西南交大光纤通信课程设计报告

系统中补偿信道的SPM和G-M损伤[49]，光孤子传输系统的G-M噪声[50]；RoF的色散和非线性损伤[51]。

上述各种方法是在CO-OFDM系统当中对非线性损伤的补偿，但是有些方法还是可以应用在WDM系统中进行非线性损伤的补偿，因为在WDM系统当中也较多的采用了OFDM的调制方式。目前，WDM系统是商用的比较多光通信系统，非线性损伤也成为了限制各种WDM系统的主要因素，下面就在WDM系统当中的非线性损伤的方法进行一些介绍。

在WDM系统当中有很多经典的补偿方法例如：优化色散和色散图方法[52]、新的调制格式[53]、偏振交织[54]、先进的扩增方法[55]、光学相位共轭[56]、后处理[57-58]

等。在上述方法当中除了光学相位共轭和后处理方法以外，其它所列举的方法都是设计一种具有高的非线性容限的系统。最近，一种集总的非线性后处理方法被提了出来，该方法是在假设系统当中的色散比较低的基础之下形成的[59]。近年来，随着相干光通信的发展[60]，全电信号在数字域能够进行访问。随着全电信号的介入，即使在光纤中的色散很大的情况下也能够进行非线性的补偿，这就导致信号在传输时的波形变化是不能够忽略的[61-62]。此外，信道间的非线性损伤例如XPM、FWM也能够得到补偿。在这些补偿技术当中数字后向传播技术（Digital backward Propagation ,DBP）技术被看做是最有前途的。

在WDM系统中使用DBP技术补偿非线性在技术上具有挑战性。在WDM系统中，作为一个整体的系统遇到大量的非线性和色散，它是不可能通过单一的步骤来补偿非线性损伤的。相反，所有WDM信道的电场作为一个整体必须沿光线进行后向传输。与只需要补偿线性损伤的WDM系统相比，这显然需要更多的计算[63]。因此，先进的技术还要继续发展，使DBP在补偿非线性损伤的同时能够有一个可以容忍的计算量[64]。美国中佛罗里达大学的李桂芳教授所带领的课题组，在DBP技术进行非线性损伤的补偿的方面做出了大量研究。在2010年，该课题组提出了在WDM光传输系统当中通过使用耦合的非线性薛定谔方程，采用一种先进的分步方法来减少DBP技术对信道间的非线性损伤进行补偿时的运算负荷[65]。同年，他们通过试验验证了通过采用解耦合的非线性薛定谔方程进行分布式XPM补偿的DBP技术的可行性[66]。在2011年，他们又提出了一种改进的分步方法（split-step method，SSM）用于偏振复用的波分复用（PDM-WDM）系统中。在该系统当中，一个由Manakov方程所推导出的耦合系统的非线性偏微分方程被用在DBP技术的计算过程当中[67]。同年，在PDM-WDM系统当中，提出了一种选择性的后补偿（Selective post-compensation）的方法，该方法由Manakov方程所推导出的耦合系统的非线性偏微分方程被用在DBP技术的计算过程当中而得到。同时他们还在具有不同的信道粒度（channel granularities）的WDM系统当中进行了仿真以估计该方法在不同的信道内或者信道间采用DBP技术进行非线性损伤的补偿时的性能和计算负荷方面的实用性[68]。在2012，李桂芳教授的课题组，提出了一种色散折叠（dispersion-folded）的DBP技术用来减少在进行非线性补偿时产生的庞大的计算量[69]。

由上面的分析可以知道，无论是在纯净的CO-OFDM系统还是在WDM系统中，非线性损伤都是一个关键的因素，必须进行进一步深入的研究。上述研究只是开始，随着光通信的迅猛发展，以及技术的日新月异，更有效的技术将会越来越多的应用于光通信系统当中进行高效简单的非线性损伤的补偿。

三 结论

5

首先，OFDM技术由于其高效的抗色散性能、高的频谱利用效率的独特优势，同时伴随着先进的DSP技术的发展，其必将受到越来越多的关注。同时CO-OFDM系统也会成为研究的焦点，同时该系统当中的非线性损伤也会得到更为广泛的研究，新的补偿算法也必将会用于该系统当中进行非线性损伤的补偿。

其次，目前对于如何解决高速传输链路中的残余色散问题，尚未达成共识，也没有成熟的解决方案。所以残余色散对非线性损伤的抑制作用是一个值得深思的东西，将来势必引起巨大的关注。另外值得注意的是电均衡技术可以同时补偿链路残余色散和光纤非线性，也势必受到较多关注。由于在电域对信号进行处理，具有灵活方便的优势。基于DSP技术的相干检测系统可提高系统灵敏度和谱效率，并且具有强大的电信号处理能力，为电均衡技术的发展和应用提供了新平台，成为近来的研究热点。

最后，WDM系统是目前商用的主要系统，随着业务流的日益增多以及人们要求的不断调高，现有的WDM系统已经越来越不适应现实生活的需要，更大容量的系统将会应运而生。但是，只能在现有的通信网络的基础之上进行升级才能够保证更好的性价比，所以OFDM这种调制格式在WDM系统当中的应用已经成为了势不可挡的趋势。结合之后的OFDM-WDM系统的通信容量将会得到飞速的提升，并且频谱利用效率也会得到很大的改善。但是，由于系统当中的通道内、通道间的非线性损伤与以前纯净的系统相比只会有过之而无不及，所以对该系统信道内外非线性损伤的补偿算法的研究将会受到越来越多的关注。

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