Joint nonlinearity and chromatic dispersion pre-compensation for coherent optical orthogonal frequency-division multiplexing systems

This paper introduces a joint nonlinearity and chromatic dispersion pre-compensation method for coherent optical orthogonal frequency-division multiplexing systems. The research results show that this method can reduce the walk- off effect and can therefore equalize the nonlinear impairments effectively. Compared with the only other existing nonlinearity pre-compensation method, the joint nonlinearity and chromatic dispersion pre-compensation method is not only suitable for low-dispersion optical orthogonal frequency-division multiplexing system, but also effective for high- dispersion optical orthogonal frequency-division multiplexing transmission system with higher input power but without optical dispersion compensation. The suggested solution does not increase computation complexity compared with only nonlinearity pre-compensation method. For 40 Gbit/s coherent optical orthogonal frequency-division multiplexing 20 × 80 km standard single-mode fibre system, the suggested method can improve the nonlinear threshold （for Q 〉 10 dB） about 2.7, 1.2 and 1.0 dB, and the maximum Q factor about 1.2, 0.4 and 0.3 dB, for 2, 8 and 16 ps/（nm.km） dispersion coefficients.

Improved pilot data aided feed forward based on maximum likelihood for carrier phase jitter recovery in coherent optical orthogonal frequency division multiplexing

Pilot data aided feed forward （PAFF） carrier recovery is essential for phase noise tracking in coherent optical receivers. This paper describes a new PAFF system based on new pilot arrangement and maximum likelihood （ML） to estimate the phase jitter in coherent receiver- induced by local oscillator＇s lasers and sampling clock errors. Square M-ary quadrature amplitude modulation （M-QAM） （4, 16, 64, and 256） schemes were used. A detailed mathematical description of the method was presented. The system performance was evaluated through numerical simulations and compared to those with noisefree receiver （ideal receiver） and feed forward without ML. The simulation results show that PAFF performs near the expected ideal phase recovery. Results clearly suggest that ML significantly improves the tolerance of phase error variance. From bit error rate （BER） sensibility evaluation, it was clearly observed that the new estimation method performs better with a 4-QAM （or quadrature phase shift keying （QPSK）） format compared to three others square QAM schemes. Analog to digital converter （ADC） resolution effect on the system performance was analyzed in terms of Q-factor. Finite resolution effect on 4-QAM is negligible while it negatively affects the system performance when M increases.

Beyond 100 Gb/s Optical OFDM Transmitted in Optical Long-Haul System

100 G Ethernet is considered to become the next generation Ethernet standard for IP networks.Typical 100 Gb/s transmission systems and their performance are presented.Comparision and analysis for 100 Gb/s transmission systems have been discussed.It is demonstrated that optical OFDM can be used in future 100 Gb/s/ch and long-haul system.

Joint timing and frequency synchronization in coherent optical OFDM systems

In this paper,we review our joint timing and frequency synchronization algorithms in coherent optical orthogonal frequency division multiplexing(CO-OFDM)systems.We first present a timing estimation method by designing the pattern of the training symbol,whose timing metric has a sharp and clear peak,to ensure accurate timing offset(TO)estimation performance.Then we provide both data-aided(DA)and blind(BL)approaches to estimate the carrier frequency offset(CFO).For the DA algorithm,we utilize the same training symbol structure as the timing estimation does,while for the BL algorithm,we utilize the zero-subcarrier power(ZSP)to achieve full-range CFO estimation.Note that our joint timing and frequency synchronization approaches require only one OFDM symbol,which ensure not only the data transmission efficiency,but also the TO and CFO estimation performance.A modified BL ZSP algorithm is proposed to further improve the CFO estimation performance by taking the power average over a series of OFDM symbols.The effectiveness of the TO estimation algorithm,and both the DA and BL CFO estimation algorithms are verified and demonstrated in both simulations and experiments.

Fiber Nonlinearity Mitigation by Improved Zero-Interpolation Method in Coherent Optical OFDM Systems

An improved zero-interpolation method with signal clipping to improve fiber nonlinearity tolerance in the long-haul coherent optical orthogonal frequency division multiplexing system （CO-OFDM） is presented. The new technique is implemented by interpolating zero subcarriers and selecting the odd subcarriers to carry data and clipping conventional CO-OFDM signal at zero. With such a scheme,the effect of fiber nonlinearity can be miti-gated,and the wanted signal carried on odd subcarriers is or-thogonal to clipping noise,which falls on even frequencies. Simu-lation shows that the system Q value is improved by more than 2 dB at the length of 960 km

Timing synchronization algorithm based on FH sequence for coherent optical OFDM systems with carrier frequency offset

A novel symbol timing synchronization algorithm based on Frank-Hermiller(FH) sequence is proposed for coherent optical orthogonal frequency-division multiplexing(CO-OFDM) systems with carrier frequency offset. The timing synchronization is realized by using only one training symbol which is composed of conjugated symmetric sequences.The timing estimation of the proposed algorithm is robust to poor frequency offset. The proposed algorithm is shown to eliminate the timing sidelobes of Park’s algorithm and has a more accurate timing estimation. In the condition of frequency offset, the timing metric of the proposed method still maintains its peak value at the correct timing point,while the values are almost zero at all other positions.

基于特征向量构建的相干正交频分复用无源光网络系统

设计基于特征向量构建的相干正交频分复用无源光网络系统,避免无源光网络受到码间串扰的影响。设置光梳状谱发生器作为系统光源,生成光波信号,利用波分复用器分离生成的光波信号,完成分离后信号传送至IQ调制器,IQ调制器利用正交频分复用信号调制与解调过程调制分离信号,通过波分复用器集合完成调制的两路光波,集合后光波利用传输光纤传送至光节点;光节点中的相干接收器接收光波信号后,利用光电二极管传送至SVM均衡器,SVM均衡器利用特征向量构建方案均衡光波信号,实现信号的高效传输。系统测试结果表明,驱动功率为11 dB、本振光功率为6 dBm时,系统具有最佳的通信性能,所设计系统可有效改善码间串扰情况,均衡通信信号。

Fourier domain optical coherence tomography with ultralong depth range

The depth ranges of typical implementations of Fourier domain optical coherence tomography （FDOCT）, including spectral domain OCT （SDOCT） and swept source OCT （SSOCT）, are limited to several millimeters. To extend the depth range of current OCT systems, two novel systems with ultralong depth range were developed in this study. One is the orthogonal dispersive SDOCT （OD-SDOCT）, and the other is the recirculated swept source （R-SS） interferometer/OCT. No compromise between depth range and depth resolution is required in both systems. The developed OD-SDOCT system realized the longest depth range （over 100 mm） ever achieved by SDOCT, which is ready to be modified for depth-encoded parallel imaging on multiple sites. The developed R-SS interferometer achieved submicron precision within a depth range of 30mm, holding potential in real-time contact-free on-axis metrology of complex optical sys- tems.

一种联合正交带复用与限幅方法的CO-OFDM峰均比降低策略

为了降低相干光正交频分复用(coherent optical orthogonal frequeng division multiplexing,CO-OFDM)系统的高峰均值比(peak to averqge vatio,PAPR),提出一种正交带复用(orthogonal band multiplexing,OBM)和限幅相结合的策略,该策略通过正交带复用将输入的子载波进行分带来减少IFFT的采样点数达到降低CO-OFDM系统的高峰均值比的目的,正交带复用与限幅方法相结合进一步降低CO-OFDM系统的高峰均值比。仿真结果表明,该策略能有效地降低信号幅值超过门限值的概率,从而达到进一步降低CO-OFDM系统的峰均值比,提高系统性能。

双模光场与级联三能级原子在非旋波近似下的量子纠缠

利用全量子化理论,在非旋波近似下对双模相干态光场与级联型三能级原子相互作用的量子纠缠进行了精确求解.讨论了初始时刻原子能级的叠加和平均光子数对量子纠缠演化的影响.数值计算结果表明:初始时刻原子处于单一能级时,量子纠缠演化曲线的周期随着平均光子数的增加逐渐变大;初始时刻原子能级的叠加导致初始阶段纠缠度显著降低,纠缠达到最大值的时间随着平均光子数的增大逐渐变长,且初始时刻原子能级的叠加使得量子纠缠的周期性消失;无论初始时刻原子能级处于哪种能级的叠加态,随着平均光子数的增大,由虚光子效应引起的小锯齿状的振荡逐渐增强.

基于CCA与POD的气动光学畸变分析

利用典型相关分析和本征正交分解方法,对计算流体力学得到的导弹红外窗口绕流三维密度场数据进行研究.取三维密度场沿展向两个相邻截面的数据先进行典型相关分析提取出两个截面的相关性最强部分,再利用本征正交分解消除两个截面之间数据的冗余信息,获得了能够包含两个截面密度场主要的信息部分.利用提取出的数据重构流场,并进行气动光学畸变仿真分析,仿真结果能较好地近似原始流场的情况.

皮肤微循环无损伤光学成像技术的新进展

微循环信息的获取对于各类疾病的诊断和治疗有着及其重要的作用。皮肤微循环无损伤成像技术取得了很大的进展,出现了各种非光学方法(核磁共振成像、正电子发射断层成像术、超声波成像等)和光学方法(共焦显微镜、光学相干断层摄影术、新型的正交偏振光谱成像技术等)。主要介绍了皮肤微循环无损伤光学成像技术的新进展。

双模光场与Λ型三能级原子量子纠缠的精确解

在非旋波近似下,对双模相干态光场与A型三能级原子相互作用系统的量子纠缠进行了精确求解,讨论了平均光子数及失谐量对量子纠缠演化的影响。数值计算结果表明:量子纠缠的周期随平均光子数的增大而增大;量子纠缠最大值随失谐的增大逐渐降低,且达到最大值所需时间逐渐增加;当失谐较大时,量子纠缠在较短时间内具有较好的周期性;随着平均光子数及失谐的增大,虚光子效应使量子纠缠演化曲线小锯齿状的振荡逐渐增强。

基于相干光正交频分复用的WDM传输系统及其性能分析(英文)

相干光正交频分复用(CO-OFDM)由于其良好的传输性能成为近年来光传输领域的研究热点,波分复用技术可以在光纤中通过增加并行波长的数量来提高系统的容量,将CO-OFDM和WDM技术结合,可以构造出高速率、大容量、低成本的光传输网络。首先对基于CO-OFDM的WDM传输系统的理论模型和基本原理进行了研究,对基于CO-OFDM的100Gb/s×32-信道WDM传输系统进行了仿真分析。并研究了该系统的传输性能。结果表明:在没有任何光纤的色散及非线性补偿的情况下,当信号速率为3.2 Tb/s时,系统Q因子高于16.0 dB,在标准单模光纤中的传输距离可达1500 km。

CO-OFDM系统中基于线性预处理的新相位噪声抑制算法

针对相干光正交频分复用(coherent optical orthogonal frequency division multiplexing,CO-OFDM)系统中相位噪声引起的载波间干扰(inter-carrier interference,ICI)问题,提出了一种基于线性预处理的新判决反馈相位噪声抑制算法。该新算法改进了线性预处理部分,利用循环前缀与OFDM符号固有的相关性,在时域进行简单的线性组合运算,充分利用了OFDM符号中冗余信息。仿真分析表明,在激光器线宽为200 k Hz且误码率(bit error rate,BER)为10-4时,与判决反馈相位噪声抑制算法和一次迭代的判决反馈相位噪声抑制算法相比,该新算法BER曲线的信噪比(signal to noise ratio,SNR)分别改善了3 d B和1 d B,有效地降低由ICI引起的错误平层。

CO-OFDM系统中一种改进的基于线性内插的相位噪声补偿算法

为补偿CO-OFDM系统中相位噪声引起的干扰,提出了一种改进的基于线性内插的相位噪声补偿算法。该算法利用共轭导频估计相位噪声的均值,然后通过线性内插获得相位噪声的估计值,最后通过频域补偿来恢复出调制信号。此外针对求幅角的非线性运算中存在的相位周跳的问题,文章提出的修正方法能够有效地改善周跳情况的发生。分析与仿真表明:该算法在不同线宽条件下均能提升一定的SNR增益且算法的复杂度低。

相干光OFDM系统中的相位估计分析

相干光正交频分复用(Coherent Optical Orthogonal Frequency Division Multiplexing,简称CO-OFDM)可有效降低光纤色散和偏振模色散影响,由于系统采用相干检测,对相位噪声的分析及信道估计尤为重要。文章对CO-OFDM系统数学模型进行了具体分析;OFDM作为一种特殊的多载波技术,利用训练序列以及插入导频来实现系统的相位估计,根据相位偏移及噪声分布,对系统相位估计实现进行了分析;通过数字仿真进一步加以验证理论推导及方案可行性。结果表明,CO-OFDM系统在光纤传输中,受到相位噪声影响系统性能下降,通过较为简单的相位估计,可有效均衡接收信号噪声影响。

一种基于限幅的降低相干光OFDM系统非线性效应的方法

相干光正交频分复用系统(Coherent Optical-orthogonal frequency division multiplexing,简称CO-OFDM)可有效降低光纤色散效应,是近年来光传输领域研究热点之一。由于OFDM信号具有较高的峰均值功率比(Peak-to-average Power Ration,PAPR)和较窄的载波间隔,系统对非线性效应十分敏感,严重影响传输性能。文章介绍了一种基于信号限幅的CO-OFDM系统,通过载波选择和信号限幅,有效提高系统非线性容限,同时消除限幅噪声带来的影响。理论分析和仿真结果表明,在不需任何在线色散补偿条件下,基于限幅的CO-OFDM系统在单模光纤中传输960km,系统Q值较传统CO-OFDM系统有2dB提高,可完全消除限幅噪声影响。

偏振正交法抑制双向传输系统瑞利干涉噪声

瑞利后向散射引起的干涉噪声是双向光纤传输系统的主要噪声来源。本文对单模光纤双向传输系统中瑞利后向散射的一般理论和偏振特性进行了分析,并提出了采用偏振正交法减小干涉噪声的方法。建立了10 GHz模拟RF信号单波长双向传输系统,并采用偏振旋转器件实现正、反向传输光的偏振近似正交传输。通过数值模拟,分析了本系统瑞利后向散射偏振状态与入射光偏振状态、反向光偏振旋转角的关系,进一步分析了系统干涉噪声抑制水平。模拟分析结果表明,在80°～90°范围内旋转偏振态,能够使瑞利后向散射降低至少一个数量级;系统信噪比提高了近15倍。利用偏振正交法减小双向传输系统的干涉噪声,有对偏振旋转的精度要求不高、容易操作的优点。

相干光正交频分复用系统偏振模色散研究

相干光正交频分复用系统(Coherent Optical Orthogonal Frequency Division Multiplexing,CO-OFDM)具有良好的色散抑制性能及简单有效的信号均衡算法,是近年来光传输领域的研究热点之一。偏振模色散严重影响高速传输,色散均衡复杂,计算负荷较大。文章介绍了一种基于偏振复用(Polarization Division Multiplexing,PDM)和偏振分集接收(Polarization Diversity Receiver,PDR)的CO-OFDM系统(MIMO CO-OFDM),可有效实抑制一阶PMD,并对系统信号传输机理及均衡算法以进行了数学分析。分析和仿真结果表明,在不需任何偏振控制器条件下,CO-OFDM系统可实现偏振光的不敏感接收,以10Gb/s的速率通过单模光纤传输800km,系统Q值保持在15dB以上。