Emission and Absorption Cross-Sections of Mg/Er-Codoped Near-Stoichiometric ${\rm LiNbO}_{3}$ Crystals

Near-stoichiometric (NS) (Mg:) Er:LiNbO₃ crystals were grown from melts ([Li₂O]:[Nb₂O₅] = 1:1) containing 0.0/0.5, 0.5/0.5, and 1.0/0.5 mol%/mol% MgO/Er₂O₃. OH absorption study shows that the optical damage threshold Mg concentration of the Er/Mg-codoped NS crystal is just near 1.0 mol%. Polarized visible and near infrared, and unpolarized mid-infrared (2.7 μm, ⁴I_11/2 → ⁴I_13/2) emission spectra of Er³⁺ ion in these crystals were measured. From measured emission spectra, the emission and absorption cross-section spectra were calculated based upon McCumber theory, and compared with the data of congruent Er:LiNbO₃ bulk material and/or Ti-diffused Er:LiNbO₃ strip waveguide and the results obtained from the direct unpolarized optical absorption measurements. The 550, 980, and 1530 nm emission lifetimes were also measured, and compared with the Judd-Ofelt theoretical values and the measured values of the congruent material. The results show that the polarized absorption cross-section data derived from the emission spectra are comparable to the unpolarized cross-section data from direct absorption measurements. The emission and absorption cross-sections, their polarization dependences as well as the lifetime all change definitely as the crystal composition shifts from the congruent point to the near-stoichiometry. In the NS crystal, the Er³⁺ ion has smaller emission and absorption cross-sections, and longer lifetimes. In contrast, MgO codoping has less effect on the emission and absorption cross-sections, and lifetime.