偏振模色散对NRZ码和RZ码的40Gbit/s系统性能影响的仿真分析

偏振模色散(PMD)是进一步提高现有光纤通信系统单信道速率必须克服的障碍之一。本文基于已铺设光纤的典型值,针对偏振模色散对采用NRZ码和RZ码的40Gbit／s光纤通信系统的性能影响进行了分析与仿真。结果表明,考虑PMD的影响时,采用RZ码的系统并不一定比采用NRZ码的系统的传输性能好,而在占空比[0,1]之间存在一个最佳值,此时,高速系统受偏振模色散的影响最小。这对于研究、设计高速光纤通信系统时选择合适的码型和RZ码的占空比,具有重要的参考意义。     

自适应一阶和二阶PMD补偿的理论与实验研究

理论上分析了两阶段补偿器能够同时完全补偿一阶及二阶PMD(去偏振项)的机理。以两个偏振控制器和两段保偏光纤组成PMD模拟器,采用DOP方法完成了10Gb／s NRZ码高速光纤通信系统自适应一阶和二阶PMD补偿的实验研究。结果表明：补偿前系统一阶、二阶PMD分别为60ps、4200ps^2,而补偿后分别为8ps、50ps^2,补偿时间小于100ms。通过PMD补偿,NRZ码眼图得到很好恢复。建立起的自适应补偿系统可以很好地同时补偿系统中的一阶、二阶PMD。     

高速波分复用无中继光纤传输实验系统

利用自行研制的模块化高稳定度光发射机、光接收机、掺饵光纤功率放大器和掺饵光纤前置放大器，实现了4×622Mb／s×200km、2．5Gb／s×200km和4．354Gb／s（1×2．488Gb／s＋3×622Mb／s）×160km的常规单模光纤无中继传输实验。带有掺饵光纤前置放大器的四路光接收机灵敏度达到－46．8dBm（622Mb／s，NRZ223－1　PRBS）和－39．5dBm（4×2．5Gb／s，NRZ27—1PRBS）。系统各信道误码率优于4×10－12～4×10－15。     

OCDMA系统二维2D-OOSC方阵码的参数优化和性能分析

给出了二维λ-t光正交方阵码2D-OOSC的设计方案, 重点讨论了2D-OOSC码字的相关性及码字参数对系统性能的影响。详细分析了2D-OOSC系统的用户容量以及由多用户干扰引起的误码性能。研究表明，在占用相同带宽的条件下，系统的用户容量远大于基于1D-OOC的WDM-CDMA混合系统，特别是在传输距离不太远的局域网中，光纤色散引起的传输损失较小。基于2D-OOSC地址码的 OCDMA系统有着现实的应用价值。     

60km已敷设光纤链路中PMD部分补偿方案

偏振模色散(PMD)在40Gb/s或更高速率的光纤通信系统中已成为限制系统性能的主要因素.为使PMD补偿经济、控制方便,具有一定的工程实用价值,提出了一套采用两级固定时延补偿结构的PMD部分补偿方案,设计并制作了PMD补偿器,在实际传榆线路中进行了测试.测试结果与光通信系统设计软件Optiwave Optisystem模拟补偿效果进行了对比,优化了补偿器的各项参数.在对实际敷设的四组长约60 km的光纤链路的补偿试验中所得结果与计算结果符合较好,实现了15～20%的补偿量.     

一种大容量的二维光正交码

首先构造一种扩展的立方线性同余码（ECLC）并分析了其码字性能。在此基础上，用该码作为时间扩频伪随机序列和以素数码（PC）作为跳波长伪随机序列。再构成一种新的二维光正交码ECLC/PC，然后分析了ECLC/PC码字的性能，并与EQC/PC作为比较，相比之下，两者的码长和码重都相等，前者的互相关性能略有下降。但码字容量大大增加。因此，ECLC/PC适合于接入用户数较多的OCDMA系统。     

2.5Gbit/s偏振模色散自动补偿试验系统

从理论上分析了光补偿法实现偏振模色散(PMD)自动补偿的原理，建立了2．5Gbit／s自适应偏振模色散补偿试验系统。实验结果表明，该系统能自动完成偏振模色散的补偿。     

一种新型不等带宽波长交错滤波器的设计

为改善常规马赫-曾德尔干涉仪型波长交错滤波器的输出特性,提出了一种将光纤环型腔加入非平衡马赫.曾德尔干涉仪的干涉臂中构成的新型波长交错滤波器,运用光纤传输理论和矩阵理论,得到该器件的输出表达式.通过数值模拟分析了光纤环型腔的耦合比和光纤臂长差对该波长交错滤波器性能的影响.结果表明:组成滤波器干涉臂的两对臂长差相等时,光纤环型腔的耦合比取一些定值可得到平坦的输出光谱,该器件在3 dB处,奇信道带宽大于15GHz用于10Gb/s的传输,偶信道带宽大于60GHz用于40Gb/s的传输.这种滤波器在10Gb/s向10Gb/s+40 Gb/s升级过程中作为复用-解复用器具有很大优势.     

一种高速多通道数据采集与串行传输电路

引言 在数字技术飞速发展的今天，将各种模拟信号转换为数字信号并进行相应的传输和处理可以大大提高数据采集与传输系统的性能，我们以新一代的现场可编程门阵列（FPGA）为数据采集与传输的核心，结合模数（A/D）与数模（D/A）转换芯片，加上自定义的数据链路层协议，采用光纤链路，为多通道数据采集，点对点高速传输提供了一种新的解决方案。本力案在数据传输速率为125Gbps、误码率低于10-12的条件下，最大允许链路衰耗为27dB，可以满足不同类型的应用需求。整个系统以XILINX公司的FPGA芯片VIRTEX Ⅱ PRO XC2VP4为核心，外围电路包括美信公司的MAX1198 （ADC）、MAX5851（DAC）和飞通公司的PT7621和PT7321全双工光收发一体化模块。     

一位码纠错编码器的可编程逻辑器件实现

纠正数字信号在传输中出现的差错,逐渐成为数字通讯系统中的一个突出问题,这就要求采取适当的编码方法,及时发现并纠正错误,来提高信息传输的可靠性。本文简单介绍了用（１２,８）码纠正一位错误码元的原理;详细讨论了用一片Ｌａｔｔｉｃｅ ｉｓｐＬＳＩ１０１６芯片实现（１２,８）码编码器和解码器的设计方法。     

All optical format conversion at 10 Gbps using SOA and optical band-pass filter

In this paper, an all-optical non return-to-zero (NRZ) and return-to-zero (RZ) modulation format converter using a single semiconductor optical amplifier (SOA) and an optical band-pass filter (OBPF) is proposed. The format converter consists of a single SOA which is acting as a nonlinear element to broaden the spectrum of the input signal and the OBPF which is used to extract the special spectrum from the broadened spectrum. By adopting the ultra-fast SOA model associated with optical system software, the 10 Gbps NRZ and RZ format conversion is successfully demonstrated with simulation. We also demonstrate the proof-of-principle experiment at 10 Gbps by using the test SOA and OBF converter. The converted NRZ is achieved with an output extinction ratio of 11.51 dB. The BER is 5.5 × 10^?9 while the power of NRZ is ?10 dBm and the BER is 1.0 × 10^?9 when the power of RZ is ?14 dBm. The experimental results coincide well with the simulated results. The proposed scheme is robust and has potential for applications in future optical networks.     

Photonic crystal fibre based all-optical modulation format conversions between NRZ and RZ with hybrid clock recovery from a PRZ signal

Several all-optical modulation format-converting schemes are described for non-return-to-zero (NRZ) and return-to-zero (RZ) modulation formats that make use of spectral filtering of either self-phase modulation (SPM) or cross-phase modulation (XPM) broadened signal spectrum in a highly-nonlinear dispersion-flattened photonic-crystal fibre. Format conversions have been performed between the most widely used modulation formats - NRZ and RZ. In addition, a hybrid clock recovery scheme is proposed to obtain the data rate of the NRZ signal for NRZ-to-RZ format conversion. All format-converting schemes are based on the extraction of the spectral components in a nonlinear phase modulation broadened signal spectrum. In NRZ-to-RZ format conversion, a periodic pulse train, at a repetition rate similar to the NRZ data rate, is used as a control that induces a nonlinear phase shift to the NRZ probe signal and broadens its spectrum. The spectral components, contributed by different time instances of the control pulse, can be extracted as the converted RZ signal output. In RZ-to-NRZ format conversion, the RZ signal serves as a control that induces a nonlinear phase shift to a continuous wave probe light, where a logic-inverted NRZ signal can be extracted by filtering out the chirped components. Format conversions between NRZ and RZ signals at 9.95328 GB/s (OC-192) are demonstrated. As the proposed optical signal-processing schemes make use of the fibre nonlinearity (SPM/XPM), it is possible to extend it to a high-speed operation <160 Gb/s. Therefore the proposed format-converting schemes can serve as a format converter between the optical time-division multiplexed networks and the wavelength division multiplexed networks     

Use of return-to-zero formats in electrical dispersion compensated optical single sideband transmission systems

The use of return-to-zero (RZ) formats is proposed to enhance the dispersion tolerance of intensitymodulated carrier-unsuppressed (IM-CUs) optical single sideband (oSSB) systems. The performance of these systems, employing electrical dispersion compensation (EDC), is accessed using unipolar and polar RZ signalling formats, and compared with common non-return-to-zero (NRZ). Simulation results at 10.7 Gb/s reveal that oSSB systems with EDC using both NRZ and unipolar RZ signalling formats are limited to low extinction ratio (ER) values. However, the use of polar RZ allows increased ER and transmission distances, because of higher inter-symbol interference tolerance and the absence of discrete spectral tones. Considering a minimum ER of 9 dB and resort to forward error correction schemes, polar RZ allows single-channel error free transmission over 1400 km of standard single-mode fibre, substantially improving the 980 km obtained for NRZ with ER of 6 dB. Dense wavelength-division multiplexing transmission was also assessed, retrieving improved resilience of IM-CUs oSSB polar RZ.     

Advanced modulation formats for fiber optic communication systems

Choice of modulation format plays a critical role in the design and performance of fiber optic communication systems. We discuss the basic physics of electro-optic phase and amplitude modulation and derive model transfer functions for ideal and non-ideal Mach-Zehnder modulators. We describe the generation and characteristics of the standard nonreturn-to-zero (NRZ) modulation format, as well as advanced formats such as return-to-zero (RZ), carrier-suppressed RZ (CSRZ), duobinary, modified duobinary, differential phase-shift keyed (DPSK), and return-to-zero DPSK (RZ-DPSK). Finally, we discuss the relative merits of these formats with respect to a variety of system impairments.     

Performance of line codes in optical direct detection continuous phase frequency shift keying transmission system impaired by polarization mode dispersion

Small deviations from perfect circular symmetry in the core region of single mode fiber (SMF) cause optical pulses to become broadened as they propagate. This phenomenon is referred to as polarization mode dispersion (PMD), which leads to intersymbol interference and becomes a major impediment for the high speed long-haul fiber-optic links. We present here the theoretical complement for evaluating the performance of a line-coded continuous phase frequency shift keying (CPFSK) optical transmission system with a direct detection receiver. The analysis is carried out for two different line-coding schemes, i.e. alternate mark inversion (AMI) and order-1 coding, to investigate the effectiveness of the line coding in counteracting the effect of PMD in a CPFSK direct detection transmission system in the presence of group velocity dispersion (GVD) and receiver noise in a single mode fiber. The average bit error rate (BER) performances are evaluated without and compared to that of line codes at a bit rate of 10 Gb s^?1 considering Maxwellian distribution for the mean differential group delay (DGD). We found that the amount of power penalty improvement of line-coded systems is within 0.65 to 2.25 dB with respect to NRZ data at a BER of 10^?9.     

Comparison and optimisation of optical single sideband transmitters

A detailed comparison and optimisation of three different techniques to generate optical single sideband (OSSB) signals is presented. Three techniques are investigated: phase-shift (PS), optical filtering and Weaver's method (WM). The conventional binary and alternate mark inversion (AMI) coding formats are used to assess the transmitter performance for signals with different spectral properties. Different duty-cycles are considered for each format to analyse the generation of OSSB signals from signals with enhanced tolerance to inter-symbol interference. The optimisation of each transmitter parameter is carried out by maximising the amount of accumulated dispersion, using 40 Gb/s bit rate signals, for a specific eye opening penalty. With the binary format, an increase by more than 150 ps/nm of the accumulated dispersion is observed using the WM and NRZ pulses, compared to a reference double sideband signal with NRZ pulses. The optical filtering and PS techniques present lower accumulated dispersion improvements, although the PS allows significant sideband suppression near the carrier frequency. With the AMI format, an increase of 35 ps/nm is achieved by most OSSB transmitters and considerable high sideband suppression is observed using the optical filtering technique     

Impact of second-order PMD in a coherent optical orthogonal frequency division multiplexing system

The second-order, polarization-mode dispersion (PMD) effect for coherent optical orthogonal frequency division multiplexing (CO-OFDM) system in long-haul transmission with single-mode fiber was analyzed in detail. The closed-form expression is presented for the time impulse response of an OFDM signal affected by second-order PMD. Analysis and simulation results show that, compared with polarization-dependent chromatic dispersion (PCD), CO-OFDM system performance is mainly influenced by depolarization rate (DR) which can moderate phase noise induced by chromatic dispersion. For a fiber link of 800 km and DR of 30 ps, the system Q value is improved by about 1.3 dB over the case of no DR.     

Simulation and performance analysis of duobinary 40 Gbps optical link

This paper presents a system model to simulate and evaluate the performance of duobinary RZ (DB-RZ) and duobinary NRZ (DB-NRZ) modulation format in a 40?Gbps optical communication link. The design has been optimized to estimate the electrical and optical filter parameters for optimum performance under dispersion and self-phase modulation induced impairments. The presented model has been operated under various OSNR conditions to investigate the limitations of GVD and SPM on the BER-equivalent Q-factor for DB-NRZ and DB-RZ encoded optical data. The analysis reports the superiority of DB-NRZ over DB-RZ at higher OSNR. However, for a typical OSNR value a crossover optical filter bandwidth has been observed beyond which DB-RZ outperforms DB-NRZ.