J. L. Santos

Quantum dots as self-referenced optical fibre temperature probes for luminescent chemical sensors

The use of semiconductor nano-particles as temperature probes in luminescence chemical sensing applications is addressed. Temperature changes the intensity, the peak wavelength and the spectral width of the quantum dots luminescent emission in a linear and reversible way. Results are presented that show the feasibility of implementing a self-referenced intensity-based sensor to perform temperature measurements independent of the optical power level in the sensing system. A resolution of 0.3 °C was achieved. In addition, it is demonstrated that self-referenced temperature measurements at multiple points could be performed using reflection or transmission based optical fibre configurations.

Luminescence-Based Optical Fiber Chemical Sensors

Abstract A scheme for the simultaneous determination of temperature and analyte concentration for application in luminescence-based chemical sensors is proposed. This scheme is applied to an optical oxygen sensor, which is based on the quenching of the fluorescence of a ruthenium complex. Temperature measurement is performed using the excitation radiation and an absorption long-pass filter. Preliminary results are presented that show the viability of an oxygen measurement that is independent of temperature and optical power level. The possibility of self-referenced temperature measurements with semiconductor nanoparticles is also investigated. In order to optimize the sensor design, several different optical fiber probe geometries for oxygen sensing are tested and compared, including different methods of coupling radiation into the optical fiber system. Polyvinyl alcohol (PVA) and polyacrylamide membranes are tested as supports for sensor immobilization in fiber-optical pH sensing devices in aqueous solution. Some results are presented that show the feasibility of using fiber-optical pH indicators for remote monitoring.

Study of the properties of arc-induced long-period gratings and Bragg gratings in B/Ge doped fibers

We have investigated the possibility to erase arc-induced long period gratings through uniform exposure to UV laser radiation and their potential recover by thermal annealing. The temperature stability of UV/arc-induced gratings is also presented.

Characterization of response of a dual resonance of an arc-induced long-period grating to various physical parameters

We present, for the first time to our knowledge, results on the characterization of response of a dual resonance observed in the spectrum of a single long-period grating arc-induced in a B/Ge co-doped fiber to different physical parameters. The dual resonance is formed by two overlapping resonances corresponding to coupling of the core mode to symmetric and antisymmetric cladding modes. Therefore, these resonances may behave differently when strain, bending, torsion, or temperature is applied to the grating. We show that the bending, strain, and torsion sensitivities of the two resonances are very different, while the temperature sensitivities are almost the same.

Self-referenced intensity based optical fiber temperature probes for luminescent chemical sensors using quantum dots

The use of semiconductor nano-particles as temperature probes in luminescence chemical sensing applications is addressed. Temperature changes the intensity, the peak wavelength and the spectral width of the quantum dots luminescent emission in a linear and reversible way. Results are presented that show the feasibility of implementing a self-referenced intensity based sensor to perform temperature measurements independent of the optical power level in the sensing system. Additionally, it is demonstrated that self-referenced temperature measurements in multiple points could be performed using reflection or transmission based optical fiber configurations.

Intensity based luminescent optical fiber oxygen sensor using quantum dots

The suitability of semiconductor nanoparticles to provide a reference signal in luminescence based chemical sensors is addressed. A CdSe-ZnS nanocrystal, with emission peak at 520 nm is used to provide a reference signal. Measurements of oxygen concentration, which are based on the dynamic quenching of the luminescence of a Ruthenium complex, are performed. Both dye and the nanocrystal are immobilized in a sol-gel matrix and are excited by a blue LED. Results are presented showing that the ratio between the reference and the sensor signals is highly insensitive to fluctuations of the excitation optical power. Preliminary results show that nanocrystals could be used to measure temperature and provide a reference signal.

Optical fiber refractive index sensor with reduced thermal sensitivity based on Superimposed Long-Period Gratings

In this paper, we report the development of a reduced temperature sensitivity optical fiber sensor for refractive index measurement based on Superimposed Long-Period Gratings (SLPG) inscribed by the electric arc technique in standard fiber. The reduced sensitivity to temperature is achieved by calculation of the difference between resonance wavelengths of two guided cladding modes.

Reflection-based phase-shifted long-period fiber grating for cryogenic temperature measurements

In this work, we propose a compact sensor head to perform cryogenic temperature measurements based on a long-period fiber grating. The presented configuration enables the sensor to be interrogated in reflection since a phase-shifted is produced by Fresnel reflection on the end-face of the fiber, cleaved at a quarter-period separation distance from the end of the grating.

All-fiber sensor based on a metallic coated hybrid LPG-FBG structure for thermal characterization of materials

In this paper it is presented an all-fiber implementation of the hot-wire needle probe concept, widely used to measure the thermal properties of materials, particularly the thermal conductivity. It is based on the heating of a metal thin film deposited on the surface of the fiber induced by the coupling of laser light into the cladding via a long period grating, and determination, using a fiber Bragg grating, of the time dependence of the temperature of the surrounding medium at a fixed distance of the fiber. The medium considered in this research was the air and the results obtained indicate the feasibility of this approach and point out future developments.

Fibre Refractometer based on a Fabry-Pérot Interferometer

It is described a fibre optic sensor for liquid refractive index measurement based on the visibility variations of a Fabry-Perot interferometer with interfering waves generated in a short Bragg grating and in the fibre tip (Fresnel reflection) in contact with the liquid. The sensor was characterized emerging the fibre tip in distilled water with different concentrations of ethylene glycol. A linear relation was obtained, with a readout resolution of ≈ 10 -3 . It was also observed that the temperature direct cross sensitivity is residual.