GHz高重频飞秒光纤激光脉冲形成机理和噪声抑制的研究

With the decrease in the length of the laser resonant to centimeter level, the Kerr nonlinear effect (mainly self-phase modulation) that originally plays a role in the pulse formation is significantly weakened, while the other effects in the laser cavity emerge. As a result, the pulse formation mechanism is not clear and the phase noise is deteriorated, becoming two key questions for GHz femtosecond fiber lasers. In this project, the dominant factors of pulse formation mechanisms including gain filtering and the absorber induced nonlinear phase shift are studied, respectively. Firstly, a hybrid model of GHz femtosecond laser is proposed by establishing the relationship between the dynamic rate equation and the nonlinear Schrödinger equation, to clarify the role of gain filter effect in pulse formation. Secondly, the influence of the nonlinear phase shift in the absorber in pulse formation is investigated by introducing the linewidth enhancement factor in the hybrid model, to clarify the pulse duration evolution in the new mechanism, which opens a new way to simultaneously realize femtosecond-duration and GHz-repetition-rate. Finally, a high-repetition-rate ultrafast laser dominated by the saturation effect of the absorber using a hybrid silica fiber structure is experimentally constructed, to realize a repetition rate of >3GHz and a pulse duration of <200fs. The narrow pulse duration and the phase-locked loop technology are exploited to suppress the quantum noise and environmental noise, respectively. As a result, the quantum-limited phase noise performance is expected in this project. Therefore, this research provides a novel idea and theoretical supports towards building stable high-repetition-rate femtosecond lasers.

随着激光谐振腔长度缩短至厘米级，原本影响脉冲形成机理的Kerr非线性效应（主要是自相位调制）显著减弱，谐振腔内其他效应作用凸显，致使GHz高重频飞秒光纤激光面临脉冲形成机理不清楚和相位噪声劣化两方面问题。本项目分别以增益滤波和吸收体的非线性相移作为脉冲形成的主因开展研究。首先，通过建立动态速率方程和非线性薛定谔方程的关联，提出一种适用于高重频激光的复合模型，揭示增益滤波在脉冲形成中发挥的作用；其次，在复合模型中引入线宽增强因子，研究吸收体的非线性相移对脉冲形成机理的影响，揭示新机理下脉冲宽度的影响规律，开启同步实现飞秒脉宽和GHz重频的新途径；最后，采用复合石英光纤构建吸收体的饱和效应主导的超快激光实验，实现重频>3GHz、脉宽<200fs的激光输出。用实现的窄脉宽抑制量子噪声，结合锁相环技术抑制环境噪声，以实现量子极限的相位噪声。本项目为获得稳定的高重频飞秒激光源提供新思路和理论依据。

针对GHz高重频光纤激光面临的脉冲宽度宽、噪声劣化以及不稳定机制增多的问题，本项目开展了理论和实验两方面的研究：在理论方面，利用非线性薛定谔方程和增益动态速率方程构建复合模型，研究了GHz高重频超短脉冲光纤激光的孤子动力学模型和锁模前后的孤子动力学演化规律。分析了增益和孤子扰动引起的脉冲不稳定机制，发现了存在于直流锁模内部的新型脉动不稳定机制。研究了谐振腔参数对超短脉冲激光源的光谱和脉冲的影响，揭示了光谱保持能力和脉冲宽度的演变规律。在实验方面，实现了1μm波段基础重复频率1.6GHz、脉冲宽度814fs、相位噪声时间抖动291fs（积分范围1kHz–10MHz）、以及相对强度噪声0.08%的全保偏低噪声全光纤脉冲激光输出。通过旋转保偏镀膜插芯和保偏增益光纤插芯的夹角，可以实现锁模光谱形状的调控机制。此外，实现了基础重复频率3.3GHz、脉冲宽度266fs、相对强度噪声0.09%、长期稳定运转的高重频耗散孤子。该项目的结果不仅解决了为此类光源此一直面临的上述科学问题，而且为其进一步发展提供了理论依据和新思路。