Page 54 - Fiber Bragg Gratings

2.5 Densification and stress in fibers 35

Figure 2.12: Out-diffusion time (1/e of initial concentration, C 0 of hydrogen
as a function of fiber diameter at 20°C (a), and the diffusion time for a standard
fiber as a function of temperature (b) (courtesy F. Bhakti).

The direct writing of gratings in RE fibers is limited to longish lengths,
which suits the fabrication of narrow-band DFB fiber lasers. All gratings
are of Type I or Type II; Type IIA has not been reported. A summary of
the results on the most photosensitive fibers is listed in Table 2.1.

2.5 Densification and stress in fibers

There is increasing evidence that densification and stress increase in opti-
cal fibers contributes to the change in the refractive index [84-87]. Surface