基于光子晶体光纤的大功率高效超宽谱光源研究

【作者】 赵磊；

【导师】 隋展； 朱启华；

【作者基本信息】 中国工程物理研究院 ， 光学工程， 2009， 硕士

【摘要】 光子晶体光纤的出现极大丰富和扩展了人们对于光纤波导的认识,优于常规光纤的显著特点使其在光传输、放大等方面都有极大的应用潜力。基于光子晶体光纤的超宽谱光源就是其特殊波导特性的产物,是光子晶体光纤的一个最激动人心的应用。由于超宽谱光源在医学、生物、化学、通信、精密测量、军事应用等领域的极大应用前景,一经出现便受到了广泛重视,基于光子晶体光纤的超宽谱光源得到了迅猛发展。但作为新生事物,仍有许多理论和技术问题尚待解决。一方面,光子晶体光纤特殊的波导特性使得其中的脉冲传输和超宽谱产生过程非常复杂,其理论模型和数值模拟方法仍有待改进,超宽谱产生机制需要明确;另一方面,目前超宽谱光源的输出功率普遍较小,转换效率很少被关注。当然,还有其他许多需要研究的问题。本论文主要针对这两个问题从理论和实验上进行研究,目的在于明确光子晶体光纤中的脉冲传输和超宽谱产生特性,研究制定提高输出功率和转换效率的总体技术方案。主要包含以下具体内容:首先,改进了描述光子晶体光纤中脉冲传输和超宽谱产生的广义非线性薛定谔方程形式,使之更加直观地反映光子晶体光纤中的各种传输效应,同时也方便数值计算的实施。另外,对数值方法进行了改进,并分析了与算法有关的几个关键问题。在利用分步傅立叶方法数值求解时,对非线性项直接采用积分处理,而不采取任何数学近似。数值计算时将其视为卷积,通过方便的傅立叶变换将卷积变为傅立叶变换的乘积,再做逆变换得到,巧妙而又精确的解决了非线性项的计算。整个过程没有任何人为的近似,从而保证了计算模型的精确度。对因步长选择引起的计算精度进行了分析,提出了根据频域窗口选择时间步长,根据时域窗口判断时间窗口合理性、选择采样点数的方法,再根据频谱图上伪四波混频等边带调制确定空间步长选择合理性。最后,针对非线性项的处理,对比分析了差分方法和龙格—库塔方法的计算精度,表明龙格—库塔法在这里处理非线性项有较好的计算精度。这些结论为分步傅立叶方法求解广义非线性薛定谔方程等数值求解过程中的窗口选择、步长选择、计算精度等问题提供了直观参考标准,也为精确描述光子晶体光纤中的脉冲传输和超宽谱产生问题提供了很好的理论指导。利用改进后的模型及算法详细分析了脉冲在光子晶体光纤中传输过程中的色散和非线性效应。以影响超宽谱光源系统性能的光子晶体光纤各参数和泵浦源各参数为变量,从理论和实验上分别研究它们对超宽谱光源输出特性的影响。通过这一研究,进一步明确了不同情况下超宽谱的产生理论和影响因素,并分反常色散区和正常色散区泵浦两种情况分别设计了超宽谱光源优化设计方案。将超宽谱光源的效率区分为某波段转换效率和全系统转换效率。理论研究表明,某波段转换效率可以通过光子晶体光纤与泵浦源参数优化提升;全系统转换效率与泵浦光对光子晶体光纤的耦合效率和泵浦光吸收转化为超连续谱的程度有关。前者通过设计高效的耦合系统解决,后者则通过选择最优化的光子晶体光纤长度达到。在理论研究基础上,分别采用普通固体激光器和光纤激光器做泵浦源对不同类型的光子晶体光纤进行超宽谱产生实验研究。分别获得了较好的预期实验结果,验证了理论模型和算法的准确性,进一步明确了超宽谱产生的物理机制和关键实验技术。着重提升超宽谱光源的全系统转换效率,探索了不同泵浦源条件下的高效耦合方式,为下一步发展大功率高效超宽谱光源提供了实验指导。在系统的理论和实验研究基础上,提出了发展大功率高效超宽谱光源的总体技术方案。针对限制其输出功率和转换效率的因素,分别提出了对泵浦源、光子晶体光纤、耦合方式等的优化设计方案,并探索了其他可能的替代技术路线。更多还原

【Abstract】 The advent of a new class of optical fiber waveguides in the form of the photonic crystal fiber (PCF) in the late 1990s has enriched and extended the knowledge of fiber waveguide. The PCFs have showed many advantages in fiber delivery, amplification and many more similar applications compared with conventional fibers. The generation of ultra-broadband high brightness supercontinuum based on the special waveguide characteristics is one of the exciting applications. Because of the significance in fields such as medicine, biology, chemistry, telecommunication, precision measurement, military and so on, the supercontinuum light source based on PCFs has attracted widespread interests throughout the scientific community. Although the supercontinuum light source based on PCFs has received prosperity, there has left many unresolved theoretical and experimental problems. On one hand, because of the special waveguide characteristics, the process of pulse propagation and supercontinuum generation seems to be more complicated. The theory model and numerical simulation method need to be improved. Physical mechanisms under supercontinuum generation also need to be clarified. On the other hand, the output power of the present supercontinuum light source is so low-level. Attention should be paid on the conversion efficiency. Besides, there remains many other problems need to be researched, but those are beyond this thesis. Here, we concentrate our attention on the above two mentioned problems. Theoretical and experimental research has been careful took in aim to clarify the characteristic of pulse propagation and supercontinuum generation. A systematic technical scheme is proposed to enhance the output power and conversion efficiency. The content of this thesis is summarized as follows.The format of General Nonlinear Schrodinger Equation (GNLSE) which describes pulse propagation and supercontinuum generation in PCFs has been improved. This improvement makes it more facilitative to the reflection of the various propagation effects and the numerical simulation. The numerical method has also been improved. Several significant techniques related to the numerical simulations have been analyzed too. We treat the nonlinear term straightly as an integral instead of some mathematic approximation during the numerical calculation by the Split-Step Fourier Method (SSFM). Resolving of the integral is achieved by the Counter-Fourier -Transform of the product of the two Fourier-Transforms. The artful calculation of the nonlinear term avoided man-made approximation insures the precision. Numerical precision associated with step size choice is studied. Criteria are proposed as follows. The temporal and spatial step size can be appropriate chosen from the spectrogram. And the temporal figure provides a criterion for the temporal step size. These conclusions present some intuitionistic reference criteria for the intractable step size choice in solving the GNLSE. We have also analyzed the numerical precision of the Difference Method and the Runge-Kutta Method when calculating the nonlinear term. Results shows the Runge-Kutta Method can achieve much higher precision. All of these conclusions provided useful guidance in describing and numerical calculating the pulse propagation and supercontinuum generation in PCFs.Detail analysis of dispersion and nonlinear effect in the process of pulse propagation and supercontinuum generation in PCFs is carried out by the improved model and numerical method. PCFs parameters and pump parameters which may influence the supercontinuum light source are treated as variables to analyze their impact on the supercontinuum output. Theoretical and experimental research has clarified the mechanisms and affections under different cases. Optimized supercontinuum light source projects are proposed separately in the normal and anomalous dispersion region.The conversion efficiency of supercontinuum light source is divided into the whole system conversion efficiency and the efficiency conversion at certain band. Theory study shows results as follows. The efficiency conversion at certain band can be enhanced by optimizing the PCFs and the pump source. The whole system conversion is related to either the coupling efficiency or the extent of pump laser translated into the supercontinuum. The former can be enhanced by designing a high efficiency couple system. And the later can be achieved by optimizing the PCFs length.On the basis of these theory studies, supercontinuum generation experiments are carried out by pumping different PCFs by a conventional solid-state laser and a fiber laser. Preferable anticipated experimental results are obtained. So the veracity and validity of our model and numerical method is proved. And the physical mechanisms as well as the key experimental techniques are clarified. Emphasis is put on enhancing the whole system conversion efficiency. High efficiency coupling methods under different pumping conditions are also explored. These studies provide useful experimental guidance for developing the high power and high efficiency supercontinuum light source.A whole system project of high power and high efficiency supercontinuum light source is proposed based on the systematic theoretical and experimental studies. Optimized designs of pumping source, PCFs, coupling system and other related are proposed too in order to overcome the limitation of the output power and conversion efficiency.更多还原