Key words: germaniumsilicate glass, subcritical and supercritical water, etching, fibreoptic reflectometry
Etching of germaniumsilicate singlemode optical fibre edge in sub- and supercritical
water was studied by means of fibreoptic reflectometer (FOR) and scanning electron
microscopy (SEM). It was shown that in the case of germaniumsilicate core edge the
etching rate is much higher than for silica cladding edge. This leads to the formation
of a “well” at the edge of the fibre, which becomes deeper as etching goes forward.
The oscillatory character of the FOR signal is caused by an interference effect of the
light reflected from the edge of the core and the edge of the cladding. This interference
allows one to calculate the etching rate at different pressures and temperatures of
water and, therefore, in its different states (liquid, vapour and supercritical). The
lowest measured etching rate (10 -3 nm/s) was measured at 200 °C and 54 atm; the
maximum etching rate (~30 nm/s) was reached at 400 °C and 246 atm. The etching
rate increases with temperature in accordance with Arrhenius law (apparent activation
energy 58 ± 3 kJ/mole). The character of the FOR signal changes at the late stages of
etching from regular oscillating to random noisy due to the formation of defects of
different size on the fibre edge, which are easy to observe using the SEM imaging.