GHz high-repetition-rate femtosecond fiber lasers integrate the characteristics of femtosecond-level ultra-short pulses with GHz-level high repetition rate, playing an irreplaceable and core supporting role in key fields such as optical frequency comb construction, precise micro-nano processing, high-speed biological imaging, terahertz technology, and aerospace communication. Their development has resolved the long-standing contradiction between high quality and high efficiency in traditional femtosecond laser processing while promoting innovations in frontier disciplines such as frequency metrology and ultrafast science. It holds significant strategic importance for the domestic substitution of high-end manufacturing, breakthroughs in frontier research, and the upgrading of information technology. It is currently one of the key research hotspots and core development directions in the field of ultrafast lasers.

Passive mode-locking, due to its simple structure and excellent stability, has become the mainstream solution for generating GHz-level ultra-short pulses. Among them, fundamental frequency mode-locking dominates the preparation of high-reliability and high-repetition-frequency lasers owing to its low noise and high stability advantages.

In the 1 μm band, in 2012, the research group led by KÄRTNER developed a fundamental mode-locked Yb-doped fiber laser with a repetition frequency of 3 GHz and a pulse duration of approximately 206 fs (Fig.6). In 2019, the research team led by YANG achieved a breakthrough in parameters with tens of GHz repetition frequency using semiconductor saturable absorption mirror (SESAM) mode-locking, obtaining fundamental repetition frequencies of 3.1 GHz, 7.0 GHz, and 12.5 GHz in passive mode-locked ytterbium fiber lasers (Fig.8). It is known that the 12.5 GHz fundamental repetition frequency is the maximum for 1 μm mode-locked fiber lasers; In 2024, the research group LED by ZHANG achieved a synergistic breakthrough of 1.2 GHz repetition frequency and 38 fs pulse width using nonlinear polarization evolution(NPE) mode-locking (Fig.3). The spectral width exceeds 70 nm, with an average output power greater than 1 W. This represents the highest repetition frequency and shortest pulse width achieved so far in Yb-doped NPE mode-locked lasers.

In the 1.5 μm band, in 2011, YAMASHITA achieved all-fiber lasers with fundamental repetition frequencies of 4.24 GHz, 9.63 GHz, and 19.45 GHz using carbon nanotubes (CNT) saturable absorption mode-locking (Fig.10). In 2021, the team led by YANG achieved a breakthrough of 4.9 GHz repetition frequency and 63 fs pulse width using SESAM mode-locking. In 2024, the team developed an all-fiber laser with a fundamental repetition frequency as high as 21 GHz (Fig.14). In 2025, the ZHANG research group reported the first 1 GHz femtosecond ring cavity erbium-doped fiber laser using NPE mode-locking, which directly generated an 81.8 fs pulse width without external cavity compression. This represents the highest repetition rate achieved in a ring cavity erbium-doped fiber laser (Fig.16).

Currently, continuous improvements in laser performance have been made through techniques such as resonator cavity length reduction, precise dispersion management, and nonlinear control, laying a solid foundation for applications in multiple fields.

In conclusion, significant progress has been made in GHz high-repetition-frequency femtosecond fiber lasers in aspects such as mode-locking technology, band expansion, and parameter optimization. However, they still face core challenges, including the suppression of nonlinear effects in short cavities, the synergistic optimization of high power and narrow pulse width, and the long-term stability of the system. Future research should focus on the development of core components such as highly doped gain fibers, breakthroughs in all-fiber integrated structures, and a deeper integration of “light source-application” coordinated design. It is essential to promote their in-depth integration with downstream fields such as optical atomic clocks and dual-comb spectroscopy, thereby facilitating their broader commercialization in industries, healthcare, and precise measurement. This will provide core light source support for the high-quality development of related industries and disciplines.