Isabelle Riant

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.

Study of the photosensitivity at 193 nm and comparison with photosensitivity at 240 nm influence of fiber tension: type IIa aging

The photosensitivity in germanosilicate fibers at 193 nm is studied and compared with that at 240 nm. As previously shown, it is found that, without hydrogen sensitization, the initiation of the phenomenon is a one-photon process for a high germanium content fiber and a two-photon process for a low germanium content fiber. However, when the low germanium content fiber is previously loaded with hydrogen, it is observed that the photo-induced index change initial growth rate varies linearly with the fluence, invoking a one-photon process. It is shown that external strains applied to the fiber strongly influence the mechanisms of the photosensitivity at 193 nm. Finally, an aging study is carried out.

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%).

Comparison of isochronal and isothermal decays of Bragg gratings written through continuous-wave exposure of an unloaded germanosilicate fiber

We compare the results of thermally induced isochronal and isothermal decays of fiber Bragg gratings written through cw exposure of an unloaded germanosilicate fiber. We show that isochronal step decays can be used to predict isothermal decays, provided that some corrections are carried out to take into account a reversible change in grating reflectivity induced by the increase of the fiber temperature. The isochronal accelerated-aging method enables one to sample most of the initial distribution of trapped site energies in a fairly short time. Taking advantage of this property of the method, we show that the initial distribution for a weak grating is similar to those for stronger gratings. The consequences of this observation are discussed within the framework of the various reaction pathway model.

Temperature-induced reversible changes in the spectral characteristics of fiber Bragg gratings

It is reported that reversible changes in the reflectivity of Bragg gratings can be induced by a change in the temperature of the grating (77 K < T < K). The changes have proved to be greater in highly doped Ge fibers than in standard fibers, whereas they could hardly be detected in hydrogenated fibers. The sign of the change for type I gratings was opposite that for type IIA gratings. The changes are likely due to a temperature-induced increase (or a decrease) in the amplitude of the refractive-index modulation. Possible mechanisms for these changes in modulation are discussed. Interestingly for the purpose of correcting data of isothermal accelerated aging experiments, a numerical relation that accounts for the temperature-induced changes in type I grating reflectivity is given.

Thermal stability of the 248-nm-induced presensitization process in standard H2-loaded germanosilicate fibers.

Grating growths through exposure of presensitized standard fibers to KrF light were recorded in various experimental condition. It is shown that there exists an optimum sensitization fluence at which the efficiency of the sensitization process is higher. Isochronal thermal annealing of pre-exposed fibers led to a decrease in the sensitization-induced enhancement of photosensitivity. IR-absorption spectroscopy was carried out in fibers or preform plates to monitor the attenuation ascribed to H-bearing species in the same samples. The annealing-induced decay in photosensitivity cannot be correlated with those of the H-bearing species in the whole temperature range (110 degrees C-800 degrees C). This indicates that the enhancement of photosensitivity comes from the transformation of more than one species.

Gain equalization with optimized slanted Bragg grating on adapted fibre for multichannel long-haul submarine transmission

Slanted Bragg grating and fiber were specially designed for erbium amplifier gain flattening. Results on 11 cascaded 50-km amplified spans show 0.1 dB gain excursion per amplifier on bandwidth as wide as 17.8 nm.

Widely Vernier tunable external cavity laser including a sampled fiber Bragg grating with digital wavelength selection

We demonstrate an easy and efficient step tunable sampled fiber Bragg grating (SFBG) external cavity laser. A small current variation applied to a single electrode induces a step tuning of 15 nm based on the Vernier effect between an SFBG with a comb-like reflectivity and a two-section semiconductor laser diode. Thanks to the low current variation of less than 3 mA in the phase section, the output power remains constant within 0.6 dB at 6 dBm. The side-mode suppression ratio is greater than 40 dB over the whole tuning range.

Suppression of discrete cladding mode resonances in fibre slanted Bragg gratings for gain equalisation

In a slanted Bragg grating, coupling between the fundamental guided mode and the counterpropagative cladding modes result in discrete resonances in the transmission spectrum. These resonances are a drawback when Slanted Bragg Gratings are used for gain flattening of fibres amplifiers. A new method based on a chemical etching of the cladding is proposed leading to an overlap of the resonances and a reduction of the amplitude of the modulation. This method can be applied for any value of photo induced modulation amplitude in the SBG.

New and efficient technique for suppressing the peaks induced, by discrete cladding mode coupling in fiber slanted Bragg grating spectrum

A new design of fiber slanted Bragg grating exhibiting a smooth transmission spectrum without the need of a specific packaging to transform the cladding modes into a continuum of radiation modes is reported.