High-Power Broadly Tunable 3- $\mu\hbox{m}$ Fiber Laser for the Measurement of Optical Fiber Loss

We demonstrate a
$\hbox{Ho}^{3+}$
,
$\hbox{Pr}^{3+} $
co-doped fluoride fiber laser that produces an output power of 7.2 W, a 150-nm tuning range, and a narrow linewidth of
$<\ 0.14$
nm using a power-scalable
$\hbox{Yb}^{3+} $
fiber-laser-pumped 1150-nm Raman fiber laser as the excitation source. The maximum output power is achieved at a slope efficiency of 29%, with wavelength tuning between 2825 and 2975 nm that overlaps with the OH absorption region in many mid-infrared transparent glasses. The system is used to provide accurate measurements of the background scattering loss and the degree of water incorporation in the rare-earth-doped core of commercial double-clad fluoride fiber; the high power allows accurate core loss measurements for quite lossy fibers. Furthermore, the spectral location of the OH absorption feature was observed to be dependent on glass composition shifting from 2872 nm in undoped ZBLAN to 2896 nm upon
$\hbox{Ho}^{3+}\hbox{Pr}^{3+}$
co-doping, whereas
$\hbox{As}_{2}\hbox{S}_{3} $
glass was observed to have an OH peak location of 2911 nm.