Narrow pulse width mode-locked mid-infrared 2.8 μm fiber laser based on a weakly absorbing heterojunction saturable absorber

To address the lack of suitable saturable absorbers for weak absorption in mid-infrared 2.8 μm mode-locked lasers with weak gain, this study proposes a method to achieve weak absorption by modifying the band structure using a heterojunction. Benefiting from the transition mechanism associated with interlayer coupling, the MoTe 2 /MoS 2 heterostructure exhibits weak absorption with moderate modulation depth of 5.7% and a low saturation intensity of 0.59 MW/cm 2 . Based on the saturable absorber, the laser operation evolves from continuous-wave to Q-switched mode-locking and eventually to stable continuous-wave mode-locking within the single cavity. The mode-locked laser has a pulse width as narrow as 8.68 ps and operates at a repetition frequency of 43.5 MHz, and the maximum average output power of 201.8 mW with a signal-to-noise ratio of 59.6 dB. The obtained pulse width is comparable to those of previously reported mode-locked fiber lasers operating in this wavelength region. The generation of ultrashort pulses demonstrates that the weak absorption induced by heterojunctions is effective for achieving high-quality mid-infrared mode-locking.