1.7 μm period-doubling mode-locked fiber laser (<italic>invited</italic>)

ObjectiveMode-locked fiber lasers operating in the 1.7 μm wavelength region have attracted increasing interest due to their important applications in biomedical treatment, gas sensing, and mid-infrared spectroscopy. However, the 1.7 μm band does not correspond to a typical emission region of rare-earth-doped fibers. Because of the strong reabsorption effects and gain competition, the mode-locking operation states and pulse dynamics are more complex. As an important phenomenon in nonlinear dissipative systems, the period-doubling phenomenon can reflect the stability evolution and intrinsic dynamical mechanisms of mode-locked fiber lasers under parameter modulation. However, existing studies are mainly concentrated in the 1.0 μm and 1.5 μm wavelength regions, and experimental reports in the 1.7 μm band are still limited. This paper aims to construct a 1.7 μm mode-locked fiber laser based on nonlinear optical loop mirror (NOLM), focusing on the period-doubling dynamics of dissipative solitons and bound solitons. It reveals the generation conditions and physical mechanisms of the period-doubling phenomenon, providing experimental basis for a deeper understanding of the nonlinear evolution law and performance optimization of 1.7 μm band mode-locked fiber lasers.MethodsA NOLM-based all-fiber laser using thulium-doped fiber as the gain medium was constructed. A bandpass filter was employed to suppress long-wavelength amplified spontaneous emission, while an ultra-high numerical aperture fiber was introduced for intracavity dispersion management. Stable dissipative soliton operation was achieved by adjusting the pump power and intracavity polarization state. The period-doubling regimes were induced through further pump enhancement and fine polarization tuning. The laser dynamics were systematically characterized using an optical spectrum analyzer, oscilloscope, radio-frequency spectrum analyzer, autocorrelator, and time-stretched dispersive Fourier transform (TS-DFT) technique.Results and DiscussionsStable dissipative soliton operation with a central wavelength of 1737.76 nm and a 3-dB spectral bandwidth of 15.13 nm was achieved, corresponding to a pulse duration of 6.88 ps. With increasing pump power and appropriate polarization adjustment, clear period-doubling behavior was observed, characterized by alternating pulse intensities in the pulse train and a distinct spectral peak at half of the fundamental repetition frequency in the RF spectrum. TS-DFT measurements further revealed periodic variations in spectral intensity and shape under the period-doubling regime. At higher pump powers, bound-state solitons and their corresponding period-doubling states were also obtained, with an inter-pulse separation of 42.07 ps and good operational stability. These results indicate that the period-doubling phenomenon originates from enhanced nonlinear phase shifts and strengthened effective saturable absorption modulation induced by pump power and polarization variations.ConclusionsIn conclusion, the period-doubling dynamics of dissipative solitons and bound-state solitons in a NOLM-based 1.7 μm mode-locked fiber laser have been experimentally demonstrated and systematically analyzed. The results enrich the understanding of nonlinear pulse dynamics at 1.7 μm and provide an experimental foundation for further performance optimization and application development of mid-infrared mode-locked fiber lasers.