ECOC 1975 - ECOC 2013

33
THE DEVELOPMENT OF OPTICAL FIBRE USING SODIUM BOROSILICATE GLASSES AND THE
DOUBLE CRUCIBLE TECHNIQUE
C R Day, K J Beales, J E Midwinter and G R Newns
Introduction
We wish to report work carried out in developing the double crucible fibre
drawing process to the point at which it is limited primarily by the bulk
glass loss. The double crucible method has been described previously1,2.
It is now possible to draw many kilometres of fibre in a continuous
process from platinum crucibles with very high reproducibility of fibre
attenuation. During the course of the work a number of problems have been
identified and controlled independently, and these are described below,
followed by some of the results achieved.
Recontamination
Operation of the crucibles at high temperatures causes leaching of trace
impurities, in particular iron and copper, from the platinum. This effect
was observed by Newns et a13 using the sodium calcium silicate (NCS) glass
system. By careful choice of glass compositions in the sodium borosilicate
(NBS) system,compatible glass pairs can be found which have viscosities
suitable for fibre pulling at temperatures low enough to reduce the leaching
rate to a negligible level. The operating temperature can thus be
reduced from -1100 0 C for NCS glasses to -850 C for NBS glasses. In
current fibre there is no increase in loss with time which can be
attributed to recontamination.
Loading
Relatively large gas bubbles may be trapped in the melt by a poor glass
loading technique. The glasses used for loading the crucibles were in the
form of canes 6 to 8 mm in diameter, pulled directly from the melt, and
subjected to a minimum of handling and exposure to the atmosphere. The
canes must be fed in at low speed so that a positive meniscus is formed
between the melt and the rod. This prevents cold rod from being thrust into
the pool of glass and drawing in gas bubbles. The rod should a1so enter
the melt at a fixed position to prevent fold-over effects which could also
trap gas.
Atmosphere control
When the crucibles are operated open to the air, streams o·f minute
scattering centres can be observed at the core-claddingiriterface of the
fibre. These were postulated to be gas bubbles released as a result of an
electrolytic cell formed be 4 ween the core and cladding glasses and the tip
of the core crucible nozzle. The driving force of the cell appears to be
oxygen in the atmosphere which diffuses through the cladding glass. The
The authors are with the Post Office Research Centre,
Martlesham Heath, Suffolk.