Laser diode-pumped holmium-doped fluorozirconate glass fiber laser in the green (λ∼544-549 nm): power conversion efficiency, pump acceptance bandwidth, and excited-state kinetics

The green (544-549 nm) Ho-doped fluorozirconate (ZBLAN) glass fiber laser, pumped in the red (/spl lambda//spl sim/6;15 nm) by a high-power (/spl sim/30 mW) InGaAlP laser diode or a ring dye-laser, has been characterized with regard to power conversion efficiency, fiber core-diameter and length, cavity output coupling, and pump acceptance bandwidth. Fibers doped with /spl sim/1200 ppm (by weight) of Ho and having core diameters of 1.7, 3, and 11 /spl mu/m, and lengths ranging from 12.5 to 86 cm, have been studied in Fabry-Perot resonators having output couplings ranging from 1.545 to 96%. For a 1.7-/spl mu/m core-diameter fiber, 21 cm in length, the threshold-launched pump power for the diode-pumped fiber laser is 1.9 and 3.5 mW for cavity output couplings of 1.5% and 24%, respectively. These values are the lowest for any upconversion-pumped fiber laser reported to date. Also, the noise and threshold-pumping power properties of the diode-pumped fiber laser are superior to those for its dye-laser-pumped counterpart. The highest laser slope efficiency (>22% with respect to launched pump power) was measured for a 3-/spl mu/m core-diameter fiber and a cavity output coupling of 24%. The spectral interval over which the launched threshold pump power for this laser is <10 mW is almost 20 nm (637-656 nm). Studies of the fiber laser waveform as a function of pump power reveal competition for population between the /sup 5/S/sup 2/ and /sup 5/F/sub 4/ states and among the Stark sublevels of the /sup 5/F/sub 4/ manifold. Also, measurements of the output power on individual laser lines of the /sup 5/F/sub 4/, /sup 5/S/sub 2//spl rarr//sup 5/I/sub 8/ (ground) transitions of Ho/sup 3+/:ZBLAN as a function of pump power demonstrate the existence of a loss mechanism at the fiber laser wavelength, presumably due to absorption from ground or the /sup 5/I/sub y/, /sup 6/S/sub 2/ or /sup 5/F/sub 4/ excited states of the ion.