Experimental evidence of two types of photorefractive effects occuring during photoinscriptions of Bragg gratings within germanosilicate fibres

Abstract Photoinscriptions of Bragg gratings have been carried out within six germanosilicate fibres. Real time measurements of the spectral transmission, of the fwhm bandwidth and of the resonance wavelength of the Bragg gratings permit to estimate the refractive index change photoinduced in the core of the fibre. It has been observed that the refractive index changes were strongly nonlinear as a function of the number of ultraviolet (UV) laser pulses used for writing the gratings. Experimental results show that, at least, two photorefractive effects should be responsible for the complicated dynamical evolution of the grating growth observed within some fibres.

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