Polarisation mode dispersion tolerance of 10 Gbit/s NRZ and RZoptical signals

The authors have evaluated the performance of an optical receiver with polarisation mode dispersion (PMD) for non-return-to-zero (NRZ) and return-to-zero (RZ) signals at 10 Gbit/s, and found that the RZ format is more tolerant to PMD than is the NRZ format     

Simultaneous Compensation of Polarization Mode Dispersion and Chromatic Dispersion Using Electronic Signal Processing

In this paper, we study the performance of 10.7-Gb/s nonreturn-to-zero (NRZ) and return-to-zero (RZ) on-off keying signals in the joint presence of first-order polarization mode dispersion (PMD) and chromatic dispersion (CD) based on optical signal-to-noise ratio penalty measurements. Our investigations show that the tolerance of RZ to first-order PMD is severely reduced by the presence of typical values of residual CD. Three different receiver strategies are studied: 1. unequalized threshold detection; 2. combined feed-forward and decision-feedback equalization; and 3. maximum-likelihood sequence estimation (MLSE). In all three cases, the presence of CD eliminates the advantage of RZ over NRZ in terms of PMD tolerance. For NRZ, we find that the MLSE improves the tolerance to first-order PMD by 60%-70%, even in the presence of residual CD.     

RZ码和NRZ码的偏振模色散自适应补偿特性比较

建立了自适应偏振模色散补偿系统,利用偏振度作为反馈信号,对40 Gb/s的RZ和NRZ码分别进行了PMD补偿的数值模拟,结果显示,采用DOP作反馈信号、用二段补偿器和三段补偿器对二种码型的PMD补偿均是有效的.但由于高阶PMD的影响对NRZ码的补偿效果要优于RZ码,特别是存在偏振相关色散的影响时,对RZ码的补偿的影响在明显大于NRZ码,这说明,对于RZ码补偿偏振相关色散是必要的.     

偏振模色散所致光纤链路传输损伤分析

为了研究偏振模色散所致光纤链路传输损伤,采用基于耦合非线性薛定谔方程的分步傅里叶法数值分析了1阶和2阶偏振模色散对光纤链路中非归零码、归零码及啁啾归零码脉冲信号的影响。结果表明,偏振模色散造成光纤链路性能的严重恶化,成为限制其性能的最主要因素之一。而且随着工作速度的提高,高阶偏振模色散的影响更大;采用合适的码型能在一定程度上减轻偏振模色散的影响,其中啁啾归零码略优于归零码,而归零码优于非归零码。     

不同调制码型的偏振模色散补偿研究

在40Gb/s的光纤通信系统中,用三段模拟器模拟光纤传输中的一阶与二阶偏振模色散(PMD),采用单偏振态的偏振度(DOP)作为多级PMD补偿器的反馈信号,对归零(RZ)码、非归零(NRZ)码、载波抑制归零(CSRZ)码和啁啾归零(CRZ)码分别进行四个自由度的一级、六个自由度的二级和十个自由度的三级PMD补偿器的补偿.仿真得到了四种码型的DOP与误码率(BER)的关系,同时补偿后的BER、DOP和PMD的变化关系表明,对RZ和NRZ码采用二级补偿器的效果比一级和三级补偿器要好得多,对CSRZ码和CRZ码采用单偏振态的DOP作为反馈信号进行PMD补偿的效果并不理想.     

Impact of chromatic and polarization-mode dispersions on DPSK systems using interferometric demodulation and direct detection

We study the impact of chromatic dispersion (CD) and first-order polarization-mode dispersion (PMD) on systems using binary differential phase-shift keying (2-DPSK) or quaternary DPSK (4-DPSK) with nonreturn-to-zero (NRZ) or return-to-zero (RZ) formats. These signals are received using optical preamplification, interferometric demodulation, and direct detection. We consider the linear propagation regime and compute optical power penalties at fixed bit-error ratio (BER). In order to evaluate the BER precisely taking account amplifier noise, arbitrary pulse shapes, arbitrary optical and electrical filtering, CD, and PMD, we introduce a novel model for DPSK systems and compute the BER using a method recently proposed by Forestieri for on-off keying (OOK) systems. We show that when properly applied, the method yields highly accurate results for DPSK systems. We have found that when either the NRZ or RZ format is used, 2-DPSK exhibits lower power penalties than OOK in the presence of CD and first-order PMD. RZ-2-DPSK, as compared with NRZ-2-DPSK, incurs smaller penalties due to PMD, but offers no advantage in terms of CD. 4-DPSK, as it has twice the symbol duration of OOK or 2-DPSK for a given bit rate, incurs much lower CD and PMD power penalties than either of these techniques. RZ-4-DPSK is especially promising, as it offers CD and PMD penalties significantly smaller than all other techniques, including NRZ-4-DPSK.     

可补偿二阶偏振模色散的两级自适应补偿器研究

研制了对于10Gbit／s非归零码(NRZ)和归零码(RZ)光纤通信系统的二阶偏振模色散(PMD)自适应补偿实验系统。实验中运用光纤链路中的偏振度(DOP)作为反馈控制信号，采用粒子群优化算法(PSO)作为偏振模色散自适应补偿的搜索和跟踪控制算法，粒子群优化作为补偿搜索算法具有收敛速度快、抗噪声和避免陷入局部极值的优点；作为跟踪算法可以快速跟踪偏振模色散的随机变化。实验证明，该补偿系统可以同时补偿一阶和二阶偏振模色散。不论对于非归零码还是归零码，补偿后眼图恢复很好。补偿搜索时间为几百毫秒。跟踪系统对于链路中突发的偏振模色散变化的响应恢复时间小于20ms，实现了准实时的一阶和二阶偏振模色散自适应补偿与跟踪。     

The Effects of Polarization-Mode Dispersion on the Phase of the Recovered Clock

The impact of polarization-mode dispersion (PMD) on the phase of the recovered clock in the receiver is analyzed. The effects of first-order PMD on different clock-recovery configurations utilized for return-to-zero (RZ) and non-RZ (NRZ) formats are studied. Closed-form expressions relating the PMD-induced sampling time shift with the differential group delay and the power ratio between the principal states of polarization are obtained for each high-Q filter-based clock recovery. An experimental validation at 10 Gb/s is also shown for the case of NRZ data format     

Novel C-IPDM Signal Format for Suppression of Polarization Mode Dispersion

A novel chirped intra-bit polarization diversity modulation (C-IPDM) signal format is proposed. The transmission performance of C-IPDM is compared to NRZ, RZ and the common IPDM in terms of the PMD tolerance by simulation in a 40 Gb/s system. The results show that the C-IPDM format can reduce the effects of second-order PMD significantly due to the chirping characteristic and the system Q-factor is increased especially in high PMD systems.     

System performance evaluation in terrestrial systems with highpolarization mode dispersion and the effect of chirping

We evaluate the system power penalty for different modulation formats-nonreturn-to-zero (NRZ), return-to-zero (RZ), dispersion-managed solitons, and prechirped RZ-in the presence of polarization mode dispersion (PMD) for 10-Gb/s terrestrial systems. All orders of PMD are considered by simulating the fiber using the coarse-step method, and a statistical approach is used to estimate the occasional fading of the signals. We show that pulses with lower duty-cycles perform better in general, and the system performance is improved if appropriate prechirping interacts with the residual chromatic dispersion of the fiber     

用光信号偏振度衡量偏振模色散和非线性效应对WDM系统的影响

利用非线性薛定谔方程，计算考虑偏振模色散和非线性效应情况下波分复用（WDM）系统中的RZ码和NRZ码的光信号偏振度，得到DOP随着波长变化的曲线图，同时得到各个信道的眼图。结果表明：通过光信号偏振度的曲线图与眼图的比较，不但明显看出PMD和非线性效应对各个信道的影响是不同的，并且证实偏振度可以作为WDM系统中监测信道性能的监控信号，这将为实现PMD实时最坏信道补偿提供理论依据。     

偏振模色散对NRZ码和RZ码系统影响的研究

利用分步傅里叶变换求解在双折射光纤中的耦合非线性薛定谔方程,数值计算了偏振模色散（PMI）对20Gbit/s的非归零码（NRZ）和归零码（RZ）通信系统的影响,通过数值计算发现采用一定脉冲宽度的RZ可以有效地降低PMD对通信系统的影响,并在此基础上,计算了系统传输最大距离时所满足的脉冲最佳宽度。     

Numerical comparison between distributed and discrete amplificationin a point-to-point 40-Gb/s 40-WDM-Based transmission system with threedifferent modulation formats

We describe a detailed numerical investigation on the relative merits of gain flattened distributed Raman amplification (DRA) and discrete gain flattened amplifiers. We simulate a system with forty 40-Gb/s channels spaced at 100 GHz and compare the performance of three different modulation formats nonreturn-to-zero (NRZ), return-to-zero (RZ) and carrier-suppressed RZ (CS-RZ). Three types of amplifiers, multifrequency backward- and forward-pumped DRAs, and an idealized discrete gain flattened amplifier are examined for various signal powers and transmission distances. For the backward-pumped DRA, we also describe calculated tolerance limits imposed by incomplete dispersion slope compensation and polarization mode dispersion (PMD) level     

A comparison between different PMD compensation techniques

We quantify the benefits of using different techniques for compensation of polarization mode dispersion (PMD) in fiber-optic communication systems by means of numerical simulations. This is done both with respect to PMD-induced pulse broadening and in terms of system outage probability for different data formats [nonreturn-to-zero (NRZ) and return-to-zero (RZ)]. Attention is focused on simple and relevant single- and double-stage post-transmission compensators with a few degrees of freedom (DOF). It is generally believed that a PMD compensator with a polarization controller and a variable delay line can only compensate the PMD to the first order. We show, from analytical results, the counterintuitive fact that this scheme can also partially compensate for higher order PMD. We also investigate the benefit of using a polarizer as compensation element where the optical average power can be used as a feedback signal     

Crosstalk detection schemes for polarization division multiplextransmission

Polarization division multiplex (PolDM) is a bandwidth-efficient and sensitive modulation format suitable for upgrading bandwidth-limited trunk lines. We show how control signals for polarization demultiplex can be obtained efficiently. For interleaved return-to-zero (RZ) signals, incoherent crosstalk has to be detected and minimized. In other cases, in particular for non-return-to-zero (NRZ) signals, coherent crosstalk senses penalties much better and should be detected instead. NRZ transmission experiments with either scheme are presented at a data rate of 2×10 Gb/s, with endless polarization tracking. Polarization mode dispersion (PMD) tolerance is also assessed     

Automatic PMD compensation experiment with particle swarm optimization and adaptive dithering algorithms for 10-gb/s NRZ and RZ formats

We demonstrate a 10-Gb/s polarization-mode dispersion (PMD) compensation experiment by using a 4-degree of freedom compensator for both NRZ and RZ formats. The particle swarm optimization method is used as the searching algorithm and an adaptive dithering algorithm is employed in tracking process. The results show that the searching process is finished within several hundreds of milliseconds and the response time for the compensator to recover from a sudden disturbance is less than 20 ms.     

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     

PMD致展宽及对不同码型的影响

首先从理论上系统地分析了由光纤偏振模色散(PMD)引起的传输光脉冲的展宽,给出了脉冲展宽的计算公式;然后分析、比较了PMD对采用不同码型的传输系统性能的影响,结果表明,PMD对采用预啁啾码(CRZ)的传输系统的性能影响最小;最后采用数值仿真方法对分析结果进行了验证。     

NRZ versus RZ in 10-40-Gb/s dispersion-managed WDM transmissionsystems

We compare nonreturn-to-zero (NRZ) with return-to-zero (RZ) modulation format for wavelength-division-multiplexed systems operating at data rates up to 40 Gb/s. We find that in 10-40-Gb/s dispersion-managed systems (single-mode fiber alternating with dispersion compensating fiber), NRZ is more adversely affected by nonlinearities, whereas RZ is more affected by dispersion. In this dispersion map, 10- and 20-Gb/s systems operate better using RZ modulation format because nonlinearity dominates. However, 40-Gb/s systems favor the usage of NRZ because dispersion becomes the key limiting factor at 40 Gb/s     

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