Calcium-molybdenum-cobalt-2-methylimidazole trimetallic organic framework as a saturable absorber for passive mode-locked thulium-holmium doped fiber lasers.

Ultrafast fiber lasers operating in the 2 μm spectral region require robust and highly nonlinear saturable absorbers (SAs) to achieve stable mode-locking. This study demonstrates that a calcium-molybdenum-cobalt-2-methylimidazole (CaMoCo-MIM), a trimetallic organic framework (TMOF), functions as an efficient SA, enabling stable and self-starting mode-locking pulse in a thulium-holmium-doped fiber laser (THDFL). The CaMoCo-MIM exhibits excellent nonlinear optical performance, achieving a high modulation depth of 36.2%, surpassing many previously reported MOF-based SAs, as supported by the comparative data presented in this study. Stable soliton pulse generation was realized at a pump power of 307 mW, producing a central wavelength of 1895.21 nm, a 3 dB spectral bandwidth of 3.03 nm, and a pulse duration of 1.25 ps. A time-bandwidth product (TBP) of 0.316 indicates a nearly chirp-free pulse. Long-term operation over four hours confirmed robust stability with a constant repetition rate of 14.6 MHz. This TMOF establishes a high-modulation-depth SA in next-generation lasers.