Evaluation and control of the dopant distribution in a Nd:LiNbO_3 fiber grown from the melt by the edge-defined film-fed growth (EFG) method

摘要

The quality of the fiber depends to a considerable extent on homogeneity, I.e., on the distribution of both special added and detrimental impurities. A non-uniform distribution of such impurities along the fiber length and cross-section leads to non-uniform spatial, electrical and optical properties of the crystal. Variations of the concentration of impurities are determined by the processes that take place during the fiber manufacturing. In order to eliminate non-uniformities, crystal growth experts have concentrated on the analysis of the mass transport, heat transfer, impurity distribution and shape of the crystal-melt interface. In order to evaluate the dopant distribution, a stationary numerical model - including incompressible fluid flow in the Boussinesq approximation, heat and mass transfer, and surface tension-driven flows due to the temperature gradient along the liquid free surface (meniscus) - is developed using the finite element method. A two-dimensional axis-symmetric model is implemented with COMSOL Multiphysics 3.3 software, and the Nd impurity distribution dependence on the pulling rate v_0 and on the radius of the capillary channel R_(cap) in a LiNbO_3 fiber grown from the melt by the EFG method is determined. Using this dependence, the optimal v_0 and R_(cap) which assure the best impurity distribution are chosen.