Tellurite And Fluorotellurite Glasses For Active And Passive

6. Optical properties; MDO 237
Glass MOF004 (30 mol. % ZnF2 melted for 2 hours) still has hydroxide present, as
addition of zinc fluoride caused volatilisation during melting resulting in a decrease in the
hydroxyl concentration in the final glass. This is favourable for fabrication of fibre as the
loss attributed to OH absorption was significantly reduced with increasing fluoride: 526
dB.m -1 for MOD013 (0 mol. % ZnF2), is decreased to 21 dB.m -1 for MOF004 (30% mol.
ZnF2), over 20 times less. However, as fig. (6.27) shows, the optical loss due to fluoride
content plateaux above 15 mol. % indicating using the current melting procedure (i.e.
melting for 2 hours in a fume cupboard with dry air) all of the OH cannot be completely
removed, although it can be reduced to a large extent [5].
A number of other groups has studied the drying effect of addition of ZnF2 or PbF2 to
tellurite glasses [7, 30-33]. Nazabel et al. [33] showed absorption of around 0.5 cm -1 (500
dB.m -1 ) for the band at 2900 cm -1 for a glass of composition 61TeO2-12ZnO-27ZnF2
melted for 20 min. between 650 and 800°C. Glass MOF001 (25 mol. % ZnF2, fig.
(6.30)) and MOF004 (30 mol. % ZnF2, fig. (6.29)) showed absorption lower than 0.05
and 0.03 cm -1 respectively (50 and 30 dB.m -1 ) at around 2900 cm -1 . The OH absorption
bands in spectra of the fluorotellurite glasses prepared in this study, such as MOF001 and
MOF004, are one order of magnitude lower (in absorption coefficient and dB.m -1 ) than
the glasses melted by Nazabel et al. [33].
Fig. (6.29) to (6.34) show the Gaussian deconvolution of OH bands in spectra of
glasses in the series (90-x)TeO2-10Na2O-xZnF2, mol. %, for 5 ≤ x ≤ 30 mol. % (glasses
MOF001, 004 to 008). These fluorotellurite glasses did not exhibit a strongly H-bonded
OH band at around 2300 cm -1 , which was seen in the spectra of the oxide glasses (this
band was no longer present for ZnF2 containing glasses in fig. (6.26), which also shows