Modeling the electrooptic evolution in thermally poled germanosilicate fibers

We formulate a simple quantitative three-species charge-carrier transport model, consisting of two distinct positive ions and a single negative ion, to describe the dynamics during thermal poling of a germanosilicate optical fiber. We numerically solved the equations and report one-dimensional space-time solutions for the electrooptic (EO) coefficient. In the two-cation model, our findings show the EO coefficient initially dips near the anode and then monotonically rises to a steady-state value, higher than that produced by the initial applied poling field. However, at the cathode, the electric field quickly dropped to zero where it remained zero for the poling duration. The introduction of a moving negative ion clearly shows the existence of a dead time characteristic appearing at the cathode, resulting in a gain in the initial EO coefficient. This model also reveals that the resulting EO evolution in a thermally poled germanium-boron codoped fiber can be attributed to the movement of just two ions of opposite polarity. To explain the increase in the EO coefficient in boron codoped germanosilicate fiber, we found it necessary to allow for an increase in the third-order susceptibility by a factor of ∼3.4.