【作者】 姜海明；

【导师】 谢康；

【作者基本信息】 电子科技大学 ， 光学工程， 2011， 博士

【摘要】 与奥林匹克运动提倡的“更快、更高、更强”的精神相似,更大的传输带宽、更高的传输速率和更长的传输距离一直是长途干线光纤通信系统的重要发展方向,波分复用(WDM)和光放大器技术是推动上述发展方向的重要技术基础。在各种已知的光放大器中,光纤Raman放大器(FRA)由于具有噪声系数低、增益波段灵活、可实现宽带放大等众多优点,已成为现代WDM光纤通信系统的关键技术之一。对应用于WDM光纤通信系统FRA的一个基本要求是其增益谱要平坦,本文围绕宽带FRA增益谱平坦化这个中心问题进行了以下研究工作。(一)分布式宽带FRA的增益谱平坦化研究分布式宽带FRA把各类常规传输光纤作为Raman增益介质,由于这些光纤的Raman增益效率系数很小且非常不平坦,所以一般采用多个泵浦源同时泵浦来实现其增益谱的平坦化。在该方面研究中本文主要进行了以下创新性工作:(1)提出了3种用于多波长泵浦宽带FRA数学模型求解的高效和稳定的打靶法。在这3种打靶法中,反向泵浦的迭代起始值根据光纤中受激Raman散射的物理规律和引入的有关参数来共同确定。同时,引入并改进了求解非线性方程组的牛顿-拉斐森方法作为反向泵浦迭代初始值的调整机制。多个FRA的仿真计算结果表明,相对于简单打靶法这3种打靶法的计算效率和稳定性均有极大的提高。(2)提出了一种由泵浦功率积分求解泵浦输入功率的高效方法。在该方法中,根据Raman耦合方程在给定泵浦功率积分的情况下构造了一个关于泵浦输入功率的非线性耦合方程组,这样就使泵浦输入功率的求解问题变成了非线性方程的求根问题。通过积分中值定理和牛顿-拉斐森求根方法的有机结合应用,上述关于泵浦输入功率的非线性方程组得到了高效的求解。数值仿真的结果表明,该方法比此前有关文献报道的方法的求解效率提高了约2个数量级。(3)将粒子群优化(PSO)算法引入到宽带FRA设计领域,并对其进行了改进,提出了一种新的粒子群优化(MPSO)算法。对多个高维复杂的基准测试函数的优化结果表明,MPSO算法能在保持原PSO算法简单易行优点的同时大幅提高算法的优化效率和收敛率。(4)在将PSO、MPSO和所提出的3种打靶法、平均功率法进行有机结合的基础上,提出了3种多波长泵浦宽带FRA的增益谱平坦化方法。多个FRA增益谱的优化结果显示,这些优化设计方法的有关性能较有关文献中报道的方法性能均有很大的提升。(5)在详细分析多波长泵浦增益谱平坦宽带FRA传统实现方法缺点的基础上,提出了一种智能实现方法。该方法把FRA看成一个黑箱系统,不再根据具体的数学模型精确分析其内部究竟发生了什么、是如何发生的。而是把决定各泵浦功率的驱动电流看成黑箱系统的输入,相应的增益谱看成黑箱系统的输出,通过智能优化方法调整泵浦驱动电流从而实现宽带FRA增益谱的平坦化。为验证所提方法的可行性,文中给出了详细的软、硬件实现方案,并进行了实验验证。多个实验的结果表明,该方法是智能高效的多波长泵浦增益谱平坦宽带FRA的实现途径。(二)分立式宽带FRA的增益谱平坦化研究第(一)部分提到多波长泵浦法对于分立式FRA的增益谱平坦化同样适用,但由于分立式FRA的增益介质可以有很多种选择,不像分布式FRA那样必须使用传输光纤作为增益介质,这就给分立式FRA的增益谱平坦化设计留下了很大的灵活性。本文在分立式宽带FRA的增益谱平坦化方面主要进行了如下创新性工作。(1)提出了一种具有优良Raman增益属性的新型双芯光子晶体光纤(NPCF),通过对其横截面结构的合理设计,使其Raman增益效率系数在一定的波长范围内接近常数,进而为增益谱平坦宽带FRA提供优良的增益介质。数值仿真结果表明,在C波段(1530 ~ 1565 nm)该NPCF的Raman增益效率系数的波动率从普通单模光纤的49.6%骤降为3.3%,同时其数值上也比普通单模光纤有所提高,长波长方向提高约1.3倍,短波长方向提高约2.7倍的。(2)利用所提出的上述NPCF设计了一个C波段FRA。数值仿真的结果表明,在仅用一个泵浦的情况下,该NPCF-Raman放大器的平均增益可达8.7 dB,且增益谱波动小于0.9 dB,相对于普通单泵浦FRA的性能有很大的提升。NPCF-Raman放大器对于降低FRA的制造成本和简化FRA的设计复杂度具有重要意义。更多还原

【Abstract】 As the Olympic motto of“Faster, Higher, Stronger”inspires, a broader transmission bandwidth, a higher transmission rate and a longer transmission distance have always been the target long haul optical fiber communication systems aspire after. Wavelength division multiplexing (WDM) and optical amplifier technologies are the crucial boosters for the evolutions of development. Among various optical amplifiers, fiber Raman amplifier (FRA) is an enable technology for WDM optical fiber communication systems due to their low noise figure, flexibility of gain band and broad bandwidth amplification. In this dissertation, the following progress was made in aiming to flatten the gain spectrum of a broadband FRA.(1) Flattening of gain spectrum of a distributed broadband FRA.A distributed broadband FRA uses conventional fibers with small and uneven Raman gain efficiency coefficient as gain media. Multiple pumps are usually needed to achieve a flat gain spectrum of an FRA. Regarding this issue, the following creative works are carried out.(1.1) Three shooting algorithms were proposed for stably and efficiently solving the Raman coupled equations with multiple counter pumps. In these shooting algorithms, the starting values of counter pumps are determined by the analyses of physical pictures of stimulated Raman scattering in fiber and some newly introduced parameters. In addition, the Newton-Raphson method for solving a set of nonlinear equations was modified to adjust the initial values of counter pumps. The simulations of multiple FRAs showed that the calculation efficiency and stability of these shooting algorithms are improved dramatically when compared with the conventional shooting algorithm.(1.2) An efficient method for extraction of input pump powers from pump power integrals was proposed. In this method, a set of coupled nonlinear equations of input pump powers for given target pump power integrals are constructed. Finding the input pump powers corresponding to the target pump power integrals is mapped to finding solution of this set of nonlinear equations. By combined application of Newton-Raphson root-finding method and the mean value theorem for integrals the set of nonlinear equations was solved efficiently. The calculation results showed that the proposed method reduces the computational effort by two orders of magnitude when compared with those previously reported techniques.(1.3) The particle swarm optimization (PSO) was modified and introduced to the design of FRA. The application to multiple benchmark functions showed that the modified PSO (MPSO) improves the efficiency and convergence of the program greatly, in the mean time keeps the simple and elegent feature of the basic PSO.(1.4) By organic combination of PSO, MPSO and the 3 shooting algorithms, average power analysis, three design methods for a multiple-wavelength-pumped broadband gain-flattened FRA were proposed. The design results of multiple FRAs indicated that the performances of these methods are all improved when compared with the old, traditional methods in literatures.(1.5) An intelligent implement method of FRA was proposed based on the thorough analyses of drawbacks of conventional implement methods. In this new method, an exact mathematical model of FRA to determine what happens and how it happens is not needed. Instead, a FRA is regarded as a black-box. The driving currents that determine the pump powers are treated as the input of the black box and the corresponding gain spectrum are treated as the output of the black box. The method achieves the flattening of gain spectrum of a FRA by adjusting the driving currents via an intelligent approach. In order to verify the feasibility of this intelligent implement method, detailed schemes of software and hardware were presented and design of multiple intelligent FRAs were carried out. The experimental results showed that the proposed method is an efficient and intelligent approach for the implementation of multiple-wavelength-pumped broadband gain-flattened FRA.(2) Flattening of gain spectra of a discrete broadband FRA.The above-mentioned methods of flat-spectrum design of a distributed FRA are also applicable to a discrete broadband FRA. On the other hand, a discrete FRA has its own methods to achieve flat gain spectrum because fibers of various special feature can be adopted as gain media in this case, in contrast with the distributed FRA where the transmission fiber is the gain media. On the flattening of a gain spectrum of a discrete broadband FRA, the following creative works were made in this dissertation.(2.1) A novel twin-core photonic crystal fiber (NPCF) was proposed. This NPCF possesses a higher and flatter Raman gain efficiency (RGE) spectrum over a specified band of wavelength than a conventional fiber. This feature is achieved by appropriate design of its cross-section. Therefore, it is a good candidate of gain medium for a flat, broad gain band FRA. It was numerically demonstrated that the relative fluctuations of RGE of the NPCF reduces to 3.3% in the C (1530 ~ 1565 nm) band, while that of a conventional single mode fiber (SMF) is 49.6% in this band. Moreover, its value of RGE is 1.3 times and 2.7 times larger than that of a SMF respectively at the long and short wavelength end.(2.2) By using the NPCF as gain medium a C-band FRA was designed. It was numerically demonstrated that when pumped with a single source, an average gain of 8.7 dB with a fluctuation of less than 0.9 dB is achievable. It is a remarkable improvement over a conventional FRA. The NPCF reduces the complexity and cost of an FRA greatly.更多还原