Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 320
quantified to determine if any material was removed. Nitric acid, however produced a
relatively poor etch.
From these initial studies HF appeared to be the best etchant for fluorotellurite glasses,
however over short times introduced a significant amount of OH into the glass. If this OH
was confined to the glass surface, it may not greatly affect the optical loss of a clad
preform; however, it may detrimentally alter the fibre drawing process. Therefore, high
concentration HF (e.g. 40 %), for short times, at 21°C would seem to be the best choice
of etchant to try to minimise surface degradation.
7.3.3. Ion exchange
The general phenomena of ion-exchange in glasses has been long studied [26], and
specifically Ag + /Na + ion-exchange for waveguides in silicates [1, 27] and tellurites [28]
also. Fig. (7.36) shows the diffusion profile of glass T08 (80TeO2-9ZnO-10Na2O-1Er2O3
mol. %), which had a silver layer evaporated on both polished sides (100 and 300 µm),
followed by heat treatment at 285°C for 12 hours under inert atmosphere (argon). The
thicker silver layer (300 µm) resulted in deeper penetration (at least 2 µm) into the glass,
compared to the 100 µm layer (≈ 1 µm). However, large errors may be involved due to
the similarity between: the errors of the EDX experiment (+1 at. %) and the silver levels
measured (for heat treated samples particularly), and the depth of penetration of silver (1
to 2 µm) and the electron beam interaction volume (≈ 1 µm 3 ). At the surface the silver
content was around 2.3 and 2.5 at. % for the 100 and 300 µm layers respectively, and the
sodium content of the glass was 6.8 at. % batched; therefore total replacement of Na +