Preparation And Characterization Of Locally Er-doped High-solubility LiNbO₃ Crystal

Er:LiNbO₃ crystal combines the excellent electro-optic, acousto-optic and nonlinear optical properties with the laser properties of Er ion, and it is a potential substrate material for active optical waveguide devices. But the solubility of Er ion in Er:LiNbO₃ crystal prepared by conventional method is limited, and it limits further development of related devices. In this dissertation, the feasibility of preparing locally Er-doped high-solubility LiNbO₃ crystal by simultaneous work of Er in-diffusion and Li-poor vapor transport equilibration (Li-poor VTE) treatment was demonstrated for the first time.The main work of the dissertation can be summarized as following aspects:1. A group of Z-cut MgO(5 mol%):LN crystals were treated by Li-poor VTE procedure, and the transformation of their OH- absorption bands was discussed. It shows that OH- absorption band at ³481 cm-1 generate directly, but not obtain from the OH- absorption band at ³535 cm-1.2. Four X-cut and four Z-cut congruent LiNbO₃ substrates (Li2O content≈48.5 mol%) were used as initial samples. Using a radio-frequency magnetically-controlled sputtering system, pure Er metal was coated onto the surface of each plate. The samples underwent a Li-poor VTE of 1030℃/240 h, 1050℃/180 h, 1070℃/180 h and 1100℃/180 h, respectively. After that, the Li2O content in the crystal, determined from measured birefringence, decreases to (48.0-48.2)±0.1 mol%. Secondary ion mass spectrometry was used to study the Er diffusion parameters. The results show that all of the Er diffusion reservoirs were not exhausted, and Er ions in all of the samples followed the complementary error function profile. Using diffusion depth, diffusivity for each samples was calculated. From the Arrhenius law, the diffusion constant and activation energy are evaluated to be (1.2±0.2)×107/(2.2±0.3)×104 m2/h and 2.32±0.08 eV / 1.51±0.08 eV for X-/Z-cut plate. Based on law of mass conservation, the Er solubility in the X-/Z-cut plate is evaluated to be (1.42±0.24)/(1.14±0.15), (2.18±0.27)/(2.01±0.19), (2.62±0.26)/(2.68±0.19) and (2.54±0.20)/(2.30±0.16)×1020 ions/cm3 for the VTE temperature of 1030, 1050, 1070 and 1100℃. Comparison shows that under the same temperature the solubility of the diffusion in the Li-poor atmosphere is considerably higher than that of the diffusion without Li-poor VTE procedure.3. To have a reference of parameters setting, result prediction and result verification, a numerical simulation method based on the experimental data of the Z-cut plate underwent 1100℃/180 h treatment was utilized to investigate the variation of Li2O content and Er diffusion process in the plate. Er diffusion mechanism was also discussed according to the reported reference. Based on Fick law, diffusion equation and boundary conditions were treated by finite difference method, and two general models about Li-poor VTE procedure and Er ions diffusion procedure were set up. Next, Er diffusion model was used to obtain the time when all the Er ions formed reservoir and the time when reservoir was exhausted. From the model, figures and fitted curves about 0-100 nm thick Er metal depleted time, as well as 1/e diffusion depth as a function of diffusion duration from 0 h to1000 h were also obtained.In summary, we get the the transformation of OH- absorption bands in MgO:LN treated by Li-poor VTE procedure. A group of locally Er-doped high-solubility LiNbO₃ crystals are obtained by simultaneous work of Er in-diffusion and Li-poor VTE treatment, and models of Li-poor VTE procedure amd Er in-diffusion are built. This method could be fitted for improving the performance of related devices such as waveguide amplifiers.