Highly nonlinear Pb₂P₂O₇-Nb₂O₅ glasses for optical fiber production

In this study we present the production and characterization of Pb₂P₂O₇-Nb₂O₅ glasses and optical fibers. The dependence of Nb₂O₅ content on thermal, structural and optical properties were investigated by thermal analysis (DSC), Raman spectroscopy, UV-Visible absorption, M-Lines and Z-scan techniques. Glass transition temperature (T_g) increased linearly with Nb₂O₅ content up to 60 mol%, while thermal stability against crystallization (ΔT) reached a maximum value of 225°C at 40 mol% of Nb₂O₅. Raman spectra showed a significant structural change by the insertion of NbO₆ octahedral units in the glass network. The increase of Nb₂O₅ concentration shifts the glasses absorption edge to lower energies, and also increases the linear refractive indexes (n₀) due to the high polarizability of niobium atoms and formation of non-bridging oxygen. Similarly to n₀, an increase in the positive values of nonlinear refractive indexes was observed using Z-Scan technique with increase of Nb₂O₅ content, based on structural changes caused by the replacement of Pb₂P₂O₇ instead Nb₂O₅. The average of n₂ values at 500-1500 nm raised from 2.2 × 10^-19 to 3.8 × 10^-19 m²/W, when the Nb₂O₅ content was increased from 10 to 60 mol%. Lastly, a core-cladding preform was produced by suction method and the optical fiber drawn. The sample containing 40 mol% of Nb₂O₅ was used for presenting the highest thermal stability against crystallization and n₀ values > 2 from green to near-infrared wavelengths. Multimode step index fiber with good core circularity and concentricity was produced and the optical losses were determined by cut-back method at visible and near-infrared ranges.