飞秒极端脉动孤子的产生与控制

Discoveries of new types of solitons contribute significantly to the development of femtosecond sources. Femtosecond extremely pulsating solitons as a new type of soliton, exhibit excellent properties including high energy, low noise and self-tuning of its parameters in a wide range, which can have potential applications in frequency combs and tunable THz emission. How to generate such nonlinear pulses is an open question. This project will build discrete model based on nonlinear Schrödinger equation and experimentally use passively mode-locked fiber laser as a platform to focus on physical mechanisms accounting for the generation of extremely pulsating solitons; since current methods fail to capture the dynamic spectrum of such solitons, dispersive Fourier transformation technology will be employed to solve this issue; besides, traditional method used to characterize the coherence of femtosecond lasers is complex and cannot measure coherence in real time, this project will propose a new method to measure coherence in real time. In short, this project will aim at mastering the principle and technique of designing femtosecond extremely pulsating soliton laser and paving the way for its applications.

飞秒光源的发展与新型孤子的发现密不可分。飞秒极端脉动孤子是一类新型非线性脉冲，具有高能量低噪声，脉冲参数可大范围自调谐的优异性质，在飞秒光梳以及可调谐太赫兹领域有潜在应用前景。如何从实验上产生这类非线性脉冲是一个开放性问题。本项目将建立基于非线性薛定谔方程的分立模型，实验上利用被动锁模光纤激光器为操作平台，重点研究飞秒极端脉动孤子产生的物理机制；针对当前探测设备无法实时测量极端脉动孤子动态光谱的问题，提出利用色散傅里叶变换法解决实时探测问题；此外，传统飞秒激光器相干性测量方法复杂且无法实时测量，本项目提出一种可实时探测相干性的方法。通过本项目的研究将全面掌握飞秒极端脉动孤子激光器设计的原理与技术，为其在相关领域的应用打下基础。

经典教科书认为锁模激光器有两个泵浦阈值，第一阈值(First threshold)和第二阈值(Second threshold)。当泵浦达到第一阈值时，激光器产生连续光。当泵浦达到第二阈值时，激光器会锁模，输出超短脉冲。然而，本项目发现在这两个泵浦阈值之间还存在另一个激光状态叫呼吸子激光。但是呼吸子随着时间快速周期性变化，传统探测技术响应速度太慢，根本无法探测到呼吸子。本项目研制了超快光谱探测技术，实验上揭示了呼吸子时频演化动力学特性，发现呼吸子的能量，谱宽，脉宽呈同步变化(Peng et al., Sci. Adv. 5 (11), eaax1110 (2019))。当激光器工作在双呼吸子模式时，实验发现双呼吸子很难同步，无法形成稳定的束缚态。本项目通过精密色散调控，最终实现同步双呼吸子，产生了呼吸子分子。呼吸子激光器打破了传统锁模激光器能量的均匀化分布，某些呼吸子具有极高的能量，这为高能量激光器的设计提供了新思路。该工作为在光学领域及其它耗散系统产生呼吸子以及呼吸子分子提供了线索。