Page 57 - Fiber Bragg Gratings

38 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

However, the picture is not as simple, since another mechanism opposes
it: the relief of the internal stress frozen in during fiber fabrication, on
UV exposure [96,97]. Stress relief can only remove the effect of the frozen-
in stress and is therefore strongly dependent on the initial thermoelastic
stress at fabrication. There is correlation between fiber drawing tension
and the maximum induced index change for Type I gratings but reduced
maximum index change for Type IIA [98]. The process of densification
has been shown to occur in fibers as evidenced by scans using an atomic
force microscope of the surface of D-shaped fibers and in etched fibers
[99], and in preform samples that were drawn into a D-shaped fiber
[100]. These observations are on the surface of the sample and are unable
to replicate the stress profiles within the core of the fiber directly. Direct
optical measurement of in-fiber stress has indicated that rather than the
relief of the stress, tensile stress actually increases with an associated re-
duction in the average refractive index by —30% of the observed UV induced
refractive index change in non-hydrogen loaded, high-germania-content fi-
bers [101]. The changes in the stress profile of the fiber are consistent with
the shift in the Bragg wavelength of a grating during inscription [102].
The third mechanism for the UV induction of the refractive index
change is via the formation of Ge-H and the generation of GeE' centers;
it was proposed by Tsai and Friebele [89]. The concentration of the GeE'
have been previously correlated with the presence of the precursor states
of the Ge(I) and Ge(II) centers. However, the concentration of GeE' centers
continues to grow despite the saturation of both Ge(I) and Ge(II), indicat-
ing that the formation of the E' centers has another route. The color
center model for the changes in the refractive index is supported by the
measurements made by Atkins et al. [103],

2.7 Summary of routes to
photosentization

A method for photosensitizing silica optical fiber is based on the observa-
tion that increasing the 240-nm absorption enhances the effect [104].
Thus, reduced germania present as GeO has been shown to have a good
photosensitive response. Other defect formers, such as europium [80],
cerium [81], and thulium [43], also fall in this category, albeit with a
smaller effect. Phosphorus, which is used extensively in the fabrication
of germania-doped silica planar waveguides, shows a weak response with