2.4        Experimental results

2.4.4       Characterization of the photosensitivity

a)   Method based on the variation of the refractive index modulation amplitude

In this method, the measurement of a FBG reflectivity at different irradiation values gives the amplitude Dn of the refractive index modulation through the following equations

where R is the reflectivity, k the coupling coefficient, L the length of the grating, h the overlap integral between the LP₀₁ mode and the fiber core, and l_b the Bragg wavelength. The above equations are only useful for gratings that are not too strong. For this reason, very small grating are used for this measurement (around 0.2 mm). If the fringe visibility is 100 %, the mean index is equal to the amplitude. In case of a smaller fringe visibility, the mean index has to be measured independently.

Fig. 2-16 Photosensitivity of a standard fiber (Inset: FBG transmission spectra for different number of total pulses).

The photosensitivity of a standard fiber (Spectran SMT-A1310H) has been determined with this method. The fiber was hydrogen loaded (7 days at 150 bars) and irradiations were performed at 193 nm. Different transmission spectra of the FBG (Fig. 2-16 inset) are used to retrieve the refractive index modulation amplitude Dn as a function of the total dose (proportional to the number of pulses). Figure Fig. 2-16 shows four of these transmission spectra and the photosensitivity curve. The maximal refractive index modulation amplitude is 2×10^-3.

b)   Method based on the variation of the mean effective refractive index

To measure the mean index, a weak FBG (as small as possible) is used. The exposure of the grating region with a homogeneous laser beam increases the refractive index of the core and thus the mean effective refractive index , which can be measured with the shift of the Bragg wavelength l_b

The photosensitivity of the polarization maintaining fiber (Fibercore HB1250P) has been measured with this method. The fiber is also hydrogen loaded. Different transmission spectra of the FBG are used to retrieve the mean effective refractive index as a function of the total dose (proportional to the number of pulses). Fig. 2-17 shows the photosensitivity curve. The photosensitivity of the polarization maintaining fiber saturates at 7×10^-3.

Fig. 2-17 Photosensitivity of the polarization maintaining fiber Fibercore HB1250P.