Elimination of platinum inclusions in phosphate laser glasses

Results from small-scale glass melting experiments aimed at reducing the density of platinum particles in phosphate laser glasses are discussed. The platinum particles originate from the crucibles used to melt the laser glass and can cause optical damage in glasses used in high-peak-power lasers; this problem was particularly acute in the LLNL 120 kJ, 100 TW Nova laser. The melting experiments examine the effects of (i) N{sub 2}, O{sub 2}, and Cl{sub 2} gas atmospheres; (ii) temperature and temperature gradients; (iii) processing time; and (iv) platinum alloys on the formation and dissolution of platinum inclusions in LHG-8 and LG-750 phosphate laser glasses. Results show that most platinum inclusions originate early in the melt cycle, with thermal gradients within the melter being one of the major causes. By using oxidizing gas conditions (O{sub 2}, Cl{sub 2}, or O{sub 2} + Cl{sub 2}), the platinum inclusions can be dissolved into the glass during the course of the melt cycle. The dissolution rate of platinum under oxidizing conditions has been measured, and a model is used to quantify the description of the dissolution process. The effect of ionic platinum on the transmission spectra of the laser glasses produced under various oxidizing conditions hasmore » also been measured. Results from the above laboratory-scale melting experiments have been incorporated into proprietary laser-glass melting processes. The laser glasses prepared under these conditions have an average of less than 0.1 platinum inclusions/liter, which represents a 1000-fold reduction over the previously available phosphate laser glasses. 52 refs., 56 figs., 15 tabs.« less