Significance High-power Ytterbium-doped femtosecond fiber lasers have rapidly developed over the past few decades due to their all-fiber structural design, excellent beam quality, and reliable system stability. They are widely used in industrial processing, biomedicine, military defense, and frontier science. Fiber lasers utilizing chirped pulse amplification (CPA) technology are the predominant solution for these applications. Although fiber laser systems based on CPA are capable of generating femtosecond pulses with mJ level pulse energy, the pulse widths are typically above 200 fs. Nevertheless, a wide variety of applications require relatively low pulse energy and significantly shorter pulse durations. To date, the technique of nonlinear pulse amplification (NPA) has been employed to achieve femtosecond pulses with durations less than 50 fs. NPA technology includes self-similar pulse amplification, pre-chirp management amplification, and gain management nonlinear (GMN) amplification. In self-similar pulse amplification, however, the spectrum tends to exceed the gain bandwidth at higher energies, resulting in reduced pulse quality after compression. Pre-chirp managed amplification often requires precise control of the seed pulse dispersion, increasing system complexity and reducing stability. Compared to the first two technologies, GMN technology relies on nonlinear attractors and is insensitive to the temporal distribution of the seed pulse. Additionally, it surpasses the limitations of the gain narrowing effect, producing highly compressible pulses with spectral bandwidths exceeding hundreds of nanometers. Compared to CPA technology, the GMN technology does not require additional stretchers, ensuring an extremely compact structure.
Progress Firstly, the mechanism of GMN amplification technology is detailed. GMN amplification primarily utilizes the dynamically evolving gain spectrum as a degree of freedom, generating spectra that far exceed the gain spectrum bandwidth while maintaining an approximately linear chirp and can be compressed to ~50 fs. In GMN amplifiers, when the nonlinear phase shift accumulates to a certain extent, spatiotemporal deterioration (STD) occurs, resulting in a rapid decline in pulse quality and a sharp increase in pulse duration after compression. Optimizing the pump and seed source configurations can further increase the STD threshold and enhance GMN amplifier performance.
Secondly, the research progress of high peak power femtosecond fiber lasers based on GMN amplification technology, both domestically and internationally, is summarized from different design structures. Currently, two main design structures of GMN systems have been reported. One design involves pre-compressing the seeds output by the oscillator through a space compression device, injecting them into the fiber amplifier for GMN amplification, and then de-chirping through the space compression device. The other design involves pre-compressing the seeds by the optical fiber device, injecting them into the optical fiber amplifier for GMN amplification, and then de-chirping through the spatial compression device. This design maintains the advantages of the all-fiber structure and then de-chirps through the spatial compression device. Although some GMN systems use fiber optic devices as pre-compressors, the main compressor is still a free-space device. Additionally, our research group demonstrated the design structure of an all-fiber integrated GMN system. Hollow-core photonic-bandgap (HC-PBG) fiber is used to replace the grating pair compressor. This structure significantly simplifies the complexity of the GMN system.
Finally, the expansion of GMN amplification system applications is analyzed. Currently, GMN amplification systems are widely used in various studies, such as optical parametric amplification and multi-modal nonlinear optical imaging. Additionally, our research group demonstrated the use of the GMN system for application research on supercontinuum light source generation.
Conclusions and Prospects GMN amplification technology has garnered widespread attention since its proposal in 2019. This paper details the STD and influencing factors of the GMN amplifier, proposes a GMN system with an all-fiber integrated structure. The system is compact and can output a femtosecond laser with a pulse duration of 45 femtoseconds, a single pulse energy of 163 nanojoules, and a peak power of 3.6 megawatts. Due to its advantages of short pulses and high peak power, GMN amplification systems are employed in research on optical parametric systems, nonlinear optical imaging, and supercontinuum light sources. As laser technology advances, GMN systems will demonstrate significant value in fields such as high-precision processing, biomedical imaging, and frontier science.
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