Germania nanocrystals in Modified Chemical Vapour Deposition

The Modified Chemical Vapour Deposition process for fabrication of silica-based optical fibres is used to generate nanoscale soot particles which are deposited on a substrate by thermophoresis and then sintered and collapsed into a preform from which optical fibre can be drawn. To increase the refractive index of the fibre core silica and germania soots are often generated together and co-deposited. The silica soot is invariably amorphous in character but for certain configurations of the preform lathe it has been found that germania soot particles may be crystalline. A systematic study has identified parameters controlling germania crystallinity in soot nanoparticles and transition from crystalline germania nanoparticles to glassy germania has been observed. The implications of possible retention of germania nanocrystals in preform and fibre are discussed with regard to fibre quality and optical performance.