Page 14 - Fiber Bragg Gratings

Preface

The field of fiber Bragg gratings is almost exactly twenty years old, dating
back to its discovery by Ken Hill and co-workers in Canada. It grew slowly
at first, but an important technological advance by Gerry Meltz and co-
workers 10 years later, renewed worldwide interest in the subject. I was
instrumental in setting up the first International Symposium on photosen-
sitivity of optical fibers, jointly with Francois Ouellette in 1991, a meeting
with 22 presentations and attended by approximately 50 researchers.
Since, we have seen three further international conferences solely devoted
to fiber Bragg gratings, the last of which was attended by approximately
300 researchers. As the applications of Bragg gratings are numerous,
publications appear in widely differing conferences and journals. Surpris-
ingly, apart from several review articles covering the most elementary
aspects, no monograph is available on the subject and the quantity of
available literature is spread across a number of specialist journals and
proceedings of conferences. Thus, progress and the current state of the
art are difficult to track, despite the approaching maturity of the field.
More recently, poling of glass optical fibers has resulted in an electro-
optic coefficient almost rivaling that of lithium niobate.
Germanium, the core dopant of low loss, fused silica optical fiber, is
a rich defect former; ultraviolet radiation can strongly modify the nature
of the defects causing large changes in the local refractive index. The
mechanisms contributing to photosensitivity are complicated and still
being debated. They depend on the types of defects present, dopants, and
the presence of hydrogen whether in the molecular or in the ionic state.
The lack of a thorough understanding has not, however, prevented the
exploitation of the effect in a large number of applications. The very
large index changes reported to date (~0.03) allow, for the first time,
the fabrication of ultra-short (~100 m long) broadband, high-reflectivity
Bragg gratings in optical fibers. The maximum index change may be an

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