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• The multiple pulses mode-locking operations with different formation mechanisms are realized respectively in 1933 nm and 2000 nm. • Simulation agrees with the experimental results for the multiple pulses mode-locking operation. • The interaction of the background noises on the multiple pulses formation is theoretically analyzed. • The Tm-doped fiber oscillator delivers the > 400-MHz, > 300-mW, and less than 400-fs laser pulse in the 2000-nm wavelength region. The multiple pulse mode-locking operations with various pulse formation mechanisms are observed in a Tm-doped fiber oscillator with the nonlinear polarization rotation mode-locking method. In the 1933-nm wavelength region, the mode-locking operation evolves from the fundamental soliton to soliton molecules and further to noise-like pulses. Theoretical simulation revels that the interactions between background noises and multi-pulses contribute to the soliton molecules generation and noise-like pulse evolution. Simulated results match well with the experimental results. In the 2000-nm wavelength region, the multiple pulses, 4th-order harmonic solitons (409.6 MHz), are realized with the 355-mW average output power. The shortest pulse duration is 340 fs, resulting in a pulse peak power of >2 kW. [ABSTRACT FROM AUTHOR]

• First demonstration of an all-polarization-maintaining continuous-wave fiber master oscillator power amplifier (MOPA) system at ∼1540 nm. • Experimental results closely followed simulation results based on Erbium-Ytterbium co-doped fiber (EYDF) design. • Achieved >20 W output power, ∼1.4 kHz linewidth, excellent power stability (<0.2 % RMS), and low intensity noise (−110 dBc/Hz). We have developed both theoretically and experimentally an all polarization-maintaining fiber-based master oscillator power amplifier (MOPA) system, which allows for amplifying the single-frequency continuous-wave (CW) fiber laser to >20 W without compromising the laser intensity noise (−110 dBc/Hz@ relaxation oscillation peak) at the specific wavelength of ∼1540 nm. The experimental results agree well with the simulated results. To the best of our knowledge, this is the first demonstration of a ∼1540-nm single-frequency all fiber laser and reaching such a high-power level. The amplified laser features a narrow linewidth of ∼1.4 kHz and a spectral side-mode-suppression ratio of >38 dB, even as the laser wavelength is tuned from 1538.216 nm to 1540.616 nm. It is believed to be the narrowest laser linewidth realized in the <1550-nm wavelength band of an Erbium-Ytterbium co-doped fiber MOPA system. The all polarization-maintaining configuration of the MOPA ensures the amplified laser linearly polarized with the polarization extinction ratio of >22 dB and power root mean square fluctuations of <0.2 %@1h. This high-performance single-frequency fiber laser is expected to be an ideal pump source in the spin-exchange relaxation-free (SERF)-based atomic sensors after second harmonic generation. [ABSTRACT FROM AUTHOR]

• An adjustable Tm-doped gain-switched fiber laser with variable repetition frequency and pulse duration by implementing an adjustable QML self-pulsing fiber laser as the pumping laser for the first time. • A QML pulse with a maximum repetition frequency of 72.5 kHz is generated due to the weak NPR effect. • A stable gain-switched pulse with a minimum duration of 366.2 ns, and an adjustable repetition frequency of 37–73 kHz is generated. • Simulation agrees with the experimental results for the gain-switched fiber laser. This paper reports on the first gain-switched fiber laser in the 2 μm region that uses an adjustable self-pulsing Er-doped QML fiber laser as a pumping source. By adjusting the output of the self-pulsing laser, a stable gain-switched pulse with a minimum duration of 366.2 ns and an adjustable repetition frequency of 37–73 kHz at 1959 nm is generated. The maximum output power achieved is 26.5 mW, with a pulse energy of 0.35 μJ and a slope efficiency relative to the pumping power of 4.6 %. Our study demonstrates the potential of an adjustable self-pulsing fiber laser as an effective pumping source for gain-switched fiber lasers. [ABSTRACT FROM AUTHOR]