Widely tunable passively Q-switched Er-doped ZrFfiber laser in the range of 3.4–3.7 μm based on a Fe:ZnSe crystal

We report the first（to the best of our knowledge） tunable passively Q-switched Er3+-doped ZrF4 fiber laser around 3.5 μm. In this case, a Fe2+:ZnSe crystal is used as the saturable absorber, and a plane-ruled grating in a Littrow configuration acts as the tuning element. At the tuned wavelength of 3478.0 nm, stable Q-switching with a maximum average power of 583.7 mW was achieved with a slope efficiency of 15.2% relative to the launched 1981 nm pump power. Further power scaling is mainly limited by the available 1981 nm pump power. The corresponding pulse width, repetition rate, and pulse energy are 1.18 μs, 71.43 kHz, and 7.54 μJ, respectively. By rotating the grating, the Q-switching can be continuously tuned in the region of 3.4–3.7 μm. To the best of our knowledge, this is the first pulsed rare-earth-doped fiber laser tunable in the region beyond 3.4 μm.