Tunable passively Q-switched Dy³⁺-doped fiber laser from 2.71 to 3.08  μm using PbS nanoparticles

We present the first widely tunable passively Q-switched Dy³⁺-doped ZBLAN fiber laser around 3 μm pumped at 1.1 μm using PbS nanoparticles as the saturable absorber (SA) to our knowledge. At 2.87 μm, the modulation depth and saturation intensity of the SA were measured to be 12.5% and 1.10  MW/cm², respectively. Stable Q-switching was achieved over a wavelength range of 2.71-3.08 μm (∼370  nm), which is a record tuning range from a pulsed rare-earth-doped fiber laser, to our knowledge. A maximum output power of 252.7 mW was obtained, with a pulse energy of 1.51 μJ, a pulse width of 795 ns, and a repetition rate of 166.8 kHz. This demonstration implies that Dy³⁺ is a promising gain medium for tunable pulsed sources in the 3 μm band and shows for the first time, to the best of our knowledge, the potential of PbS as a mid-infrared SA.