Widely tunable single-frequency Er-doped ZBLAN fiber laser with emission from 3.37 to 3.72 µm

We demonstrate a widely tunable single-frequency Er-doped ZBLAN fiber laser operating on a ⁴F_9/2→⁴I_9/2 transition band. An uncoated germanium (Ge) plate serves as a narrow-bandwidth etalon and is inserted in the cavity to achieve a single longitudinal mode selection. Wavelength tuning from 3373.8 nm to 3718.5 nm was demonstrated by using a blazed diffraction grating at 3.5 µm. At the emission peak of 3465.6 nm, the laser yields over 100 mW single-frequency output power, with a 3 dB linewidth <6.9 MHz, and a slope efficiency (with respect to the incident 1990 nm pump power) of 20.3%. Such a tunable mid-infrared single-frequency fiber laser may serve as a versatile laser source in spectroscopy and sensing applications.