Page 45 - Fiber Bragg Gratings

26 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

Figure 2.6: Growth of the transmission dip with fluence for different types
of reduced germania fibers. For the 20 mol% germania fiber, a reduction in the
reflectivity is probably due to Type IIA grating formation, (from: Griiner-Nielsen
L. and Hiibner J., "Photosensitive fiber for highly reflective Bragg gratings," in
Tech. Digest ofConf. on Opt. Fiber Commun. OFC'97, paper WL16, p. 178, 1997.)

1390,1500, and 1550 nm, respectively [40]. One major advantage of fibers
that have been reduced is that they are rendered permanently photosensi-
tive and require the minimum of processing, compared with hydrogenated
fibers (see following sections).
The incorporation of 0.1% nitrogen in germanium-doped silica fiber
by the surface plasma assisted chemical vapor deposition (SPCVD) process
[51] has been shown to have a high photosensitivity [52], The effect on
the 240-nm absorption is dramatic, raising it to 100 dB/mm/mol% GeO 2,
doubling it compared to the equivalent for germanium doping alone. The
3
induced refractive index changes are reported to be large (2.8 X 10~ )
and much larger (0.01) with cold hydrogen soaking of 7 mol%Ge;0 mol%N
fiber. The Type IIA threshold is reported to increase by a factor of ~6
over that in nitrogen-free, 20 mol% Ge fibers. However, there is evidence
of increase in the absorption loss in the 1500-nm window with the addition
of nitrogen. The next most photosensitive fibers are the germania-boron
or tin-doped fibers.