Page 49 - Fiber Bragg Gratings

30 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

seen, an atmosphere of hot hydrogen during the collapse process or hot
hydrogen soaking of fibers enhances the GeO defect concentration [37].
The presence of molecular hydrogen has been known to induce increases
in the absorption loss of optical fibers, since the early day of optical
fibers [50]. Apart from being a nuisance in submarine systems, in which
hydrogen seeps into the fiber, causing a loss that increases with time of
exposure, cold high pressure hydrogen soaking has led to germanium-
doped fibers with the highest observed photosensitivity [66]. Any germa-
nia-doped fiber may be made photosensitive by soaking it under high
pressure (800 bar) and/or high temperature (< 150°C). Molecular hydrogen
in-diffuses to an equilibrium state. The process requires a suitable high-
pressure chamber into which fibers may be left for hydrogen loading.
Once the fiber is loaded, exposure to UV radiation is thought to lead to
a dissociation of the molecule, leading to the formation of Si-OH and/or
Ge-OH bonds. Along with this, there is formation of the Ge oxygen-
deficient centers, leading to a refractive index change. Soaking the fiber
at 200 bar at room temperature for ~2 weeks is sufficient to load the
125-micron diameter fiber at 21°C [66].
UV exposure of standard hydrogen fibers easily yields refractive index
changes in excess of 0.011 [67] in standard telecommunications fiber, with
a highest value of 0.03 inferred [68]. Almost all Ge atoms are involved in
the reactions giving rise to the index changes. Figure 2.8 shows the
changes in the refractive index profile of a standard fiber before and after
2
exposure to pulsed UV radiation at 248 nm (—600 mJ/cm , 20 Hz, 60-
minute exposure) [68], The growth of gratings is long with CW lasers
(duration of 20 minutes for strong gratings with refractive index changes
3
of ~l-2 X 10~ ). The picture is quite different with the growth kinetics
when compared with non-hydrogen-loaded germania fibers. To date, Type
IIA gratings have not been observed in hydrogen-loaded fibers. There is
also no clear evidence of the stress dependence of grating growth [44].
Whereas in Type IIA the average UV-induced refractive index change is
negative, in hydrogen-loaded fibers the average refractive index grows
unbounded to large values (>0.01).
Heating a hydrogen-loaded fiber increases the refractive index rap-
idly, even in P 2O 5 and P 2O5:Al 2O3-doped multimode fibers [68], although
pure silica is not sensitized.
The dynamic changes that occur in the process of fiber grating fabrica-
tion are complex. Even with hydrogen-loaded fibers, there are indications
that as the grating grows, the absorption in the core increases in the UV,