Optimizing a 1.9 μm mode-locked laser by controlling the concentration of reduced graphene oxide-magnesium oxide

In this work, a pulsed laser was achieved through mode -locking in a Thulium/Holmium co-doped fiber laser (THDFL) utilizing a saturable absorber (SA) made of reduced graphene oxide/magnesium oxide (rGO/MgO) composite. The composite material was first synthesized using a simple hydrothermal method with graphene oxide and magnesium chloride as the precursor for rGO and MgO, respectively. Pulsed fiber laser sources have extensive applications in a diverse array of material diagnostics, encompassing the examination of semiconductor materials employed in telecommunications components, as well as the analysis of biomedical phenomena. The influence of SA concentration on its performance and its impact on pulse generation due to nonlinear interactions were explored. The results demonstrated that the 5 mg/mL rGO/MgO SA exhibited a modulation depth of 46.32 %, while the 10 mg/mL rGO/MgO SA had a value of 51.93 %. As the rGO/MgO concentrations increased from 5 mg/mL to 10 mg/mL, the central wavelength of the stable mode -locking experienced a blueshift from 1907 nm to 1902 nm. Notably, the 10 mg/mL rGO/MgO-based SA outperformed the 5 mg/mL counterpart in various aspects, including signal-to-noise ratio (SNR), pulse duration, average power, pulse energy, peak power, and efficiency. This investigation sheds light on the critical role of SA concentration in optimizing the performance of modelocked THDFL, providing valuable insights for the development of advanced ultrafast laser sources for diverse applications.