Page 27 - Fiber Bragg Gratings

8 Chapter 1 Introduction

The refractive index of glass depends on the density of the material,
so that a change in the volume through thermally induced relaxation of
the glass will lead to a change AM in the refractive index n as

where the volumetric change AV as a fraction of the original volume V is
proportional to the fractional change e in linear dimension of the glass.
We now have the fundamental components that may be used to relate
changes in the glass to the refractive index after exposure to UV radiation.
Other interesting data on fused silica is its softening point at 2273°C,
and the fact that it probably has the largest elastic limit of any material,
—17%, at liquid nitrogen temperatures [36].

1.4 Overview of chapters

The book begins with a simple introduction to the photorefractive effect
as a comparison with photosensitive optical fibers in Chapter 2. The
interest in electro-optic poled glasses is fueled from two directions: an
interest in the physics of the phenomenon and its connection with
photosensitive Bragg gratings, and as the practical need for devices that
overcome many of the fabrication problems associated with crystalline
electro-optic materials, of cutting, polishing, and in-out coupling. A fiber-
compatible device is an ideal, which is unlikely to be abandoned. The
fiber Bragg grating goes a long way in that direction. However interesting
the subject of poled glasses and second-harmonic generation in glass
optical fibers and nonlinear behavior of gratings, they are left for another
time. With this connection left for the moment, we simply point to the
defects, which are found to be in common with the process of harmonic
generation, poling of glass, and Bragg gratings. The subject of defects
alone is a vast spectroscopic minefield. Some of the prominent defects
generally found in germania-doped fused silica that have a bearing on
Bragg grating formation are touched upon. The nature and detection
of the defects are introduced. This is followed by the process of photosen-
sitizing optical fibers, including reduced germania, boron-germanium
codoped fibers, Sn doping, and hydrogen loading. The different tech-
niques and routes used to enhance the sensitivity of optical fibers,