Ismael Chiamenti

Plasmonic optical fiber sensors: enhanced sensitivity in water-based environments.

In this work, we demonstrate a refractometric fiber sensor with improved sensitivity for refractive indices ranging from 1.3629 to 1.4479. The device relies on the coupling between a long period grating (LPG) transducer and a localized surface plasmon resonance (LSPR). Sensor operation is based on the transference of energy from LPG cladding modes at the visible spectral range to the LSPR. The transducer consists of a long period grating 3.15-mm-long covered with 2-7 nm gold nanoparticles. The sensor is intensity coded at a 568 nm wavelength, presenting sensitivity of 208%/unit of refractive index for a refractive index of 1.39 and resolution better than 0.0003 for the dynamic range of refractive indices from 1.3629 to 1.4479.

Broadband Optical Active Waveguides Written by Femtosecond Laser Pulses in Lithium Fluoride

Broadband waveguiding through light-emitting strips directly written in a blank lithium fluoride crystal with a femtosecond laser is reported. Light guiding was observed at several optical wavelengths, from blue, 458 nm, to near-infrared, at 1550 nm. Visible photoluminescence spectra of the optically active F2 and F3+ color centers produced by the fs laser writing process were measured. The wavelength-dependent refractive index increase was estimated to be in the order of 10−3−10−4 in the visible and near-infrared spectral intervals, which is consistent with the stable formation of point defects in LiF.

Characterization of femtosecond-inscribed fiber Bragg gratings in the visible spectral region

The femtosecond-laser pulse inscription and characterization of fiber Bragg gratings for operation at visible wavelengths was performed using several types of optical fibers, including single-mode and graded-index fibers designed for near-infrared wavelengths. The obtained bandwidths are very narrow (∼0.12-0.36  nm) for the used exposure conditions, even in graded-index fibers. Thermal and strain characterization was performed, with results about half of those found for C-band gratings. The wavelength dependence of the sensitivity is compared with a Sellmeier model.

Dimensions and refractive index estimates of deeply buried optical waveguides in Lithium Fluoride

A recursive procedure is applied to the measured near-field profiles of buried optical waveguides recorded in a lithium fluoride (LiF) crystal by femtosecond laser pulses in order to estimate the core dimensions and the refractive index increase. Albeit the waveguides transversal section geometry is quite complex it is possible to obtain the horizontal and vertical widths and the average refractive index maximum increase assuming a simplified rectangular transversal section in the simulation. The procedure is validated by comparing the simulated results with the experimental near-field profiles and the maximum refractive index values of two commercial optical fibers. Typical dimensions of ~(8x10)µm2 and refractive index changes of ~(2-10)x10-4 were obtained for the LiF waveguides at several wavelengths.

Visible wavelength fiber Bragg gratings: thermal and strain sensitivities

The thermal and deformation properties of fiber Bragg gratings (FBG) in the visible range were characterized for the first time in our knowledge. The FBG were written in silica single mode (cutoff in the visible and infrared range) and multimode fibers, using a phase-mask (460 nm period) illuminated by a 248 nm femtosecond laser.

Polymer channel waveguide: Prospects of production and characterization

This work shows the production of optical channel waveguides in polymethyl methacrylate slabs. A writing technique employing an Argon laser at 488 nm was used. Parameters in the writing step were adjusted to obtain waveguides with different characteristics. Writing scanning speeds of 5 mm/s, 3 mm/s and 1 mm/s resulted in waveguides widths of 0.4 mm, 0.5 mm and 0.8 mm, respectively. Transversal profiles of refractive index of waveguides were measured by means of Optical Coherence Tomography. Guiding capability was determined by the near field pattern diffraction.

Functionalization of a long period grating coated with gold nanoparticles for glyphosate detection

This work describes a method for producing a nano structured fiber optic device for sensing of pesticides in water environment. The device consists of a long period grating with a coating of gold nanoparticles functionalized with cysteamine. The LPG shows attenuation bands near the phase matching turning point at the visible spectral range. A bottom-up production route was used to deposit gold nanoparticles on the fiber surface. Sensitivity to the refractive index of the external medium was measured before and after the layer deposition. Cysteamine was used as a ligand for glyphosate present in water at a concentration of 100 μM.

Thermal sensitivity increase of RFBG in the visible range

Thermal sensitivity increase of regenerated fiber Bragg gratings (RFBG) in the visible range is reported. The FBGs are produced by direct illumination under the phase mask using UV light from two different laser sources at 248 nm. After regeneration the thermal sensitivity is obtained at 500°C to avoid spectra vanishing for temperatures near that of regeneration. At 500°C, the determined thermal sensitivities are 5.74pmK-1 before and 7.65pmK-1 after regeneration (single mode fiber in the visible, femtosecond laser), and a similar pattern is presented for all regenerated FBGs.

Refractive index sensitivity in etched FBG in the visible range

A visible fiber Bragg grating (Vis-FBG) with wavelength peak centered at 673.07 nm was inscribed in a multimode fiber designed for infrared (IR) operation using a femtosecond (fs) laser emitting at 248 nm. The fiber cladding is removed by chemical etching in hydrofluoric acid solution (40%). The sensor refractive index response is determined by dipping the sensor into diluted glycerin solution at different concentrations with refractive index range from 1.3328 to 1.4607. The Vis-FBG performance is compared with an IR etched FBG (EFBG) with similar diameter. The sensitivity found for the Vis-FBG sensor is 15.71nm/RIU with a 6.34 x 10-3 RIU resolution for a refractive index of 1.4607.

Regeneration characteristics of FBG writen for the visible spectral range

We report regeneration studies of fiber Bragg gratings with resonance band in the visible spectral range. The FBG were produced by direct illumination under the phase mask using UV light from a fs-laser pumped optical parametric amplifier. Some seed gratings were not so saturated as common C-Band gratings written by other methods but their regeneration efficiency is quite high. The final line-widths are narrower than the similar C-band regenerated gratings.