Energy transfer in rare-earth doped lasers

Summary form only. In optical pumping of solid-state lasers only very rarely a perfect spectral correspondence is found between the emission of the pump and the absorption of the lasing ion. Both sides have to be developed: emitters with optimum spectral characteristics and absorbers adapted to the best pump source. In the case of flashlamp pumping fluorescent components around the flashtube can transform undesired wavelengths to those that can be absorbed in the crystal and crystals have been co-doped. In the case of laser-diode pumping mainly two emission bands can be used. Some rare earth-doped crystals or glasses are very well suited to absorb these wavelengths. Others show no spectral overlap with these pump sources and have therefore to be sensitised by co-doping of a good absorber. This absorber has to be able to transfer the excitation energy to the desired laser ion. The mechanisms leading to efficient excitation transfer are a fascinating field for spectroscopists. A few examples are discussed. Co-doping is not only helpful in a better excitation of the upper laser level, but it can also be used to better depopulate the lower laser level. Besides spectral sensitisation it would be very favourable also to generate a spatial sensitisation in order to get a better overlap of pumplight with the mode of the laser. An example is a Gaussian like distribution of the absorber across the diameter of a laser rod. It is extremely difficult to reach this goal with crystal growth techniques. Possibilities to achieve such a distribution of the absorber with the aid of optically induced colour centres are discussed and the energy transfer from colour centres to Er/sup 3+/ ions is demonstrated for Er:YAlO/sub 3/.