J.F. Bayon

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.

UV induced densification during Bragg grating inscription in Ge:SiO2 preforms

Abstract The only investigation of a Bragg grating in Ge:SiO 2 in the direct space known to date is a view from an optical microscope. Here, we show surface level modulations associated with the writing of the grating. Our conclusion is that a densification takes place. We estimate the corresponding peak to peak change of the refractive index to be 1.4−1.7 × 10 −4 whether a pulsed or a CW irradiation mode is used. Compared to the refractive index change obtained by optical measurement, this accounts for a non-negligible part (7% for pulsed and 26% for CW irradiation). We discuss the possible application of the observations to fiber and the differences in photo-chemical reactions according to the irradiation mode.

Behaviour of spectral transmissions of Bragg gratings written in germania-doped fibres: writing and erasing experiments using pulsed or cw uv exposure

Abstract Photoinscriptions of Bragg gratings have been carried out within several germanosilicate fibres via uv side writing near 244 nm. The gratings have been written with a pulsed or a continuous-wave (cw) laser. It has been observed that the refractive index changes were strongly non linear as a function of the exposure time whatever the nature of the source may be. Uv pulsed or cw photobleaching experiments have been performed using gratings written under various conditions in fibres for which complex evolutions of spectra could be observed. Uv fringeless illumination of a grating firstly can erase the first order transmission spectrum and then build a new spectrum. Results of thermal annealing of gratings written using pulsed exposure or using cw light are analysed. We render a phenomenological account of the experimental results by assuming that, at least, two photorefractive effects are responsible for the grating formation or erasure.

UV induced densification during Bragg grating inscription in Ge:SiO2 preforms: interferometric microscopy investigations

The only investigation of Bragg grating in Ge:SiO2 in the direct space known at this date is a view from an optical microscope. Here, we show surface level modulations associated with writing of grating. Our conclusion is that a densification takes place which corresponds to a refractive index increase. Compared to the refractive index change obtained by optical measurement, this accounts for a non-negligible part (7%).

TEM characterization of structural changes in glass associated to Bragg grating inscription in a germanosilicate optical fibre preform

Abstract Bragg gratings were written in a slice of an optical fibre preform using two interfering uv pulsed laser beams at 243 nm. Microstructural changes within the germanosilicate glass were studied by transmission electron microscopy (TEM) and infrared spectroscopy. TEM investigation shows that the glass is densified at the grating grooves. Infrared absorption spectra have been recorded on a germanosilicate glass sample before and after in irradiation by a fringeless uv pulsed beam. Our conclusion is that uv irradiation-induced bond breaking allows structural relaxation of the non-equilibrium glass network which tends to densify without any noticeable change in coordination.

Study of organized Ξ(2) susceptibility in germanosilicate optical fibers

Abstract Poling has been achieved in a specially designed germanosilicate fiber. The Ξ(2) susceptibility was detected and measured through refractive index changes measurements using an interferometric method within the fiber. Two methods for poling have been used: the thermally-assisted electrical method and the UV-assisted electrical method. Both the poling techniques and the results concerning the linear electro-optic effect are presented.