Page 51 - Fiber Bragg Gratings

32 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

wavelength shifts 0.05 nm to longer wavelengths at a grating reflectivity
of—1.4 dB. When the grating has grown to —27 dB (different grating but
same fiber), the shift is 0.1 nm, equivalent to an equilibrium temperature
increase of the fiber of ~80°C. At the start of grating growth (<1 dB), the
shift is not noticeable.
The formation of OH" ions with UV exposure increases the loss in
the 1500-nm window. There are two peaks associated with the formation
of Si-OH (1.39 /mi) and Ge-OH (1.42 /mi) on UV exposure [71]. A concen-
tration of 1 mol% of OH~ increases the loss at 1.4 /am by 5 dB. This is
avoided by soaking the fiber in deuterium, which shifts the first overtone
OD of the water peak to —1.9 /mi [74].
Another feature of hydrogen loading is the increased loss at wave-
lengths less than 1 /am after UV exposure of hydrogen-loaded fibers. The
loss has a wavelength dependence at <0.95 /mi of e~ 46/A , where A is in
microns [74].

Hydrogen loading of optical fibers
The loading of optical fibers with hydrogen is both temperature and pres-
sure dependent. The diffusion coefficient of hydrogen is [50]

where R = 8.311 J/(K-mol) and T is the temperature in degrees Kelvin.
The concentration of hydrogen in the fiber is calculated by solving the
diffusion equation [72] as a function of radial position p and time t:

where jm n are the nth zeroes of the J 0 Bessel function, D is the diffusion
coefficient, and R is the radius of the fiber. C 0 is the initial concentration,
and <f{p) = 1 if hydrogen is diffusing into the fiber and — 1 for out-diffusion.
The dynamics of the concentration have been modeled by Bhakti et al.
[73] to study the effect on the drift of the resonance wavelength of a long-
period grating. Under an ambient pressure of 200 bar, the in-diffusion of
hydrogen is shown in Fig. 2.10 as a function of the radius of the radial
position for different times. The out-diffusion of hydrogen is shown for
different times as the fiber is removed from the high-pressure chamber
[74] in Fig. 2.11. The first overtone OH~ peak at 1.245 /urn can be used