Advances in mid-infrared fiber lasers

The current state of the art in mid-infrared fiber laser research is reviewed. The relevant glass and ceramic fiber host matkrials and the fiber, pump, and resonator geometries are introduced. Lasers operating on transitions ranging from 1.9 to 4 pm occurring in the rare-earth-ions $Tm^{3+}$, $HO^{3+}$,$Er^{3+}$ and $Dy^{3+}$ and their population mechanisms are discussed on the basis of the fundamental spectroscopic properties of these ions. Continuous-wave fundamental-mode power levels ranging from a few mW near 4 pm up to 85 W near 2 pm have been demonstrated in recent years. Power-scaling methods and their limitations, the possibilities to optimize the population mechanisms and increase the efficiencies of these lasers, novel concepts, as well as the prospects of future mid-infrared fiber lasers at transitions in the wavelength range beyond 3 pm and extending to 5 pm are described.