Tellurite And Fluorotellurite Glasses For Active And Passive

6. Optical properties; MDO 244
zinc species are less polarisable than oxygen and tellurium [40], respectively, so the
index decreases with ZnF2 replacement of TeO2.
Fig. (6.49) shows the variation in refractive index at 633 nm with increasing oxide to
fluoride ratio of glasses in the series 65TeO2-(25-x)ZnF2-xZnO-10Na2O mol. %, for 5 ≤ x
≤ 25 mol. % (MOF009 to 013). Glass MOF001 (65TeO2-25ZnF2-10Na2O mol. %) used
for oxide / fluoride = 0. The index increased linearly (with a correlation coefficient
0.90430) from 1.88740 (ZnO / ZnF2 = 0) to 1.95167 (ZnO / ZnF2 = 1). This is to be
expected as fluorine is less polarisable than oxygen [40], therefore the index increased as
ZnO replaced ZnF2.
6.4. Summary
Core / clad choices for oxide tellurite and fluorotellurite compositions
For fibre fabrication from tungsten-tellurite glasses studied here, a possible core / clad
pair would be MOD014 (90TeO2-5WO3-5Nb2O5 mol. %) for the cladding (n = 2.147),
and MOD016 (70TeO2-25WO3-5Bi2O3 mol. %) for the core (n = 2.157). Using equation
(2.5) the numerical aperture, NA = 0.21, and acceptance angle, θ = 12°, for guiding light
at 632.8 nm was calculated. However, this is a rather small acceptance angle and may be
problematic when attempting to launch light into the fibre. The index difference for
glasses MOD010 (75TeO2-10ZnO-10Na2O-5PbO mol. %, n = 2.024) and MOD012
(75TeO2-10ZnO-10Na2O-5GeO2 mol. %, n = 1.992) was higher, and therefore would be
more suitable for an oxide core / clad pair. Using equation (2.5), this gives NA = 0.34,