EXPERIMENTAL-EVIDENCE OF 2 TYPES OF PHOTOREFRACTIVE EFFECTS OCCURRINGDURING PHOTOINSCRIPTIONS OF BRAGG GRATINGS WITHIN GERMANOSILICATE FIBERS

Photoinscriptions of Bragg gratings have been carried out within six g ermanosilicate fibres. Real time measurements of the spectral transmis sion, of the fwhm bandwidth and of the resonance wavelength of the Bra gg gratings permit to estimate the refractive index change photoinduce d in the core of the fibre. It has been observed that the refractive i ndex changes were strongly nonlinear as a function of the number of ul traviolet (UV) laser pulses used for writing the gratings. Experimenta l results show that, at least, two photorefractive effects should be r esponsible for the complicated dynamical evolution of the grating grow th observed within some fibres.