光子晶体光纤激光器和超连续光源的研究

【作者】 孙婷婷；

【导师】 袁树忠； 刘艳格；

【作者基本信息】 南开大学 ， 光学， 2008， 博士

【摘要】 光子晶体光纤起源于光子带隙思想,却又随着不断发展而高于带隙理论。时至今日,它正以极快的速度影响现代科学的多个领域。在基于PCF的众多新型光纤器件之中,稀土掺杂光纤激光器件和超连续光源是比较具有代表性的两种。本文从实验和理论两个角度研究了光子晶体光纤在以上两方面的应用:首先总结了光子晶体光纤激光器件和超连续光源的研究现状;然后利用有限单元方法分析了大模面积光纤中的双折射特性;理论和实验研究了稀土掺杂光子晶体光纤激光器和放大器;利用分步傅里叶方法数值求解非线性薛定谔方程,分析了宽脉冲泵浦在非线性光子晶体光纤中传输时的光谱展宽过程;利用准连续/连续光源作为泵浦,实验研究光子晶体光纤中的光谱展宽现象并分析了连续谱的频域和时域特性。主要内容概括如下:1.详细讨论了有限单元方法的基本思想和求解过程,利用该方法分析了大模面积光纤中的双折射特性,提出两种结构致双折射大模面积PCF的设计:(1)通过在纤芯中引入两个小于包层空气孔尺度的小孔以破坏光纤截面几何形状的对称性,在有效模场面积大于100μm2的光子晶体光纤中获得10-4的双折射度;(2)在光子晶体光纤的包层中引入两个大尺度空气孔的同时,在光纤纤芯中引入椭圆形的小空气孔以降低光纤的多重轴对称性。通过这种具有复合不对称结构的双折射晶体光纤设计,可以使光纤在具有较大模场面积的情况下获得10-4量级的双折射。2.理论分析了高折射率Bragg光纤的模式特性和色散特性,并讨论了其在掺稀土光纤激光器件方面的应用。对拉制出的掺Er3+Bragg光纤进行了放大特性的测试,测量了前、后向泵浦方式下放大器的小信号增益和噪声系数。使用环行器和光栅构成的环行腔结构,得到了1553.7nm的单波长激光输出。由于这种掺Er3+的Bragg光纤之前未见报道,所以无论在工艺上还是在特性上,都有大量研究工作尚待继续进行。3.对掺Yb3+双包层光子晶体光纤激光器进行了实验研究。采用前向端面泵浦方式,选用二向色镜和增益光纤端面构成F-P腔的结构构建了线性腔掺Yb3+光子晶体光纤激光器。激光最大输出功率11.69W,斜率效率87%。在此基础上,利用GaAs晶体作为饱和吸收体,进行了被动调Q包层泵浦PCFL的研究,获得了脉冲宽度小于90ns、最大平均功率为5.86W的激光输出。4.利用有限单元方法,研究了光子晶体光纤非线性特性的基本理论。系统地分析了光子晶体光纤有效模场面积、光纤非线性系数和其结构参量的关系,并讨论了非线性系数随波长的变化。研究表明,利用石英/空气大的折射率差,可以设计具有小模场面积的光纤来提高非线性系数。但是,当纤芯直径过小时,能量已经不能够很好地被限制在纤芯中,形成泄漏。在此基础上,研究了掺锗纤芯PCF增强的非线性特性,计算了掺杂浓度和掺杂区域半径对光子晶体光纤非线性系数、模场分布和色散等性质的影响。这对于制造具有高非线性系数的PCF具有指导意义。5.利分步傅里叶方法求解非线性薛定谔方程,对低峰值功率、宽脉冲在光子晶体光纤中的传输特性进行了数值模拟和比较,分析了脉冲传输和演化的非线性机理和超连续谱展宽特性。通过改变光纤参数和初始入射条件,发现在低峰值功率、宽脉冲条件下引起光谱展宽的主要因素是调制不稳定性。在光纤反常色散区,噪声可以作为调制不稳作用的探测波加速入射脉冲的破裂,使之形成无序的超短脉冲,进而在光纤中继续传输实现光谱的展宽。此外,还分析了脉冲功率、光纤非线性系数、脉冲宽度等因素对连续谱的影响。6.以脉冲宽度80ps的激光输出作为泵浦,通过两极放大器引入自发辐射噪声,在70m高非线性PCF中获得了通信波段的超连续谱。在实验中,观察到了调制不稳定现象的出现,与第四章的理论分析很好地吻合。此外,还将光纤拉曼激光器输出的连续光耦合入70m高非线性光子晶体光纤,在入纤功率为4.14W时得到了1450-1650nm的光谱展宽,输出平均功率为2W。7.使用脉宽为0.62ns的调Q固体激光器和20m光子晶体光纤,获得了600-1750nm的超宽带连续谱,分析了光谱展宽的机制并测量了连续谱的频域和时域特性。利用上述宽带光源,测量了全固光子晶体光纤的带隙,并利用高双折射环境滤波的方式获得了S+C+L波段的多波长信号源。更多还原

【Abstract】 Photonic crystal fibers, also known as microstructured or holey fibers, have recently generated great interest in the scientific community thanks to the new ways provided to control and guide light. Among the varieties of novel PCF-based apparatus, Rare-Earth Doped fiber laser and supercontinuum light source are relatively representative. In this dissertation, applications of PCF to the above mentioned fields are theoretically and experimentally investigated. Firstly, new occurrence on rare-earth doped fiber laser and SC generation are summarized with the focus mainly on photonic crystal fibers. Properties of birefringence in large mode area PCF are studied with a finite element method. As for PCF laser apparatus, Er3+ doped Bragg fiber based amplifier and Yb3+ PCFL are demonstrated. The last part of our work is concentrated on the propagation of quasi-continuous/continuous wave in PCF with high nonlinearity, which including both theoretical and experimental researches. The details are described as follows:1. The basic idea and analyzing process of finite element method are discussed. By means of this method, birefringence in PCF with a large mode area is studied and two fiber designs are proposed to realize form-induced birefringence: (1) A design of microstructure fiber containing small circular air holes in the center of the solid core is proposed, and it can have structure-induced birefringence with an order of ~10-4 whilst a large mode area of 100μm2. (2) By introducing two big air holes in the fiber cladding region and a small elliptical hole in the fiber core region at the same time, the cross section of the fiber shows twofold symmetry and group birefringence of ~10-4 can be achieved. These fibers can be applied to generation and propagation of high power laser.2. Modal properties and dispersion properties of high-index core Bragg fiber are theoretically investigated. Its application on rare-earth doped fiber laser apparatus is also discussed. Using the actual Er3+-doped Bragg fiber, we demonstrated a fiber amplifier and investigated its properties of gain. Result shows it have potential application in automatic gain control.3. A high power cladding pumped Yb3+ PCFL has been demonstrated using Fabry-Perot cavity configuration. The highest output power and the slope efficiency are 11.69W and 87%, respectively. In the latter experiment, a GaAs crystal was inserted into the cavity acting as a saturable absorber. Passive Q-switched Yb3+ PCFL was achieved with a pulse width less than 90ns.4. The fundamental properties of fiber nonlinearity are theoretically investigated with finite element method. Compared with the conventional fiber, the enhancement of the nonlinear coefficient in PCF can be considered from two aspects. One is the decreased mode area benefit from the large index difference between silica and air. However, the mode area can not be reduced unlimitedly and the energy will penetrate into the air holes when the diameter of the core is sufficiently small. Thus another way for nonlinearity enhancement is discussed, and PCF with Ge-doped core is studied in details.5. Based on the NLSE and Split-step Fourier Transform Method, the propagation of long pulse in PCF with low peak power is simulated, which shows that the modulation instability derived from SPM dominates the spectrum broadening. In the anomalous dispersion region, white noise can be introduced as the MI probe for speeding up the break of the long pulse. Thus unorderly ultra-short pulses come into being and its further propagation in PCF result in SC generation. The affection of peak power, fiber nonlinear coefficient and pulse width on SC spectrum are also investigated respectively.6. Pumping 70m long PCF with an 80ps pulse output, SC generation covering communication windows is demonstrated. In the experiment, two orders of amplifier are used as the noise source and MI is observed. It is in good agreement with the simulation results. In another experiment, the continuous wave from a Raman fiber laser is used to pump the same PCF, SC generation with high average power is acquired within a wavelength range of 1450-1650nm.7. The output of an Nd3+ doped Q-switched laser with high peak power is lunched into a 20m PCF for SC generation. Properties of SC both in frequency domain and time domain are measured and analyzed. By use of the ultra wide SC light source, we measured the band gap map of an all-solid PBG fiber. And it can also be used for providing multi-wavelength signals by means of filtering.更多还原