TIME RESOLVED AND SITE SELECTIVE SPECTROSCOPY OF THULIUM DOPED INTO GERMANO-SILICATE AND ALUMINOSILICATE OPTICAL FIBERS AND PREFORMS

Optical properties of thulium ions doped in varying concentrations into germano- and alumino-silicate glass fibres and their preforms are studied. Absorption and fluorescence spectra are shown to vary little with thulium concentrations up to 12000 ppm, but vary drastically with germanium or aluminium codopant. Resonant fluorescence studies of Tm3+ show correlations in the movements of the Stark levels of the ground state, indicating systematic correlations between the ion site and Stark level position. An upper limit of 10 cm-1 for the homogeneous line width of thulium in alumino-silicate glass is established at 9 K. The lifetimes of the 1G4 and 3F4 levels are studied and are related to codopant influence and cross-relaxation which are linked to fibre drawing and TM3+ concentration. A significant lifetime shortening is observed on drawing preform to fibre. A model is presented for non-exponential decay of the 3F4 level. Three- and four-level rate equations have been successfully used to simulate the observed rise of the 3F4 fluorescence and decay of the 1D2 fluorescence, excited resonantly and by up-conversion processes, respectively. These models yield both the excitation pump rate and cross-relaxation rates.