Abstract

Blue and UV fluorescence bands have been observed in Cr:Tm garnet laser materials under cw excitation at 632.8 nm. Previous studies have attributed this anti-Stokes fluorescence to upconversion processes involving Cr-Tm energy transfer. These studies have also suggested that this Cr-Tm upconversion substantially limits the efficiency of flashlamp-pumped Cr:Tm-sensitized 2-μm lasers.¹ In the present investigation, by using a cw dye laser tuned over the 610-680 nm range, we have measured relative intensities of emission bands and the corresponding excitation spectra in garnets doped with Tm:Cr and with Tm alone. The results clearly demonstrate that the principal source of the blue and UV fluorescence is not upconversion but is excited-state absorption of Tm³⁺, which takes place in the ³F₄ → ¹G₄ level and the ³H₄ → ¹D₂ levels, respectively. The blue fluorescence, which peaks at approximately 480 nm, arises through emission from the ¹G₄ level to the ³H₆ ground state of Tm³⁺, and the UV fluorescence, peaked near 360 nm, is due to the ¹D₂ → ³H₆ transition. The results indicate that these processes have negligible effects on the efficiency of Cr:Tm-sensitized 2-μm lasers.

© 1990 Optical Society of America