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We report continuous measurements of the transmission spectrum of a fiber loop mirror interferometer composed of a Panda-type polarization-maintaining (PM) optical fiber during the diffusion of dihydrogen (H 2 ) gas into the fiber. Birefringence variation is measured through the wavelength shift of the interferometer spectrum when the PM fiber is inserted into a gas chamber with H 2 concentration from 1.5 to 3.5 vol.% at 75 bar and 70°C. The measurements correlated with simulation results of H 2 diffusion into the fiber lead to a birefringence variation of -4.25 × 10 -8 per mol m -3 of H 2 concentration in the fiber, with a birefringence variation as low as -9.9×10 -8 induced by 0.031 μmol m -1 of H 2 dissolved in the single-mode silica fiber (for 1.5 vol.%). These results highlight a modification of the strain distribution in the PM fiber, induced by H 2 diffusion, leading to a variation of the birefringence that could deteriorate the performances of fiber devices or improve H 2 gas sensors.

Plasmonic tilted fiber Bragg gratings (TFBGs) are very efficient for fast, accurate, and minimally invasive biosensing. Their transmitted amplitude spectrum is a dense comb of narrowband cladding mode resonances (full width at half maximum < 1 nm) that is usually demodulated using highly resolved (wavelength resolution < 10 pm) devices. This work demonstrates the possibility of using a coarsely resolved spectrometer (166 pm) to read out the amplitude spectrum of a gold-coated TFBG. A refined analysis of the spectral content has allowed us to develop signal processing that provides a refractometric sensitivity of 2656 nm/RIU. This is a fivefold improvement compared to previously reported read-out techniques. Biosensing has then been successfully implemented with gold-coated TFBGs implemented in reflection mode for the detection of insulin, with specific antibodies grafted on the gold surface. Our experimental work is a first step toward the industrialization of the FBG technology, as it opens the door to fast parallel biosensing, profiting from the multiple sensing channels (up to 64) of the interrogator and its high processing speed (repetition rate up to 3 kHz).

We report the inscription of highly reflective fiber Bragg gratings in perfluorinated polymer (CYTOP) optical fibers using 800 nm femtosecond pulses and a line-by-line inscription method. We demonstrate that the energy for grating inscription without damage is below 27 nJ. After the writing process, we show that the grating reflectivity continues to rise for several hours. The obtained gratings show reflectivity degradation with temperature increase, while the Bragg wavelength undergoes a linear temperature dependence of 37.7 pm/°C when the humidity is controlled and the heating/cooling rate is relatively low (≤0.14 ∘ C / m i n ). The humidity sensitivity at a fixed temperature (25°C) is 22.3 pm/%RH. Finally, the axial strain sensitivity is determined to be 1.35 pm/µɛ.

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.

In this Letter, we report the fast growth of high quality uniform Bragg gratings in trans-4-stilbenemethanol (TS)-doped poly(methyl methacrylate) (PMMA) step-index optical fibers. Grating manufacturing was obtained using a 400 nm femtosecond pulsed laser and a 1060-nm-period uniform phase mask. For 20 mW mean laser beam power, the grating reflectivity reaches 98% in ∼60  s.

An endlessly single-mode doped microstructured poly(methyl methacrylate) (PMMA) optical fiber is produced for effective fiber Bragg grating (FBG) photo-inscription by means of a 400 nm femtosecond pulsed laser and the phase mask technique. The fiber presents a uniform benzyl dimethyl ketal (BDK) distribution in its core without drastic loss increase. It was produced using the selected center hole doping technique, and the BDK dopant acts as a photoinitiator. In this Letter, we report a rapidly growing process of the grating reflection band. For an 11 mW mean laser power, the FBG reflectivity reaches 83% in only 40 s.

Bragg gratings are photo-inscribed in trans-4-stilbenemethanol doped PMMA fibers using a 325 nm He-Cd laser and a phase mask. Two distinct behaviors are reported depending on the laser power density. In the high-density regime with 637  mW/mm 2 , the grating reflectivity is stable over time after the writing process, but the reflected spectrum is of limited quality, as the grating length is limited to the laser width (1.2 mm). The beam is then enlarged to 6 mm, decreasing the power density to 127  mW/mm 2 . In this case, the grating reflectivity strongly decays after the writing process. A fortunate property here results from the recovery of the initial reflectivity using a post-inscription thermal annealing. Both behaviors are attributed to the evolution between trans- and cis-isomers.

Highly localized refractive index modulations are photo-written in the core of pure silica fiber using point-by-point focused UV femtosecond pulses. These specific gratings exhibit a comb-like transmitted amplitude spectrum, with polarization-dependent narrowband cladding mode resonances. In this work, eccentric gratings are surrounded by a gold sheath, allowing the excitation of surface plasmon polaritons (SPP) for radially-polarized light modes. The spectral response is studied as a function of the surrounding refractive index and a maximum sensitivity of 50  nm/RIU (refractive index unit) is reported for a well-defined cladding-mode resonance among the spectral comb. This novel kind of plasmonic fiber grating sensor offers rapidity of production, design flexibility, and high temperature stability.

In this Letter we report a fast thermal regeneration of Type I fiber Bragg gratings inscribed with a UV laser in up to four different optical fibers: hydrogenated standard fiber, hydrogenated highly Ge-doped fiber, hydrogenated photosensitive fiber, and nonhydrogenated fiber. The thermal treatment consists in directly introducing the optical fiber into a preheated oven. The preheat temperature depends on the type of fiber used and is high enough to erase the grating and regenerate it afterward. The best results are obtained with hydrogenated photosensitive fiber and highly Ge-doped fiber, whereas no satisfactory results were obtained with hydrogenated standard fiber and nonhydrogenated photosensitive fiber. A regenerated grating with only 1.6 dB of loss was obtained in 10 min, reducing the time needed by a factor of 5.7. By adjusting the temperature of the oven, regenerated gratings of 13.7 dB of loss in 31 s and 5.8 dB of loss in 3 min were obtained. The factors of improvement in time are 110.3 and 19, respectively.

We report highly birefringent fiber Bragg gratings in standard single-mode optical fiber realized with UV femtosecond pulses and line-by-line inscription. By controlling the three-dimensional positioning of the focused laser beam with respect to the fiber core, we achieve very high birefringence at the grating location in a single exposure. A maximum birefringence value of 7.93×10(-4) has been reached for 10th-order gratings when using 2 μJ pulses, which is to our knowledge the highest birefringence value reported so far. This birefringence results from UV-induced high-densification lines shifted from the center of the core, increasing the asymmetry of the induced-stress lines. With a Bragg wavelength spacing reaching more than 800 pm between polarization modes, such gratings are particularly well suited for selective filtering or, as demonstrated here, for temperature-insensitive transverse-strain measurements.