Femtosecond Tm-Ho co-doped fiber laser using a bulk-structured Bi₂Se₃ topological insulator

We experimentally demonstrate a femtosecond mode-locked thulium-holmium (Tm-Ho) co-doped fiber laser incorporating a saturable absorber (SA) based on a bulk-structured bismuth selenide (Bi₂Se₃) topological insulator (TI). The SA was prepared by depositing a mechanically exfoliated Bi₂Se₃ TI layer onto a side-polished optical fiber platform. Unlike high-quality nano-structured Bi₂Se₃ TI-based SA, bulk-structured Bi₂Se₃ with non-negligible oxidation was used as a saturable absorption material for this experimental demonstration due to its easy fabrication process. The saturation power and modulation depth of the prepared SA were measured to be ~28.6 W and ~13.4%, respectively. By incorporating the prepared SA into a Tm-Ho co-doped fiber ring cavity, stable soliton pulses with a temporal width of ~853 fs could be generated at 1912.12 nm. The 3-dB bandwidth of the mode-locked pulse was measured to be ~4.87 nm. This experimental demonstration reaffirms that Bi₂Se₃ is a superb base material for mid-infrared passive mode-locking even under oxidation.