【作者】 张丹；

【导师】 宁提纲；

【作者基本信息】 北京交通大学 ， 信息与通信工程， 2024， 硕士

【摘要】 2μm高功率窄线宽激光广泛应用于大气遥感、光谱/相干光合成等领域,但受激布里渊散射效应（SBS）为限制窄线宽2μm激光高功率输出的主要因素。相位调制法抑制SBS效应具有系统集成度高、抑制效果显著、成本低等优势,但利用传统相位调制信号抑制光纤激光系统中SBS效应,存在输出激光线宽过大、限制相干光束合成等问题。为此,本文从窄激光线宽下抑制SBS效应出发,利用多目标优化算法,分别对无泵浦源掺铥光纤激光系统和掺铥光纤放大器系统中的相位调制波形进行优化设计。针对两个系统分别进行了理论分析、仿真讨论和对比验证,该方案具有输出信号线宽窄、SBS抑制能力强、运行效率高等优势。本论文的研究工作主要如下:（1）结合Tm³⁺速率方程以及三波耦合方程,推导出了基于SBS的掺铥光纤放大器理论模型;在2μm激光系统中,探索了不同调制信号的SBS抑制能力和信号光谱变化情况,并阐释了优化调制信号抑制SBS的原理。（2）提出了一种基于无泵浦源掺铥光纤激光系统的相位调制波形优化设计方案。该方案将掺铥光纤激光器理论模型与基于分解的多目标进化算法（MOEA/D）相结合,利用MOEA/D对相位调制波形进行迭代优化,从而得到最小化信号线宽和最小化Stokes功率两目标最佳权衡下的一组优化调制波形。利用优化调制波形进行相位调制后,在0.2～1.5 GHz信号线宽范围内,SBS阈值与未相位调制时相比最高提升了14.2倍。此外,分析了不同调制参数时,优化调制波形在窄线宽输出下的SBS抑制能力,验证了MOEA/D算法在窄线宽、强SBS抑制能力方面的稳定性。该方案为利用相干合成技术突破单根光纤激光的输出功率提供了新思路。（3）提出了一种基于掺铥光纤放大器系统的相位调制波形优化设计方案。该方案在无泵浦源掺铥光纤系统基础上考虑放大级,并对基于参考点的快速非支配排序遗传算法（NSGA-Ⅲ）进行改进,引入多设备并行运算框架,大幅度提升了运行速率。将改进NSGA-Ⅲ算法与掺铥光纤放大器理论模型相结合,信号输出功率、相对Stokes功率和信号线宽作为目标函数,最终得到了三目标最佳权衡下的一组优化调制波形,实现了在信号线宽范围为0.1～1.3 GHz时,SBS阈值最高为21.5倍的提升。在相同线宽下,相较于传统多目标遗传算法,改进NSGA-Ⅲ算法将SBS阈值提升了1.4倍,运行速度提升了3倍。通过改变相位调制信号,得到相同SBS阈值下优化调制波形相位调制时信号线宽是正弦信号线宽的1/3。该方案拓展了2μm激光在相干探测领域的应用。更多还原

【Abstract】 High-power narrow-linewidth 2-μm lasers are employed in a multitude of applications,including atmospheric remote sensing,biomedicine,and coherent photosynthesis.However,the Stimulated Brillouin Scattering（SBS）effect is the main factor limiting the high-power output of narrow-linewidth 2-μm lasers.The phase modulation method for suppressing the SBS effect offers several advantages,including high system integration,a significant suppression effect,and a low cost.However,the use of conventional phase modulation signals to suppress the SBS effect in fiber laser systems suffers from excessive output laser linewidth and limits coherent beam synthesis.Therefore,in this paper,the phase modulation waveforms in the unpumped source Tm³⁺-doped fiber laser system and the Tm³⁺-doped fiber amplifier system are optimally designed from the suppression of the SBS effect under the narrow laser linewidth using a multi-objective optimization algorithm,respectively.Theoretical analysis,simulation discussion and comparative validation have been carried out for the two systems,respectively.This solution has the advantages of narrow output signal linewidth,strong SBS suppression,and high operating efficiency.The research work is structured as follows:（1）The theoretical model of the SBS-based Tm³⁺-doped fiber amplifier is derived by combining the Tm³⁺rate equation and the three-wave coupling equation;In the 2-μm laser system,the SBS suppression ability and signal spectral variations of different modulation signals were investigated,and the principle of optimizing the modulation signals to suppress SBS was explained.（2）A design scheme for phase modulation waveform optimization based on a Tm³⁺-doped fiber laser system without a pump source is proposed.This scheme combines the theoretical model of the Tm³⁺-doped fiber laser with the decomposition-based multi-objective evolutionary algorithm（MOEA/D）,which is used to optimize the phase modulation waveforms iteratively.Then,a set of optimized modulation waveforms that exhibit the optimal trade-off between minimizing the signal linewidth and minimizing the Stokes power can be achieved.After phase modulation using the optimized modulation waveform,the SBS threshold is improved by up to 14.2 times compared to no phase modulation over the 0.2～1.5 GHz signal linewidth.In addition,the SBS suppression capability of optimized modulated waveforms at narrow linewidth outputs with different modulation parameters is investigated,which verifies the stability of the MOEA/D algorithm in terms of narrow linewidths and strong SBS suppression capability.This scheme provides a new idea for breaking through the single fiber laser output power using coherent synthesis.（3）A design scheme for phase modulation waveform optimization based on a Tm³⁺-doped fiber amplifier system is proposed.This scheme considers an amplification stage based on the unpumped source Tm³⁺-doped fiber system.And the reference point-based fast non-dominated sorting genetic algorithm（NSGA-Ⅲ）is improved by introducing a multi-device parallel computing framework,which substantially improves the operation speed.Combining the improved NSGA-Ⅲ algorithm with the theoretical model of Tm³⁺-doped fiber amplifier,the signal output power,relative stokes power and signal linewidth are used as the objective functions.A set of optimized modulation waveforms is finally obtained,which represents the best trade-off between the three objectives.An enhancement of the SBS threshold of up to 21.5 times is achieved for signal linewidths ranging from 0.1 GHz to 1.3 GHz.Under the same linewidth,compared with the traditional multi-objective genetic algorithm,the improved NSGA-Ⅲ algorithm improves the SBS threshold by 1.4times and the operation speed by 3 times.By changing the phase modulation signal,the linewidth of the optimized modulation waveform at the same SBS threshold is 1/3of the linewidth of the sine signal.This program expands the application of 2μm lasers in coherent detection.更多还原