高功率大模场光纤的光传输放大特性研究

【作者】 李晶；

【导师】 景峰；

【作者基本信息】 中国工程物理研究院 ， 光学工程， 2011， 博士

【摘要】 为了克服非线性效应、光学损伤等物理因素对光纤激光功率扩展带来的限制,目前普遍采用大模场光纤来突破这些限制。但随着光纤芯径的增大,又会激发高阶模造成激光光束质量的下降,且横向模式也会影响小宽带脉冲的光谱和时间波形,因此获得高功率、高光束质量的保真脉冲输出是大模场光纤激光系统的关键技术。本论文围绕大模场光纤的横向模式特性对小宽带脉冲的时谱特性及输出的近场分布影响展开理论和实验研究。首次论证了横向模式干涉效应对相位调制脉冲幅频效应的影响,研究了小宽带脉冲及模场的传输放大特性。以此为依据,设计了mJ级近基模输出的全光纤放大系统。首先,研究了大模场无源光纤中模场分布的解析及数值求解方法,基于这种理论模型得出了有源光纤在增益小于100dB/m的情况时模场分布可以近似为无源光纤的本征模场分布,基于模场传输及分布特性建立了横向模式干涉的理论模型。首次研究了大模场光纤放大器中模间干涉效应对近场分布及对相位调制脉冲时谱特性的影响。在大模场光纤放大器的输出端面,不同波长对应模式间的相位差不同且高阶模式在不同端面位置处的偏振不同,多个模式干涉后造成光纤端面不同位置处的光谱传递函数不同,使得光纤输出的相位调制脉冲具有时空分布特性,由于模式正交性,光纤整个端面输出的脉冲调制度的统计结果为零。首次分析了甚多模模间干涉效应对相位调制脉冲时谱特性的影响,得出模式数量越多,光谱传递函数的调制周期越小,对相位调制脉冲的幅频效应影响越大。这种结果对实验测量具有有益的指导作用,对小宽带保真传输放大的全光纤系统的设计提出了更高的模式控制的要求。结合速率方程和Ginzburg-Landau方程,分析了宽带脉冲放大过程中的时谱特性的变化情况,得出造成脉冲畸变的主要因素为增益饱和效应,这可以通过脉冲预整形或采用高斯脉冲有效抑制,而其增益不均匀性对相位调制脉冲幅频效应影响不是很大,约为3%。理论计算结果显示信号光的功率放大到数MW的高功率后,高功率激光会造成模场畸变和自聚焦特性,且高阶模并不能提高光纤的自聚焦阈值。针对阶跃大模场光纤的模式控制技术,理论分析和数值计算结果得出增益滤模是一简单有效且减小小宽带脉冲畸变的模式控制方法,配合模场适配器使用,可以有效避免高阶模的激发及模间干涉效应,减少损耗。对于弯曲选模,根据模式匹配原理提出了一种新型锥形绕纤器。最后,基于本论文的理论与实验结果,设计了mJ级近基模输出的全光纤放大系统,并进行了初步的实验验证。更多还原

【Abstract】 In order to overcome the limitations of nonlinear effects, the optical damage and other physical factors that restrict the power expansion of the fiber lasers, the large-mode-area (LMA) fibers is considered as an effective way to breakthrough these restrictions. However, with the fiber core diameter’s increasing, the higher order modes will be excited, and the laser beam quality will be degraded. Meanwhile, they will have an impact on the spectrum and the shape of the broadband pulse. So how to gain high power, high beam quality and high signal noise rate pulse is the key technology of LMA fibers laser system.In this thesis, the impact of transverse modes of the LMA fiber on the near field and the temporal, spectral characteristics of the broadband pulse was studied in theoretical and experimental. The impact of intermodal interference on the FM-AM effects of the phase modulated pulse in the broadband LMA fiber laser was first advanced. We studied the amplification and transmission characteristics of the broadband singal pulse and the transverse modes of the LMA fiber. On this basis, mJ-level quasi-fundamental mode all fiber amplifier system was predicted.We studied the analytical solution and the numerical method of the mode field expressions for the LMA passive fibers. Based on these theoretical models, we derived the mode field expressions are approximated as the eigenmodes of the passive fibers if the gain is lower than 100dB/m for the active fibers. We also derived the theoretical model of the intermodal interference based on the transmission and distribution characteristics of the mode field.The numerical simulation results show that the spectrum transfer functions are different at different positions of the LMA fiber end face owing to the intermodal interference, so there are temporal-special distribution characteristics for the output broadband pulses. We attain the similar results in the experiment when measuring the output pulse with the single mode fiber (SMF) sampling oscilloscope. Whereas there is no amplitude modulation for the output pulse when measured by the bulk detector owing to the orthogonal characteristic of the eigenmodes. For the super LMA fibers, we studied the impact of much more intermodal interference on the phase modulated pulse, and we learned that the more modes there are in the fibers, the more speckle there are for the near field, the less the modulated period of the spectrum transfer functions is, the more complex the pulse shape is. These results are of benefit to the experimental measurement, and they propose more strictly requirements of mode control for the design of all fiber amplifier system with broadband pulse shape fidelity.The pulse shape and the spectrum frame of the phase modulated pulse in the process of amplification were studied based on the rate equations and Ginzburg-Landau equation. Gain saturation is the main factor which lead to pulse distortion, and it can be suppressed by pre-shaping or adopting Gaussian pulse. Whereas, the amplitude modulation depth induced by non-uniform gain is not large, about 3% in the process of amplification. The numerical simulation results show that the mode distortion and self-focusing event will occur when the power of eigenmodes is scaled to several MW, and the self-focusing limit could not be improved by the higher order modes.Gain filtering is a method, simple, robust and reducing pulse distortion, to suppress higher order modes in the LMA fibers for broadband pulse. Gain filtering together with mode field adapters between the two modal mismatch fibers can also suppress intermodal interference and reduce splicing loss. We designed a new cone-shaped coiling device for the coiling finer based on the modes matching.Finally, we designed an mJ-level quasi-fundamental mode all fiber amplifier system based on the theoretical and experimental results of the thesis and validated it with the primary experimental results.更多还原