2.2        Fiber Bragg grating

2.2.2       Photosensitivity in fibers

a)   History

Hill and co-workers (1978, [2-3]) discovered photosensitivity of germanium-doped silica fibers. In their experiment, the 488nm laser light coupled into a fiber interfered with the Fresnel reflected beam and thus formed a weak standing-wave intensity pattern and a correspondent permanent index change. Lam and Garside (1981,[2-4]) showed that the magnitude of the photoinduced refractive index change depended on the square of the writing power, suggesting a two-photon process as the possible mechanism of refractive-index change. In 1989, Meltz et al. ([2-5]) demonstrated that a strong index of refraction change occurred when a germanium-doped fiber was exposed to UV light close to the absorption peak of a germania-related defect at a wavelength range of 240-250nm (single-photon process).

b)   Origin of the photosentivity

The mechanisms that create the refractive index change are not fully understood. Several models have been proposed. The recurrent element in these theories is that the germanium-oxygen vacancy defects, Ge-Si or Ge-Ge (the so-called "wrong bonds") are responsible for the photoinduced index changes. The main models for the photosensivities of optical fibers are :

-         The color center model [2-6, 2-7] : the breaking of the GeO defect by the UV light results in a GeE' center and the released electron is free to move within the glass matrix; when this electron is trapped, an additional absorption center appear in the glass and due to the Kramers-Kronig relation, a refractive index change is observed

-         The dipole model [2-8, 2-9] : the photo-excitation of defects forms built-in periodic space-charge electric fields

-         The stress-relief model [2-10, 2-11] : a refractive index change arises from the alleviation of built-in thermo-elastic stresses in the core of the fiber that was created during the fiber fabrication

-         The compaction model [2-12, 2-13] : the laser irradiation induces density variations of the glass that also change the refractive index

c)   Enhanced photosensitivity in silica optical fibers

i.      Dopant concentration increase

The photosensitivity is highly increased by a high concentration of germanium. Nevertheless, this kind of fiber exhibits high NA, incompatible with telecommunication devices. Then, fibers containing boron have an enhanced photosensitivity. The maximal refractive index changes are higher and achieved faster than for any other kind of fiber. Boron codoping increases the photosensitivity of the fiber by allowing photoinduced stress relaxation. Another benefit of boron co-doping is the compatible NA with standard telecommunication fibers. Other co-doping as tin has been reported.

ii.    Hydrogen loading of the fiber

Hydrogen loading is carried out by diffusing hydrogen molecules into the optical fiber at high pressures. The reaction of hydrogen molecules at the Ge sites produces germanium-oxygen deficiency centers when exposed to UV light [2-14]. This is not a permanent effect, and as the hydrogen diffuses out, the photosensibility decreases.

iii.   Irradiation with a UV laser at 193 nm

Bragg gratings fabricated silica fibers using 193nm UV light have stronger reflectivity than gratings inscribed with 248nm under similar excitation conditions [2-15].