2.5 Gb/s偏振模色散自动补偿对偏振模色散容限值的影响

在我国现有的光纤通信骨干网中,绝大部分速率为2.5 Gb/s。今后如在我国现有的2.5 Gb/s网络系统上进行密集波分复用(DWDM)升级,亦需要考虑偏振模色散(PMD)对系统容量升级的影响。采用十段高双折射光纤级联而成的偏振模色散模拟器模拟实际光纤,从信号中提取基带频率分量作为反馈信号,对2.5 Gb/s系统进行了偏振模色散自动补偿实验,并对反馈前后的系统进行了系统代价的测量和比较。实验结果表明,偏振模色散自动补偿能较大幅度地提高系统的偏振模色散容限值。     

Dynamic PMD compensator for 40 Gb/s PM-DQPSK system

The adaptive polarization mode dispersion(PMD) compensation in high-speed transmission system has become more and more necessary for the link PMD causing strong signal distortions.A dynamic adaptive PMD compensator in 40 Gb/s polar-multiplex differential quadrature phase shift keying(PM-DQPSK) system is reported.Experimental results show that the PMD compensator can track the average polarization state variation at 65 rad/s without any lost of the optimum tracking.The 1st-order PMD compensation is demonstrated experimentally,and the compensator can increase the maximal tolerable PMD value by 26 ps from 17 ps to 43 ps in an optical transmission system.     

偏振模色散的分析模型

当光纤通信系统单信道速率到40Gbit/s以上时，偏振模色散的影响已经成为严重影响系统性能的主要因素。对偏振模色散现象的研究通常可采用几种不同的分析模型：琼斯矩阵、斯托克斯空间、耦合非线性薛定谔方程；对这几种分析模型作了详细地介绍，进行了分析和比较，并对40Gbit／s光纤传输系统进行了数值模拟。     

对40 Gb/s OTDM系统中光纤光栅色散补偿器进行偏振模色散补偿的研究

改变光纤光栅紫外曝光系统 ,在相位掩模板后插入一个旋转装置 ,使得光纤在制作过程中可以进行某种旋转。通过这种方法制作的光纤光栅偏振模色散减小到平均差分群时延 (DGD)约为 0 2ps,而没加旋转制作的光纤光栅平均DGD约为 18 2 ps。采用两个这种低偏振模色散 (PMD)的光纤布拉格光栅 (FBG) ,成功地在 4 0Gb/s光时分复用 (OTDM)系统中补偿了约 2 0 4 0 ps的色散 ,该系统在经过 12 2km普通单模光纤传输后 ,未发现PMD的影响 ,传输功率代价小于 1 4dB。     

Practical considerations for optical polarization-mode dispersion compensators

For transmission systems at 10 Gb/s and beyond, polarization-mode dispersion (PMD) is one of the limiting factors. Optical PMD compensators aim at increasing the PMD value tolerated by the system; however, they do not cancel out its effects. Therefore, the performance of a PMD compensator is assessed statistically. Requirements for optical PMD compensators include a response time in the range of 1 ms in order to follow polarization fluctuations over the line. The system design should account for the interaction of other transmission impairments with the PMD compensator operation. For instance, transmitter chirp and residual chromatic dispersion have a deleterious impact on the compensator performance. While self-phase modulation is harmless, cross-phase modulation greatly reduces the compensator efficiency. System design rules have been applied to a one-year field trial, showing the compensator's efficiency and reliability. However, reducing their cost is the next challenge that will bring optical PMD compensators to be used in installed systems.     

DWDM系统中偏振模色散的影响

本文导出了四波长光波在双折射光纤中传输所满足的一般非线性耦合方程.研究了色散平坦光纤中4×10Gb/sDWDM系统的偏振模色散特性;在此基础上,进一步研究了偏振模色散对不同信道间隔的4×10Gb/sDWDM系统的影响,结果表明偏振模色散限制了DWDM系统有效带宽内的信道数量及其传输码率.     

160 Gb/s-Based Field Transmission Experiments Using Polarizer-Based PMD Compensator With Optical Power Monitor

Higher bit-rate transmission is attractive for improving network resource efficiency and reducing the complexity of network management in future transmission systems. However, chromatic dispersion and polarization mode dispersion (PMD) are one of the most serious impairments. In particular, PMD should be compensated for dynamically because it changes rapidly according to environmental variations such as temperature change and mechanical vibration. Therefore, an adaptive PMD compensator is indispensable for higher bit-rate transmission systems. In this paper, we employed a simple and bit-rate independent PMD compensator based on a polarizer with an optical power monitoring scheme in 160 Gb/s-based field experiments. By using the PMD compensator, the single channel transmission of a 160 Gb/s return-to-zero differential phase-shift-keying modulation signal over an installed fiber link with buried and aerial cable routes was successfully achieved. Approximately 1 dB of Q-factor was improved by using the PMD compensator when PMD impairment was maximized. Through these experiments, the effectiveness of the PMD compensator in the higher bit-rate transmission systems was confirmed in the field environment. Furthermore, single-polarization 8times160 Gb/s wavelength division multiplexing transmission over the installed 200 km standard single mode fiber without polarization demultiplexing was successfully achieved by using the simple PMD compensator.     

Multichannel Adaptive Polarization Mode Dispersion Compensation with One Compensator in Dense Wavelength Division Multiplexing Transmission Systems

A simple two-section polarization mode dispersion(PMD) compensator is proposed for multichannel PMD compensation, which can compensate two or even more channels simultaneously. Because of the statistical characteristics and the frequency-dependence of PMD, for current single mode fiber with moderate PMD, the probability that all channels are severely degraded at the same time is extremely small, which makes it possible to compensate a dense wavelength division multiplexing(DWDM) transmission system with moderate PMD using this compensator. It is shown that the outage probability of a 40×43 Gb/s DWDM transmission system using this compensator is decreased significantly from 3.6×10-3 to 3.6×10-5.     

Polarization mode dispersion detected by arrival time measurementof polarization-scrambled light

Polarization mode dispersion (PMD) limits optical fiber capacity. PMD compensators usually minimize the associated eye closure. This measure scales with the square of the differential group delay (DGD) and makes it difficult to detect low DGDs. However, light with a low-speed polarization modulation suffers arrival time variations, in the presence of PMD, that are proportional to the DGD. These are detected by integrating the voltage-controlled oscillator (VCO) input signal of the clock recovery phase-locked loop (PLL). This novel method has been demonstrated for 40 Gb/s nonreturn-to-zero (NRZ) and for 2×40 Gb/s return-to-zero (RZ) polarization division multiplex transmission. PMD detection sensitivities range between 2 ps and 84 fs     

MLSE Equalizers for Frequency Discrimination Receiver of MSK Optical Transmission System

We propose maximum-likelihood sequence estimator (MLSE) equalizers based on either Viterbi algorithm or template matching temple matching (TM) for the equalization of impairments imposed on the minimum shift keying (MSK) modulation formats in long haul transmission without optical dispersion compensation. The TM-MLSE equalizer is proposed as a simplified alternative for the Viterbi-MLSE equalizer. It is verified that the Viterbi-MLSE equalizer can operate optimally when noise approaches a Gaussian distribution. Simulation results of the performances of the two MLSE equalizers for optical frequency discrimination receiver-based optical MSK systems are described. The transmission performance is evaluated in terms of: (1) the chromatic dispersion (CD) tolerance for both Viterbi-MLSE and TM-MLSE equalizers; (2) transmission distance limits of Viterbi-MLSE equalizers with various number of states; (3)the robustness to fiber polarization mode dispersion (PMD) of Viterbi-MLSE equalizers; and (4) performance improvements for Viterbi-MLSE equalizers when utilizing sampling schemes with two and four samples per bit over the conventional single sample per bit. With a small number of states (64 states), the non-compensating optical link can equivalently reach up to approximately 928 km SSMF for 10 Gb/s transmission or 58 km SSMF for 40 Gb/s. The performance of 16-state Viterbi-MLSE equalizers for optical frequency discrimination receiver (OFDR)-based optical MSK transmission systems for PMD mitigation is also numerically investigated. The performance of Viterbi-MLSE equalizers can be further improved by using the sampling schemes with multiple samples per bit compared to the conventional single sample bit. The equalizer also offers high robustness to fiber PMD impairment.     

Simulation of single-channel optical systems at 100 Gb/s

With the help of a computer simulation, we have investigated the conditions under which the transmission of light pulses through optical fibers may be possible over thousands of kilometers at a bit rate of 100 Gb/s. Employing an amplifier spacing of only 20 km, nonreturn-to-zero (NRZ), return-to-zero (RZ), and dispersion-managed solitons (DMSs) may all be useful provided that certain additional conditions are met. These include dispersion management by means of a dispersion map, a reduced dispersion slope, low polarization mode dispersion (PMD), and in-line optical filters     

Subcarrier multiplexing for high-speed optical transmission

The performance of high-speed digital fiber-optic transmission using subcarrier multiplexing (SCM) is investigated both analytically and numerically. In order to reduce the impact of fiber chromatic dispersion and increase bandwidth efficiency, optical single-sideband (OSSB) modulation was used. Because frequency spacing between adjacent subcarriers can be much narrower than in a conventional DWDM system, nonlinear crosstalk must be considered. Although chromatic dispersion is not a limiting factor in SCM systems because the data rate at each subcarrier is low, polarization mode dispersion (PMD) has a big impact on the system performance if radiofrequency (RE) phase detection is used in the receiver. In order to optimize the system performance, tradeoffs must be made between data rate per subcarrier, levels of modulation, channel spacing between subcarriers, optical power, and modulation indexes. A 10-Gb/s SCM test bed has been set up in which 4 × 2.5 Gb/s data streams are combined into one wavelength that occupies a 20-GHz optical bandwidth. OSSB modulation is used in the experiment. The measured results agree well with the analytical prediction     

Large PMD Compensated by Four Free Degrees in OTDM System

By introducing a two-stage polarization mode dispersion (PMD) compensator after a optical fiber link with a large PMD,over 270 ps first-order and 2 000 ps2 high-order PMD was compensated. The results show that the two-stage compensator can be used to PMD compensation in the 20 Gb/s optical time division multiplexing system with 60 km high PMD fiber. After compensating, the 270 ps DGD is changed into max. 7 ps. Moreover,the tunable FBG has a function of dispersion compensation.     

Fibers with low polarization-mode dispersion

Fibers with low polarization-mode dispersion (PMD) enable high-bit-rate time-division-multiplexed optical communication systems. These fibers are becoming increasingly important with the growing implementation of wavelength-division-multiplexing (WDM) transmission with channel bit rates of 40 Gb/s. Important to the realization of low PMD fibers are the fiber's index and stress profiles, polarization-mode coupling, and the evolution of the polarization state with length. This paper reviews recent progress in the understanding of the fabrication and characteristics of these fibers. The paper also discusses the important fiber physical parameters, including the fiber index profile and fiber spinning parameters and their impacts on the realization of low PMD performance.     

40 Gb/s CSRZ码系统下的自适应PMD补偿实验

在采用了新型光纤的40Gb／s系统中，通常只有很小的一阶偏振模色散（PMD）。本文在40Gb/s的国家自然科学基金网（NSFCNET）上，搭建了由电控偏振控制器（PC）、保偏光纤（PMF）、信号光偏振度（DOP）检测单元以及反馈控制单元组成的PMD补偿模块，具有结构简单、反应灵敏的特点，可以对小于20ps的一阶PMD进行自适应的补偿，平均搜索时间为2ms。     

Polarization mode dispersion compensation at 10, 20, and 40 Gb/swith various optical equalizers

Polarization mode dispersion (PMD), especially in “old” fibers, is considered harmful for installation and upgrading of trunk lines. An optical PMD equalizer should have several or many differential group delay (DGD) sections with polarization transformers in between which can endlessly transform any input polarization into a principal state of the following DGD section. The sections must practically have fixed DGDs unless there is only one section. The small-signal baseband transfer function for PMD, higher order PMD, and the necessary number of sections as well as their control by the output signals of an electrical filter bank in the receiver are also discussed in this context. Several PMD equalizers have been realized and successfully tested in transmission systems with bit rates of 10, 20, and 40 Gb/s. The systems operated stably with well-opened eye diagrams for DGDs ranging between 0 and 1.7 bit durations. Best performance is obtained from a distributed PMD equalizer with one piece of polarization-maintaining fiber twisted by 64 stepper motors. The principle can also be realized in LiNbO₃     

On the second-order approximation of PMD

A second-order polarization mode dispersion (PMD) approximation based upon the pulse-width distortion has been studied. It shows that a complete second-order approximation should include the second derivative of the PR-ID vector as well as the first derivative of the PMD vector. Second-order pulse distortions are explicitly expressed including a `first-order' term involving principal states of polarization (PSP) of the pulse and a second-order term involving the beating between fiber chromatic dispersion and effective PMD chromatic dispersion. An analytical result is derived for the probability of second-order PR-ID power penalty. It shows that the mean PMD of the fiber should be restricted to 26 ps and 18 ps, respectively for an optical link with zero and 850 ps/nm chromatic dispersion, in order to maintain a one dB second-order PMD power penalty with a probability below 10^-6 at a data rate of 10 Gb/s. The analysis also indicates that a second-order PMD compensator can be used as a dynamic chromatic dispersion compensator     

Impact of Polarization Mode Dispersion in Multi-Hop and Multi-Rate WDM Rings

Fiber polarization mode dispersion (PMD) is perhaps the most critical transmission impairment in optical networks at transmission rates of 10 Gb/s and higher. Since the bandwidth-distance product, or transparency,of the optical circuit is limited by PMD, the overall network design and cost may be significantly altered by the actual fiber PMD values. The paper has three objectives. First, an accurate model for evaluating the PMD effects is presented and verified experimentally. Second, the cost increase of WDM rings due to PMD in a number of design scenarios—first generation, single-hop,multi-hop, and multi-rate networks—is assessed. Third, the polynomial-time algorithm proposed in Cerutti et al. [1] is modified to provide sub-optimal solutions for the above WDM rings, taking into account the limited bandwidth-distance product imposed by PMD. Presented results reveal that at high transmission rates, the cost of the multi-hop ring is less affected by PMD than the costs of first generation and single-hop rings.     

40Gb／s波分系统设备技术

随着40Gb／s端口路由器的出现,未来几年内40Gb／s波分系统设备将取代现有的10Gb／s波分系统设备,就像前几年10Gb／s波分系统设备取代2．5Gb／s波分系统设备一样。然而,40Gb／s波分系统有很多传输限制因素,包括光放大器自发辐射噪声、光纤非线性效应、色散、偏振模色散等等。为实现40Gb／s的波分传输,采取新型调制码型、可调色散补偿、偏振模色散补偿等措施至关重要。     

Polarization control method for suppressing polarization modedispersion influence in optical transmission systems

Recent progress in long-span optical repeater systems indicates that the polarization-mode dispersion (PMD) influence has become one of the main degradation factors in high speed and wideband systems. This paper discusses polarization-control methods for suppressing the PMD influence for both the coherent FDM system and the IM-DD optical repeater system. A principal-state transmission method, which can avoid PMD influence in coherent FDM common polarization control, has been proposed. In this scheme, FDM light is launched at one of the principal states in the fiber using two polarization controllers, located at both ends of the fiber. The feasibility of this scheme was confirmed through 2.5-Gb/s CPFSK, three-channel FDM transmission experiments. For a 150-km long fiber, a 700-GHz optical bandwidth can be used with the principal-state transmission method. This bandwidth is about three times wider than that for conventional common polarization control in a 150-km long fiber. The principal-state transmission method has been modified to apply to a long-span optical-repeater transmission system which includes optical isolators. In this method, small frequency modulation was added to the signal light to search for the principal state for the total transmission line. The modified method can avoid inter-symbol interference (ISI) degradation due to accumulated PMD in long-span optical fibers and optical components. Power-penalty-free operations were successfully demonstrated with up to one bit time PMD value in 5-Gb/s IM-DD, 1000-km transmission experiments. This method is expected to apply to a long-span undersea optical-repeater transmission system