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In this study, we studied the laser process and crystallization of 2D YAG from yttrium aluminosilicate glass.

We report on the thermal regeneration of tilted Bragg gratings (TFBGs) produced in hydrogen loaded SMF-28 optical fiber using two types of UV lasers: 266 nm femtosecond pulses and 193 nm nanosecond pulses. Different isochronal and isothermal annealing profiles are used to improve the regeneration of the cladding modes. Experimental results are in good agreement with simulation results. We show that TFBGs regenerate efficiently in the temperature range 850 °C – 900 °C when isothermal stepwise annealing profile is used. The regeneration efficiency of the cladding modes does not exceed 4.35%. The sensitivity to the surrounding refractive index is comparable to that of non-regenerated tilted Bragg gratings. These regenerated tilted Bragg gratings can be applied in several applications, like for example the follow up of high-temperature fabrication processes.

We show the possibility of using a femtosecond IR point-by-point inscribed grating as a probe to monitor the regeneration process of a UV-inscribed grating.

International audience; Femtosecond laser-induced crystallization in glasses is of interest because of its significant applications in optics and photonics. Micro-YAG crystals produced by controlled crystallization from a glassy matrix is one of the studied material to achieve this goal. In this work, a three-dimensional (3D) space-selective crystallization of YAG by femtosecond laser irradiation was investigated in yttrium aluminosilicate glass. Independent cubic YAG crystals of a tenth μm edge were grown in a glass sample depth for 20, 30 and 40 seconds irradiation. The crystallization mechanism was discussed in both static and dynamic mode by illustrating the distribution of different gradients. The YAG crystals grew on the surface and at various depths of the glass matrix with particular morphologies depending on the laser application mode.

In this study, we report on the thermal regeneration of Type-I tilted fiber Bragg gratings produced in hydrogen loaded single-mode optical fiber (SMF28). Both 193 nm excimer laser and 266 nm femtosecond pulses were used to inscribe the gratings through phase mask technique. We show that the regenerated cladding modes resist, without degradation, up to 910 ◦C and show the same refractometric properties as in non-regenerated tilted gratings.

We present a calibration procedure for a humidity sensor made of a fiber Bragg grating covered by a polyimide layer. FBGs being intrinsically sensitive to temperature and strain, the calibration should tackle three variables, and, therefore, consists of a three-variable, two-level factorial design tailored to assess the three main sensitivities, as well as the five cross-sensitivities. FBG sensing information is encoded in the reflection spectrum from which the Bragg wavelength should be extracted. We tested six classical peak tracking methods on the results of the factorial design of the experiment applied to a homemade FBG humidity sensor. We used Python programming to compute, from the raw spectral data with six typical peak search algorithms, the temperature, strain and humidity sensitivities, as well as the cross-sensitivities, and showed that results are consistent for all algorithms, provided that the points selected to make the computation are correctly chosen. The best results for this particular sensor are obtained with a 3 dB threshold, whatever the peak search method used, and allow to compute the effective humidity sensitivity taking into account the combined effect of temperature and strain. The calibration procedure presented here is nevertheless generic and can thus be adapted to other sensors.

Type-I fiber Bragg gratings photo-inscribed in hydrogen-loaded B/Ge co-doped silica single-mode optical fibers have been regenerated efficiently at 450°C, which is the lowest temperature reported so far. The mechanical strength of the annealed fiber is preserved while ensuring temperature sensing of the regenerated gratings up to 900°C. Unlike low temperature cycles (≤600°C), an annealing process at higher temperatures revealed faster regeneration for strong gratings. Changes in grating strength were also measured before the regeneration cycle. These behaviors suggest the contribution of different mechanisms to the regeneration process with different relative dynamics.

The design of the experiment is a scientific approach that provides the maximum amount of information with the minimum number of experiments. It is applicable in scientific and industrial research. We represent a three-variable two-level factorial design to assess fiber Bragg grating properties under simultaneous temperature, humidity, and strain stimuli. Three uniform gratings were inscribed in a single-mode standard optical fiber using the interferometric technique. Two gratings were recoated, one with acrylate and another with a layer of polyimide, while the third grating remained bare. With only eight measuring points, the sensitivities of temperature, strain, and humidity were computed. Moreover, with this technique, the cross-sensitivities between temperature and strain, temperature and humidity, humidity and strain, and between all three factors could also be quantified. We have proven that the results of this design are comparable with those of the classic method. For all gratings, the temperature and strain sensitivities were obtained in the order of 10 pm/°C and 1.1 pm/με, respectively. The humidity sensitivity of the polyimide recoated grating was estimated to be in the order of 4.47 pm/%RH at room temperature.

A structured sapphire-derived all-glass optical fiber with an aluminum content in the core of up to 50 mol% was used for fiber Bragg grating inscription. The fiber provided a parabolic refractive index profile. Fiber Bragg gratings were inscribed by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. Heating experiments demonstrated the stability of the gratings for temperatures up to 950 degrees C for more than 24 h without degradation in reflectivity.