Abstract

Upconversion pumped lasing in rare earth doped fluorozirconate optical fibre offers the potential for compact blue green laser sources pumped by commercially available infrared laser diodes. A promising candidate for such a source is the thulium system first demonstrated by Steve Grubb and co-workers [1], which lases in the blue at 482 nm with a single pump source at about 1130 nm. Figure 1 illustrates the relevant energy levels of the trivalent thulium ion, and the upconversion mechanism. Ions excited to the short lived H₅ and F₂₃ levels decay rapidly by phonon emission to the metastable H₄ and F₄ levels from which further upconversion transitions occur. The centre wavelengths of the three transitions are respectively 1220 nm (H₆ to H_s), 1130 nm (H₄ to F₂), and 1150 nm (F₄ to G₄), but because of the broadening of the transitions in glass it is possible to excite all three with a single wavelength pump in the range 1110 nm to 1160 nm. The next energy level above G₄ in thulium is D₂ with a transition of 1480 nm separating the two, so that further upconversion above G₄ should not occur, or only very weakly. A low phonon energy host material such as ZBLAN fluorozirconate glass is necessary so that the metastable levels do not decay rapidly by phonon emission.

© 1995 Optical Society of America