Excitation spectra of the green Ho:fluorozirconate glass fiber laser

Laser and fluorescence excitation spectra of the green Ho:ZBLAN glass fiber laser have been measured at room temperature by pumping 20-90-cm-long fibers in the 640-653-nm spectral region with a CW dye laser. Pronounced structure in the laser excitation spectrum appears to arise from Stark sublevels of the Ho/sup 3+5/F/sub 5/ and /sup 5/I/sub 8/ states rather than excited state absorption. Because the overall gain profile is inhomogeneously broadened, the laser excitation spectrum width increases with pump power and is approximately 5.7-nm FWHM when the pump power is a factor of 2.3 above threshold. For a 22-cm fiber, the threshold pump power is 128+or-5 mW and, with 5% output coupling, more than 12 mW has been extracted in the green for 318 mW of 646.7 nm input power. The slope efficiency for this oscillator (accounting for the pump launch efficiency) exceeds 14%.<>