High-speed DPSK coherent systems in the presence of chromaticdispersion and Kerr effect

The error probability for a single-channel coherent optical differential phase-shift keying (DPSK) transmission system based on repeaterless links in the presence of fiber chromatic dispersion and Kerr effect is evaluated. An accurate model for both the optical signal propagation and the probability distribution of the receiver decision variable is obtained by using a numerical solution of the nonlinear Shrodinger equation and the characteristic function method. The results show that the selection of an optimized IF filter bandwidth is crucial to obtaining the best system performance. When chromatic dispersion dominates, the best performance is achieved in the normal dispersion region whereas when the Kerr effect has the most limiting effect on system performances, the lowest error probability is attained in the anomalous regime. The maximum link length is limited by the presence of Kerr effect, independently of the amount of transmitted optical power, to be shorter than a threshold length if an error probability of 10^-9 is to be achieved     

Performance evaluation of very long span direct detection intensityand polarization modulated systems

In this paper long distances very high capacity NRZ optical transmission systems adopting direct detection are considered in links with a low average chromatic dispersion. Particular attention is devoted to polarization modulated (PM-DD) systems. Polarization modulated systems results to he more degraded with respect to intensity modulated (IM-DD) systems by the light depolarization induced by the interplay among the Kerr effect, the ASE noise of the optical amplifiers and the polarization mode dispersion. The light depolarization is particularly strong in conditions of large spectral broadening that are met when the chromatic dispersion value is maintained very low along the link. On the other hand the use of a fluctuating chromatic dispersion with a mean value equal to zero, whereas the local dispersion is different from zero, shows the double advantage to reduce the chromatic dispersion impairments and to limit the spectral broadening. The advantages offered by this dispersion management technique have been already shown in several experiments for IM-DD systems: in this work we show that this technique is very important also for PM-DD systems since the limitation in the spectral broadening reduces the light depolarization. We show that adopting a suitable dispersion management and an opportune preamplifier optical filter transmissions at 5 Gb/s can be attained in transoceanic links by means of FM-DD systems     

First-order PMD compensation in nonlinearly dispersive opticalcommunication systems via transmission over the principal states ofpolarization

Soliton propagation in the presence of polarization mode dispersion (PMD) is studied with an analytical approach. It is shown that there exist two peculiar initial states of polarization that may prevent the soliton to undergo shape distortions. These states coincide with the states that, in the absence of chromatic dispersion and nonlinearity, are usually referred to as principal states of polarization. This fact is used to show that it is possible to improve the performance of an optical transmission system operating in the presence of chromatic dispersion, PMD and Kerr nonlinearity by coupling the input pulses to one of the principal states of polarization     

一种采用光学相位分集接收技术的相干光正交频分复用远端接入系统

提出了一种采用基于光学相位分集接收技术实现远程相干光正交频分复用(CO-OFDM)信号的远程光接入方案,并进行了理论研究和仿真验证。在本方案中,没有使用色散补偿光纤(DCF)或者色散补偿模块(DCM)补偿光纤信道色散导致的负面效应,原因是CO-OFDM信号能有效抵抗传输过程中色度色散(CD)和偏振模色散(PMD)引起的负面效应。仿真结果表明,10Gbit/s CO-OOFDM信号在标准单模光纤(SMF-28)传输320km后,采用相位估计技术得到的OFDM电信号,其时域波形的相位抖动幅度更小;与采用光载波自提取技术接收相位调制COOFDM的方案进行比较,测试误差向量幅度(EVM)的结果表明,本文方案可以获得更好的COOFDM信号接收性能,星座图中星座点收敛更加紧凑,接收的CO-OFDM信号质量更高。     

Generation of optical phase-conjugate waves and compensation forpulse shape distortion in a single-mode fiber

The generation of optical phase-conjugate waves and the application of optical phase conjugation (OPC) to optical communication systems is described. The method of pulse shape distortion compensation by OPC is outlined including distortion due to both fiber dispersion and the optical Kerr effect. The generation of a forward-going phase-conjugate wave in a third-order nonlinear medium is discussed and that by a nondegenerate forward four-wave mixing in a zero-dispersion single-mode fiber (SMF) is investigated. Suppressing the stimulated Brillouin scattering (SBS) of a pump wave in the fiber prevents saturation of the generation efficiency of the phase-conjugate wave even when the pump power exceeds the SBS threshold. In transmission experiments through a 200-km standard SMF with a 16-Gb/s intensity-modulated signal and a 5-Gb/s continuous-phase FSK (CPFSK) modulated signal, it is shown the applicability of OPC is modulation independent and that OPC effectively compensates for both chromatic dispersion and the optical Kerr effect     

Evolution of the probability density functions of Gaussian ASE noise in zero-memory nonlinear fiber

The propagation of Gaussian amplified emission noise in nonlinear optical fiber with negligible dispersion is considered. It is well known that fiber Kerr nonlinearity causes a nonlinear phase noise, also known as the Gordon–Mollenauer effect. In this paper, we examine the effect of the Kerr nonlinearity on the probability density functions of the Gaussian noise quadratures. This can lead to large deviations from the Gaussian statistics of the amplified spontaneous emission noise. Analytical statistics of the noise quadratures at the output of nonlinear fiber with negligible dispersion are derived and compared with numerical simulations. The statistics of the noise squared envelope are also presented and, based on these results, the influence of Kerr nonlinearity on the direct detection and coherent detection amplitude modulated links is discussed.     

Change of field spectrum of signal light due to fibre nonlinearities and chromatic dispersion in long-haul coherent systems using in-line optical amplifiers

The change of a field spectrum of signal light in long-haul coherent optical fibre communication systems using in-line amplifiers is measured in the normal and anomalous dispersion regions of a fibre. Results show that the interplay of nonlinear Kerr effect and fibre chromatic dispersion cause a fatal change in the field spectrum in the anomalous dispersion region.<>     

Effect of the cross-phase modulation on WDM optical systems:analysis of fiber propagation

In wavelength-division-multiplexed (WDM) optical systems, cross-phase modulation induced by Kerr effect gives rise to nonlinear phase modulation of each channel, which depends on the overall power in all the other channels. In this letter, the fiber propagation of WDM optical signals affected by cross-phase modulation is analyzed. In particular, the effect due to fiber chromatic dispersion is evaluated after that cross-phase modulation has arisen and stabilized     

Impact of chirping on polarization-mode dispersion compensated systems

Investigates the effect of chirp, resulting from either the transmitter, chromatic dispersion, or Kerr effect in the fiber, on polarization-mode dispersion (PMD) compensated systems. We demonstrate both experimentally and numerically the deleterious impact of chirping on systems with first-order PMD compensation. We present a way to optimize the PMD compensator performance by minimizing the total chirp of the signal at the receiver side.     

A study on the relationship between DWDM transmission performance and chromatic dispersion

In this letter, we numerically study the relationship between 40-Gb/s-based dense wavelength-division-multiplexing (DWDM) transmission performance and chromatic dispersion in two different transmission lines. We show that the optimum chromatic dispersion region for improving the DWDM transmission performance varies with the type of transmission line. We also show that a hybrid transmission line has a greater potential to resist any change in the dispersion slope compensation characteristics than a single fiber transmission line with a dispersion compensation fiber module. Finally, we show that chromatic dispersion of more than 12 ps/nm/km in a hybrid transmission line is optimum for a 40-Gb/s-based DWDM transmission system by taking the design of a dispersion compensating fiber into account.     

色散控制孤子系统中光纤偏振模色散的滤波控制

本文研究高速OTDM色散控制孤子系统偏振模色散的影响及其滤波控制.首先建立了同时考虑偏振模色散和滤波控制时的DMS传输方程,其次采用变分法给出了滤波控制常双折射光纤中DMS传输方程的变分解,求得了DMS的特征参数的传输演化规律,然后研究了滤波控制随机双折射光纤中DMS传输稳定性问题及其定时抖动规律.最后基于变分所得定时抖动公式,分析了三种不同系统的定时抖动,显示了DMS系统中用滤波器抑制PMD时,对高速光孤子通信性能的影响是非常有效的.     

自相似抛物线脉冲的噪声扰动分析

为了研究自相似脉冲系统中的噪声扰动，我们从非线性薛定谔方程出发，推导出了时间抖动方差和能量抖动方差表达式。数值研究结果表明噪声使得自相似抛物线脉冲形状，频谱和啁啾的畸变非常严重。噪声扰动导致了脉冲振荡和频谱抖动。虽然脉冲仍具有线性啁啾，但却在前后沿出现起伏波动。在初始阶段，噪声扰动很小，可以忽略，但随着传输距离增大，噪声扰动的影响将迅速增大。不同的正色散和增益对时间抖动和能量抖动方差影响不同。     

Fiber chromatic dispersion equalization at the receiving terminalof IM-DD ultra-long distance optical communication systems

We have evaluated the effect of fiber chromatic dispersion equalization at the receiving terminal for transoceanic optical communication systems. We used a 1000-km fiber loop with 31 Er-doped fiber amplifiers for the experiments, and measured the bit-error-rate characteristics after 9000-km transmission. Accumulated chromatic dispersion originating from the discrepancy between the signal wavelength and the system zero dispersion wavelength was equalized by the equalization fiber at the receiving terminal. We used both normal and anomalous dispersion fibers at the receiving end. The results have shown that the equalization method of the fiber chromatic dispersion at the receiving terminal is useful with some limitations for ultra-long distance optical communication systems     

Feedback equalization for fading dispersive channels

Data transmission through a slowly fading dispersive channel is considered. A receiver that linearly operates on both the received signal and reconstructed data is postulated. Assuming an absence of decision errors, the receiver is optimized for a minimum-mean-square-error criterion. Transfer functions are determined and superiority over nonfeedback receivers is indicated. The feedback receiver can be realized in a slowly varying unknown environment by means of an adaptive technique that requires neither test signals nor statistical estimation. The receiver will eliminate timing jitter and Doppler shifts. In addition, the receiver provides a time-diversity effect, as the receiver probability of error averaged over the fading statistics is lower in the presence of dispersion than in its absence.     

Compensation for chromatic dispersion and nonlinear effect in highdispersive coherent optical repeater transmission system

A design for a chromatic dispersion equalizer that provides 4.4 times higher efficiency in the dispersion compensation characteristics, compared with a conventional equalizer, is proposed. In addition, the amplitude response slope in the frequency domain is less than half of the conventional characteristic. This extends the compensation limit for chromatic dispersion up to 82500 ps/nm for a 2.5-Gb/s heterodyne system, which corresponds to a 4900-km normal dispersion fiber transmission system. A compensation method for modulational instability is also proposed. The method was confirmed by a 2.5-Gb/s continuous-phase frequency-shift-keying (CPFSK) 764-km normal dispersion fiber transmission experiment, with the abovementioned chromatic dispersion equalizer. Employing computer simulations, an over-1000-km normal dispersion fiber optical repeater transmission system with 2.5-Gb/s CPFSK heterodyne detection was shown to be feasible     

Alternative modulation formats in N×40 Gb/s WDM standardfiber RZ-transmission systems

A comparison of carrier-suppressed return-to-zero (CSRZ) and single sideband return-to-zero (SSB-RZ) formats is made in an attempt to find the optimum modulation format for high bit rate optical transmission systems. Our results show that CSRZ is superior to return-to-zero (RZ) and SSB-RZ with respect to signal degradation due to Kerr nonlinearities and chromatic dispersion in wavelength division multiplexing (WDM) as well as in single-channel 40-Gb/s systems over standard single-mode fibers (SSMF). It is shown that CSRZ enables a maximum spectral efficiency of approximately 0.7 (b/s)/Hz in a N×40 Gb/s WDM system with equally polarized channels. Furthermore, the CSRZ format in N×40 Gb/s WDM systems shows no further signal degradation compared to single-channel transmission     

Chromatic dispersion limitations in coherent lightwave transmissionsystems

Computer simulation is used to evaluate the chromatic dispersion limitations for both various coherent lightwave transmission systems and direct-detection on-off keying (OOK) systems. The results show that for a 2-dB dispersion penalty the maximum modulation rate ranges from 5 to 9 Gb/s for systems operating at 1.55 μm with 15 ps/km-nm of chromatic dispersion and 100 km of fiber. The effect is less severe for OOK systems and most severe in coherent detection systems. Simulation results are in agreement with a available experimental data     

光纤脉冲分析仪的设计

针对光纤色散特性会引起传输信号的畸变,限制通信容量,指出对光纤色散参数、光信号在长距离传输后脉冲展宽程度的准确测量,可以提供可靠的设计数据来源,在实际的工程应用中有着重要意义.根据对色散基本理论的讨论,分析光脉冲在光纤中传播的展宽现象.自行研制了一套用于多模光纤色散测量的脉冲展宽测试设备,已应用在教学实验和工程实践中.     

The effect of timing jitter on MC-DS-CDMA

In this paper, we consider the effect of timing jitter on the performance of a multicarrier direct-sequence code-division multiple-access system for both uplink and downlink transmission, assuming orthogonal spreading sequences. Theoretical expressions are derived for the performance degradation caused by the timing jitter, in the presence of a multipath channel. Assuming an additive white Gaussian noise channel, perfect power control, and full load, it is shown that the performance degradation for the downlink transmission is independent of the number of subcarriers, of the spreading factor, and of the spectral contents of the jitter at the receiving mobile station, but only depends on the jitter variance. Under the same assumptions, we point out that, if the jitter spectra of all transmitting mobile stations are the same, the degradation on the uplink is the same as the degradation on the downlink.     

Comparison of the performance of optically amplified transmissionsystems

In this paper, we show a comparison among the performance of single- and multichannel optical systems considering NRZ and soliton signals. The results have been obtained by means of numerical simulations, taking into consideration the chromatic dispersion, the Kerr nonlinearity, the fiber loss, and the ASE noise of the optical amplifiers. The performance of IM-DD and coherent asynchronous ASK systems have been evaluated in terms of the Q factor. We have considered the propagation in links encompassing conventional step-index fibers, DS fibers and in links with two different dispersion management techniques in which the chromatic dispersion is varied along the propagation distance both in randomly and in deterministically way. The effects of the in-line filtering process are mainly investigated in soliton propagation