The strength of flaw-free silica fibers in the absence of water is of the order of 12-14
GPa and is essentially independent of time. (Formula available in paper). On the other hand,
in the presence of water, the time dependence is very much more serious (n - 20) and
even at relatively short testing times (tf 10 secs), the measured strength is <6 GPa.
The development of a completely satisfactory hermetic coating which will allow the
realization of the above 'water-free'
strengths is not simple, however, while the
advantages of a polymer coating in mechanically protecting the fiber and at the same
time isolating it from microbending losses is well-known, no water-impermeable
viscoelast.ic coating has yet been proposed. Two other types of coatings have been
proposed and each is successful to some extent as an hermetic coating. These materials
and their behavior are quite different and will be described below.
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