Annealing behaviour of structure and morphology and its effects on the optical gain of Er3+/Yb3+ co-doped Al2O3 planar waveguide amplifier

Using transmission electron microscopy (TEM) and x-ray diffractionanalysis, we have studied the structural and morphological evolutionof highly Er/Yb co-doped Al₂O₃ films in the temperaturerange from $600,^{circ}mkern-1mu$C--900$,^{circ}mkern-1mu$C.By comparison with TEM observation, the annealing behaviours ofphotoluminescence (PL) emission and optical loss were found to haverelation to the structure and morphology. The increase of PLintensity and optical loss above 800$,^{circ}mkern-1mu$C mightresult from the crystallization of amorphous Al₂O₃ films.Based on the study on the structure and morphology, a rate equationpropagation model of a multilevel system was used to calculate theoptical gains of Er-doped Al₂O₃ planar waveguideamplifiers involving the variation of PL efficiency and optical losswith annealing temperature. It was found that the amplifiers had anoptimized optical gain at the temperature corresponding to theminimum of optical loss, rather than at the temperaturecorresponding to the maximum of PL efficiency, suggesting that the optical loss is a key factor for determining the optical gain of anEr-doped Al₂O₃ planar waveguide amplifier.