The impact that an efficient, inexpensive and reliable visible laser source would have on data storage, display technology, undersea communications and optical processing has provided the stimulus for much recent work on solid state visible lasers. One approach that has yielded much success is the use of upconversion processes within rare earth doped materials to produce laser emission at a wavelength significantly shorter than the pump wavelength. For example, there has been a recent demonstration of visible lasing at 480 nm in Tm.sup.3+ --doped fluoride fibre (see Allain, J. Y., Monerie, M and Poignant, H.: `Blue upconversion fluorozirconate fibre laser`, Electron. Lett., 1990, 26 (3), pp. 166-168), and a demonstration of room temperature lasing at red, green and blue wavelengths in praseodymium doped fluorozirconate glass fibre (see Smart, R. G., Hanna, D. C., Tropper, A. C., Davey, S. T., Carter, S. F., Szebesta, D,: `CW upconversion lasing at blue, green and red wavelengths in an infrared-pumped Pr.sup.3+ -doped fluoride fibre at room temperature`, Electron. Lett., 1991, 27, (14), pp 1307-1309). Forced oscillation on two transitions simultaneously has also been demonstrated (see Percival, R. M., Szebesta, D., and Davey, S. T.: "Highly efficient and tunable operation of two colour Tm-doped fluoride fibre laser" Electron. Lett., 1992, 28, (7), pp. 671-672).
These demonstrations have dramatically changed the viability of such upconversion pumped laser schemes, and recently a significant amount of time has been spent investigating the infrared emission which emanates from the .sup.3 F.sub.4 manifold in fluoride fibres doped with thulium. During the course of this work, it has become common knowledge that, when pumped at around 790 nm, the fibres glow in the blue region of the electromagnetic spectrum. One explanation for this effect, is that a first pump photon results in population being excited into the .sup.3 F.sub.4 manifold (see FIG. 1 which is an energy level diagram of a thulium/terbium co-doped fluoride fibre). From this level, there are three routes for radiative decay, 0.805 .mu.m (.sup.3 F.sub.4 -.sup.3 H.sub.6), 1.475 .mu.m (.sup.3 F.sub.4 -.sup.3 H.sub.4), and 2.310 .mu.m (.sup.3 F.sub.4 -.sup.3 H.sub.5) with branching ratios of 0.893 0.083 and 0.024 respectively. The energy gap to the next level is sufficiently large that non-radiative decay is precluded. Thus besides the ground state (.sup.3 H.sub.6) and the .sup.3 F.sub.4 manifold there will be small populations in the .sup.3 H.sub.4 and .sup.3 H.sub.5 manifolds when under excitation. Moreover, the energy gap between the .sup.3 H.sub.5 and .sup.1 G.sub.4 manifolds is quite closely matched to the pump photon energy. Consequently, the sequential absorption of two pump photons could lead to a small fraction of the population excited into the .sup.3 H.sub.5 manifold reaching a high enough level (.sup.1 G.sub.4) to give rise to a small amount of blue emission when the excited ion subsequently decays back down to the ground state manifold .sup.3 H.sub.6.
The 1992 Electronics Letter paper referred to above observed that the blue emission intensity increased significantly when stimulated emission was obtained on the 2.31 .mu.m transition, since the population in the .sup.3 H.sub.5 manifold rapidly increased under these circumstances. However, this scheme is thought to be unworkable as a blue laser, since the fibre parameters for operation at mid-infrared and blue wavelengths are widely divergent.