Coexistence of noise-like pulse and high repetition rate harmonic mode-locking in a dual-wavelength mode-locked Tm-doped fiber laser

Coexistence of harmonic mode-locking (HML) and noise-like pulse (NLP) were experimentally observed in a dual-wavelength mode-locked Tm-doped fiber laser for the first time. The coexistence patterns were self-started and maintained within a wide pump range by appropriately setting the intra-cavity polarization state. The HML was obtained at 1955.3 nm with a varied repetition rate range from 324 MHz to 1.138 GHz which benefits from the dispersion compensation, while the NLP observing at 1983.2 nm can operate at either fundamental repetition rate of 4.765 MHz or second harmonic state. Experimental investigations show that the coexistence patterns are caused by the wavelength-dependent phase delay of the mode-locked fiber cavity. Moreover, dual-wavelength NLP was also observed for the first time at 2 μm spectral region by changing the intra-cavity polarization state.