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.

超快激光器已成为各个领域的强大工具，将其基本重复率提高到千兆赫 (GHz) 水平，为前沿科学和工业应用带来了巨大潜力。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鲁棒性而有望产生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，导致GHz光纤激光器的锁模机制存在悬而未决的问题。在这里，我们通过探索动态增益损耗和恢复（GDR）效应来研究GHz光纤激光器中的被动锁模，并建立了一个理论模型来全面理解其具有多GHz基本重复率的低阈值锁模机制。具体来说，GDR效应产生有效的相互作用力，从而结合数GHz孤子形成孤子晶体的对应物。研究发现，由此产生的孤子集体行为有效​​地降低了增益光纤的饱和能量，并允许连续波锁模（CWML）的脉冲能量降低几个数量级。还提出了一种在强相关长度中定义的准单孤子的新概念，以深入了解孤子组装的动力学，从而实现从当前锁模理论到现有锁模理论的交叉。具体来说，通过使用标准实时示波器和新兴时间的测量，在锁模 GHz 光纤激光器中理论表明并实验验证了分别呈现矩形和高斯形状包络的 Q 开关锁模的两种不同动力学- 镜头放大倍率。 基于所提出的 CWML 标准，我们最终实现了 GDR 介导的锁模光纤激光器，其基本重复频率高达 21 GHz，信噪比高达 85.9 dB。