Spectroscopic site determinations in erbium-doped lithium niobate.

Abstract

Rare-earth (e.g., ${\mathrm{Er}}^{3+}$, ${\mathrm{Nd}}^{3+}$) based, guided wave optical amplification in lithium niobate (${\mathrm{LiNbO}}_{3}$) integrated optic systems is a new and important addition to the field of integrated optics. The application of total site selective spectroscopy to rare-earth-doped ${\mathrm{LiNbO}}_{3}$ provides the most complete spectroscopic characterization of this class of materials to date. In a previous publication we identified six spectroscopic sites in Er:${\mathrm{LiNbO}}_{3}$ using total site selective spectroscopy, two of which are cluster sites which upconvert light using nonradiative energy transfer between Er ions within a given site. In this paper ${\mathrm{Er}}^{3+}$ site identifications are made based on a consideration of solid solution defect equations in conjunction with an experimental study of the site distribution as a function of dopant concentration (0.4--2.0 mol % Er:${\mathrm{LiNbO}}_{3}$) and the Li/Nb ratio in the crystal. The Li/Nb ratio was altered using a vapor phase equilibration technique. Our results indicate that increasing the ${\mathrm{Li}}_{2}$O content of Er:${\mathrm{LiNbO}}_{3}$ not only reduces the cluster site concentration by \ensuremath{\sim}30% but also increases the amount of light absorbed in the crystal by \ensuremath{\sim}15%. This observation is, to the best of our knowledge, the first report of post growth materials processing in rare-earth-doped ${\mathrm{LiNbO}}_{3}$ to effect a change in absorption or cluster site concentration. In addition, increasing the dopant concentration increases ${\mathrm{Li}}_{2}$O deficiency in Er:${\mathrm{LiNbO}}_{3}$ crystals. Simple solid solution defect model calculations agree with these experimental results. \textcopyright{} 1996 The American Physical Society.