Picosecond passively mode-locked mid-infrared fiber laser

Mode-locked mid-infrared (mid-IR) fiber lasers are of increasing interest due to their many potential applications in spectroscopic sensors, infrared countermeasures, laser surgery, and high-efficiency pump sources for nonlinear wavelength convertors. Er³⁺-doped ZBLAN (ZrF₄-BaF ₂-LaF₃-AlF₃-NaF) fiber lasers, which can emit mid-IR light at 2.65-2.9 μm through the transition from the upper energy level ⁴I_11/2 to the lower laser level ⁴I _13/2, have attracted much attention because of their broad emission range, high optical efficiency, and the ready availability of diode pump lasers at the two absorption peaks of Er³⁺ ions (975 nm and 976 nm). In recent years, significant progress on high power Er³⁺- doped ZBLAN fiber lasers has been achieved and over 20 watt cw output at 2.8 μm has been demonstrated; however, there has been little progress on ultrafast mid-IR ZBLAN fiber lasers to date. We report a passively mode-locked Er³⁺- doped ZBLAN fiber laser in which a Fe²⁺:ZnSe crystal was used as the intracavity saturable absorber. Fe²⁺:ZnSe is an ideal material for mid-IR laser pulse generation because of its large saturable absorption cross-section and small saturation energy along with the excellent opto-mechanical (damage threshold ∼2 J/cm²) and physical characteristics of the crystalline ZnSe host. A 1.6 m double-clad 8 mol% Er ³⁺-doped ZBLAN fiber was used in our experiment. The fiber core has a diameter of 15 μm and a numerical aperture (NA) of 0.1. The inner circular cladding has a diameter of 125 μm and an NA of 0.5. Both continuous-wave and Q-switched mode-locking pulses at 2.8 μm were obtained. Continuous-wave mode locking operation with a pulse duration of 19 ps and an average power of 51 mW were achieved when a collimated beam traversed the Fe²⁺:ZnSe crystal. When the cavity was modified to provide a focused beam at the Fe ²⁺:ZnSe crystal, Q-switched mode-locked operation with a pulse duration of 60 ps and an average power of 4.6 mW was achieved. More powerful and narrower pulses are expected if the dispersion of the cavity can be properly managed.