Page 53 - Fiber Bragg Gratings

34 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

hydrogen diffuses out of the core after fabrication of the grating, before
returning to the original wavelength. Note that the resonance of the LPG
is only dependent on the difference in the core and cladding refractive
indexes, since it is the relative difference that is important (see Chapter
4). The effect on the refractive index of the fiber as the hydrogen out-
diffuses is a complicated process, as shown by Malo et al. [76] using Bragg
gratings. Once the fibers are removed from the chamber, the hydrogen
begins to diffuse out but is fixed in the core by UV irradiation during
grating fabrication. Depleted in the core, hydrogen in the cladding diffuses
in before diffusing out. The stress changes the molecular polarizability
of hydrogen [77], as well as the Bragg wavelength, first toward long, and
then toward short wavelengths. The drift in the wavelength is found to
be 0.72 nm for an initial pressure of 100 bar. The Bragg wavelength is
sensitive to the net refractive index of the core, not to the difference
between the core and the cladding as for the LPG. The dynamics of the
coupling between the modes in LPGs has also been reported [78].
To prevent the fiber from out-gassing prematurely, it should be stored
at low temperatures (—70°C) until it is used.
The diffusion time taken to reduce the hydrogen initial concentration,
C 0/e, in the core is shown in Fig. 2.12a as a function of fiber diameter,
at room temperature (20°C). Also shown is the diffusion time for standard
fiber as a function of temperature (Fig. 2.12b).

2.4.5 Rare-earth-doped fibers
For a vast number of applications, such as fiber lasers and amplifiers, it
is necessary to fabricate gratings in rare-earth-doped fibers. It is more
difficult to write Bragg gratings in these fibers than in standard fibers.
Worse, germanium is replaced by aluminum (A1 2O 3) to reduce the effect
of quenching and lifetime shortening [79]. The lack of germanium reduces
the photosensitivity of optical fibers, even with hydrogen sensitization,
although gratings have been reported [46]. Gratings can be formed in
4
most fibers, but the index changes remain weak (<10~ ) in all cases with
240-nm irradiation except in hydrogen-loaded Al/Ce or Al/Tb. With 193-
4
nm irradiation, Al/Yb/Er fiber has shown index changes of 10~ while
3
hydrogen loading increases this figure to ~10~ . The conclusion is that
hydrogen loading improves the photosensitivity of germanium-free rare-
earth-doped fibers. However, only a small subset of Er, Nd, and Ce and
4
Tb-doped silicate fibers show reasonable photosensitivity (>10~ ) [46].