Chen Xuewen, Lin Wei, Hu Xu, Wang Wenlong, Liang Zhaoheng, Ling Lin, Yang Yang, Guo Yuankai, Liu Tao, Chen Dongdan, Wei Xiaoming, Yang Zhongmin
School of Physics and Optoelectronics; State Key Laboratory of Luminescent Materials and Devices; Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices; Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou, China.
Research Institute of Future Technology, South China Normal University, Guangzhou, Guangdong, China.
Light Sci Appl. 2024 Sep 20;13(1):265. doi: 10.1038/s41377-024-01613-z.
Ultrafast lasers have become powerful tools in various fields, and increasing their fundamental repetition rates to the gigahertz (GHz) level holds great potential for frontier scientific and industrial applications. Among various schemes, passive mode-locking in ultrashort-cavity fiber laser is promising for generating GHz ultrashort pulses (typically solitons), for its simplicity and robustness. However, its pulse energy is far lower than the critical value of the existing theory, leading to open questions on the mode-locking mechanism of GHz fiber lasers. Here, we study the passive mode-locking in GHz fiber lasers by exploring dynamic gain depletion and recovery (GDR) effect, and establish a theoretical model for comprehensively understanding its low-threshold mode-locking mechanism with multi-GHz fundamental repetition rates. Specifically, the GDR effect yields an effective interaction force and thereby binds multi-GHz solitons to form a counterpart of soliton crystals. It is found that the resulting collective behavior of the solitons effectively reduces the saturation energy of the gain fiber and permits orders of magnitude lower pulse energy for continuous-wave mode-locking (CWML). A new concept of quasi-single soliton defined in a strongly correlated length is also proposed to gain insight into the dynamics of soliton assembling, which enables the crossover from the present mode-locking theory to the existing one. Specifically, two distinguishing dynamics of Q-switched mode-locking that respectively exhibit rectangular- and Gaussian-shape envelopes are theoretically indicated and experimentally verified in the mode-locked GHz fiber laser through the measurements using both the standard real-time oscilloscope and emerging time-lens magnification. Based on the proposed criterion of CWML, we finally implement a GDR-mediated mode-locked fiber laser with an unprecedentedly high fundamental repetition rate of up to 21 GHz and a signal-to-noise ratio of 85.9 dB.
超快激光已成为各个领域的强大工具，将其基本重复频率提高到吉赫兹（GHz）水平在前沿科学和工业应用中具有巨大潜力。在各种方案中，超短腔光纤激光器中的被动锁模因其简单性和稳健性而有望产生GHz超短脉冲（通常是孤子）。然而，其脉冲能量远低于现有理论的临界值，这引发了关于GHz光纤激光器锁模机制的诸多问题。在此，我们通过探索动态增益耗尽与恢复（GDR）效应来研究GHz光纤激光器中的被动锁模，并建立一个理论模型，以全面理解其具有多GHz基本重复频率的低阈值锁模机制。具体而言，GDR效应产生有效相互作用力，从而束缚多GHz孤子形成孤子晶体的对应物。研究发现，孤子的这种集体行为有效降低了增益光纤的饱和能量，并使得连续波锁模（CWML）所需的脉冲能量降低了几个数量级。还提出了在强相关长度中定义的准单孤子这一新概念，以深入了解孤子组装的动力学，这使得从当前锁模理论到现有理论的转变成为可能。具体来说，理论上指出并在锁模GHz光纤激光器中通过使用标准实时示波器和新兴的时间透镜放大技术进行测量实验验证了分别呈现矩形和高斯形状包络的调Q锁模的两种不同动力学。基于所提出的CWML准则，我们最终实现了一种GDR介导的锁模光纤激光器，其具有高达21 GHz的前所未有的高基本重复频率和85.9 dB的信噪比。
