Page 63 - Fiber Bragg Gratings

42 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

2.7.1 Summary of optically induced effects

The chemical reactions that take place in photosensitive optical fiber
exposed to UV radiation are probably never going to be understood com-
pletely. However, several known factors influence the index change.
1. Bleaching of the 240-nm GeO band in reduced germania fibers.
This has been measured and alludes to the following picture of
chemical modification. The Ge-Si bonds break, liberating an
electron, which may be retrapped at another defect site. What
remains is the type of picture seen in Fig. 2.2 (GeE'). It is not
known whether a volumetric change occurs as well. It is likely
that, owing to the confinement of the photosensitive species
within a massive cladding, any physical relaxation or contraction
of molecular bonds will result in a stressed state. Recently, the
stress change in fiber cores has been theoretically modeled and
also measured using optical methods [110], with good agreement
between the two. Douay et al. [99] have etched fibers and
preforms previously exposed to interfering UV beams and found
that a relief grating is revealed. Although this is not direct
evidence that physical changes occur on UV exposure, it does
show that the chemistry has indeed been altered. Riant et al.
[100] performed atomic force microscopy (AFM) on the surface
of D-shaped fibers in which gratings have been written. The
surface of the fiber, a few microns above the photosensitive
core of germanin-doped silica, showed a surface relief directly
indicating stress changes. Therefore, not only does the absorption
in the UV change, but so does the density of oscillators with
UV exposure, both of which alter the refractive index based on
the Kramers-Kronig rules. However, it must be remembered
that the molecules within the core are not free to change their
shape, but are elastically coupled to the mass in the cladding
via bonds that remain predominantly unchanged. This is the
basis for the induced stress and for part of the index modification
by the stress-optic coefficient. Russell et al. [Ill] have estimated
4
that a strain of only 10~ is necessary to induce an index change
in the effective index of the mode by the same order of magnitude.
It is well known that fiber drawing conditions and stress annealing
can alter the refractive index of boron-doped glasses [112].