Mode-locked 2 μm laser with highly thulium-doped silicate fiber

We report self-starting passively mode-locked fiber lasers with a saturable absorber mirror using a piece of 30-cm-long newly developed highly thulium (Tm)-doped silicate glass fibers. The mode-locked pulses operate at 1980 nm with duration of 1.5 ps and energy of 0.76 nJ. This newly developed Tm-doped silicate fiber exhibits a slope efficiency of 68.3%, an amplified spontaneous emission spectrum bandwidth (FWHM) of 92 nm, and a gain per unit length of greater than 2 dB/cm. To the best of our knowledge, it is the first demonstration of mode-locked $2\mu \text{m}$ fiber laser using shorter than 1-m-long active fiber, which paves the way for the demonstration of mode-locked fiber laser at $2\mu \text{m}$ with gigahertz fundamental repetition rate.