A femtosecond pulse fiber laser at 1935 nm Using a bulk-structured Bi ₂Te₃ topological insulator

We experimentally demonstrate a femtosecond mode-locked, allfiberized laser that operates in the 2 μm region and that incorporates a saturable absorber based on a bulk-structured bismuth telluride (Bi₂Te₃) topological insulator (TI). Our fiberized saturable absorber was prepared by depositing a mechanically exfoliated, ∼30 μm-thick Bi₂Te ₃ TI layer on a side-polished optical fiber platform. The bulk crystalline structure of the prepared Bi₂Te₃ layer was confirmed by Raman and X-ray photoelectron spectroscopy measurements. The modulation depth of the prepared saturable absorber was measured to be ∼20.6%. Using the saturable absorber, it is shown that stable, ultrafast pulses with a temporal width of ∼795 fs could readily be generated at a wavelength of 1935 nm from a thulium/holmium co-doped fiber ring cavity. This experimental demonstration confirms that bulk structured, TI-based saturable absorbers can readily be used as an ultra-fast mode-locker for 2 μm lasers.