Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 370
particles would contribute to losses due to the similarity in their dimensions to the
wavelength of the light. These nano-particles will result in Rayleigh scattering
proportional to 1/λ 4 . Larger micro-particles give rise to Mie scattering, proportional to
1/λ 2 . The 10 hour melt does not seem to exhibit significant scattering losses, due to the
increased refining of the glass due to longer melting time.
Ryder et al. [33] showed a decrease in overall visible transmittance (400 to 750 nm)
with increasing Pt content in phosphate glasses, although around seven absorption bands
occur in the 200 to 333 nm region [34]. Therefore, the remaining background absorption
seen in fig. (8.16) for the 10 hour melt is likely to be due to losses from the unclad fibre
surface, and to noble metal (Au / Pt) absorption. Producing fibre by rotational casting [7],
should result in fluorotellurite fibre of lower loss than the rod-in-tube TZN fibre reported
by Wang et al. (0.9 dB.m -1 at 1.35 µm) [18]. Gold is known to stay in solution better than
platinum, although both can precipitate as tiny particles in the glass. These metals are
taken into the melt by a number of processes: vapour phase transport above the melt
surface, mechanical abrasion, and reaction with the melting batch [35]. Metallic particle
inclusions can result in severe failure in laser glasses, therefore solid solution is preferred.
These particles can absorb laser energy, until melting, resulting in cracking of the glass.
Precipitation is encouraged under reducing conditions, whilst solubility increased under
oxidising conditions for platinum [35]. Noble metals also impart a yellow colour to
tellurite glasses, in greater severity for Pt than Au.
Increasing melting times reduces OH absorption and scattering due to inhomogeneity.
However longer melting times will inevitably increase noble metal dissolution into the
glass. Therefore, an acceptable balance must be struck between processing times and