DR) of the modulator. A closed-form expression of a realistic DR as a function of systems' parameters and all main sources of noise at the receiver has been derived. The results of the study indicated that the derived computational expression is general and can be applied for a wide range of parametric variations.     

Dispersive properties of optical filters for WDM systems

Wavelength division multiplexing (WDM) communication systems invariably require good optical filters meeting stringent requirements on their amplitude response, the ideal being a perfectly rectangular filter. To achieve high bandwidth utilization, the phase response of these filters is of equal importance, with the ideal filter having perfectly linear phase and therefore constant time delay and no dispersion. This aspect of optical filters for WDM systems has not received much attention until very recently. It is the objective of this paper to consider the phase response and resulting dispersion of optical filters in general and their impact on WDM system performance. To this end we use general concepts from linear systems, in particular, minimum and nonminimum phase response and the applicability of Hilbert transforms (also known as Kramers-Kronig relations). We analyze three different classes of optical filters, which are currently being used in WDM systems and compare their performance in terms of their phase response. Finally, we consider possible ways of linearizing the phase response without affecting the amplitude response, in an attempt to approximate the ideal filter and achieve the highest bandwidth utilization     

Comparison of nonlinear pulse interactions in 160-Gb/s quasi-linear and dispersion managed soliton systems

The understanding and development of 160-Gb/s transmission systems requires the study of the impact of different dispersion compensation schemes on pulse propagation in nonlinear fiber. In this paper, we present an investigation of 160-Gb/s optical transmission systems, focusing on optimal propagation regimes, and in particular, we analyze different transmission limitations and dominant nonlinear effects by comparing quasi-linear and dispersion managed soliton systems. Two quasi-linear systems, one using nonzero dispersion-shifted fiber (NZDSF) and the other single-mode fiber (SMF), and one short-period (1 km) dispersion managed soliton (DMS) system are studied, both for single-channel and wavelength-division-multiplexed (WDM) transmission. First, the performance of the two quasi-linear systems in single-channel transmission are compared and it is shown that the NZDSF and SMF systems allow similar error-free transmission distances with only small differences in the intrachannel four-wave mixing (IFWM) induced amplitude jitter. The effect of pulsewidth on transmission performance in this regime was investigated and the use of shorter pulses was found to result in lower amplitude jitter. We analyzed the behavior of the DMS system and showed that the reduced pulse broadening during transmission allowed a significantly longer single-channel transmission distance with a smaller impact of nonlinearities compared to quasi-linear propagation. The sensitivity of the DMS system performance to statistical fluctuations in the fiber dispersion was studied and the results show the level of accuracy in the dispersion management map which must be ensured in these systems. Finally, the performance of the DMS in WDM transmission was investigated and it was found that it was subject to very large penalties increasing the minimum channel spacing possible because of the strong impact of interchannel cross-phase modulation (XPM).     

Effects of nonlinear crosstalk in optical PMD compensation

We report on the effects of nonlinear crosstalk on the optical polarization-mode dispersion (PMD) compensation in wavelength-division-multiplexing (WDM) systems. An analytical model is presented to describe the effect of nonlinear crosstalk on the first-order PMD compensation. We also experimentally show that the performance of WDM systems could be degraded by nonlinear birefringence, even when the first-order PMD is completely compensated.