Abstract

Upconversion lasers based on lanthanide doped host materials have been known for several years. The efficiency of their multiphoton excitation schemes depends critically on the absence of competing nonradiative processes, depleting the intermediate energy levels involved as well as the upper laser level. For this reason, fluoride host materials generally outperform oxides due to their lower phonon energies and, consequently, lower multiphonon relaxation rates. In this work, we concentrate on LiYF₄ (YLF) codoped with Pr³⁺ and Yb³⁺, allowing for a sensitized avalanche upconversion mechanism [1] to produce red (639.5 nm) output upon pumping at ~ 835 nm. The threshold pump intensity of this process is in the order of 10⁵ to 10⁶ W/cm², requiring a tightly focused pump beam. In bulk media, the high divergency of such a beam results in a very short gain medium, where only a small part of the incident light is absorbed, giving rise to a poor overall efficiency. A longer gain medium, as demonstrated with fluoride glass fibres, allows for a far better performance. In a disordered host material, however, the inhomogeneous broadening of the spectral lines reduces the peak gain cross section. Therefore, it seems promising to combine the advantages of the fibre geometry and a crystalline host by using crystalline waveguides.

© 1998 IEEE