In recent years, high-energy, ultrafast, mode-locked, 2-μm fiber laser systems have been widely applied in fields such as laser sensing, mid-infrared spectroscopy, material processing, and free-space optical communication. The generation of such mode-locked pulse outputs is primarily achieved through saturable absorber technology. Among these, Figure-9 mode-locked fiber lasers based on the nonlinear amplifying loop mirror (NALM) have the advantage of incorporating a phase shifter, which facilitates the self-starting of the laser and allows it to maintain a long-term stable mode-locking state. This enables the laser to produce high-energy outputs while effectively controlling nonlinear effects, leading to the more stable and efficient performance. By precisely adjusting the fiber length and pump power within the Figure-9 cavity, the stable, high pulse energy output can be achieved while maintaining a low repetition frequency. The repetition rates of 2 μm Figure-9 oscillators typically range from the MHz to tens of MHz levels. Such high repetition rates are unfavorable for achieving high pulse energy. Additionally, although Figure-9 mode-locked fiber lasers operating in dissipative soliton resonance mode can achieve single-pulse energy outputs ranging from hundreds of nJ to mJ, the pulse width is usually on the ns scale. However, applications such as micromachining, precision ranging, and medical fields typically require ps-level pulse widths. Therefore, achieving high-energy picosecond laser pulse output is critical in Figure-9 thulium-doped mode-locked fiber lasers.

First, a Figure-9 mode-locked fiber laser oscillator based on NALM is constructed. Ensuring that the laser oscillator can achieve stable self-starting mode locking, the lengths of the fiber segments within the cavity are adjusted to manage the dispersion characteristics of the cavity. The aim is to control the temporal and spectral widths of the output pulse, thereby minimizing pulse distortion during subsequent amplification. Subsequently, an external pulse picker is used to reduce the repetition rate of the output pulses of the oscillator from the MHz to kHz range. Finally, a two-stage master oscillator power amplifier (MOPA) scheme is adopted to increase the output pulse energy. The first-stage amplifier boosts the seed pulse power to ensure effective injection into the second-stage amplifier. The second-stage amplifier employs high-power pumping and carefully selects thulium-doped gain fibers to maximize energy amplification while maintaining good beam quality. By appropriately distributing the pump power between the two amplifiers, nonlinear effects such as self-phase modulation (SPM) and stimulated Raman scattering (SRS) are effectively controlled, ensuring high-energy output while preserving pulse quality.

By precisely managing the dispersion and nonlinearity within the Figure-9 laser oscillator cavity, a pulse output with a repetition rate of 20 MHz, pulse width of 46 ps, spectral center wavelength of 1950 nm, 3-dB spectral width of 0.09 nm, and maximum average output power of 24.6 mW, corresponding to a single pulse energy of 1.23 nJ, is achieved (Fig. 3). An external pulse picker reduces the output pulse repetition rate of the oscillator from 20 MHz to 20 kHz (Fig. 5). By adopting the MOPA scheme, the average output power increases to 1 W at a repetition rate of 20 kHz, corresponding to single-pulse energy of 50 μJ (Fig. 8). The power stability over 3 h at the maximum average output power of 1 W is measured, and the root mean square (RMS) jitter is calculated to be 1.485%, indicating that the entire laser system operates in a stable state (Fig. 9).

We report the low-repetition-rate, high-energy Figure-9 femtosecond thulium-doped mode-locked fiber laser system based on NALM. By taking an external acousto-optic modulator (AOM) as the pulse picker, the repetition rate is reduced from 20 MHz to 20 kHz. Additionally, using a self-designed all-fiber MOPA structure, the single-pulse energy at 20 kHz repetition rate increases to 50 μJ. We believe that further optimization of the dispersion and nonlinearity of the seed laser, along with improvements to the all-fiber MOPA structure, can further reduce nonlinear effects during the amplification process. For instance, advanced dispersion compensation techniques or hybrid amplification schemes can potentially enable even higher single-pulse energy outputs in the future.