The fluoride fiber laser with the longest emission wavelength, the Ho(3+)-transition at 3.9 mum in the attenuation minimum of the 3-5-mum atmospheric window, is characterized. After reviewing the importance of fluoride fibers due to their low phonon energies, we describe room-temperature fluorescence and laser action with liquid-nitrogen cooling. Continuous-wave laser action at 3.9 mum is presented for the 640- and the 890-nm pump ranges. A shift of the emission wavelength is achieved by varying the resonator mirrors. Laser characteristics and temperature dependence are discussed.

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