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Publicações

Publicações por Paulo Caldas

2015

Arc-Induced Gratings in the Turning Points

Autores
Colaco, C; Caldas, P; Chibante, R; Rego, G;

Publicação
24TH INTERNATIONAL CONFERENCE ON OPTICAL FIBRE SENSORS

Abstract
We demonstrated the inscription of arc-induced long-period fiber gratings (LPFGs) in the B/Ge co-doped fiber and in the SMF28 fiber by using grating periods shorter than 150 mu m and 200 mu m, respectively. This achievement was a result of the development of a high voltage power supply that allows for a constant and stable electric current ranging from 10.5 mA up to 21 mA. The fabricated LPFGs were characterized as a function of the external refractive index from 1.333 up to 1.420 and a refractive index sensitivity of the order of 1000 nm/RIU was obtained without further optimization.

2016

Arc-Induced Long-Period Fiber Gratings in the Dispersion Turning Points

Autores
Colaco, C; Caldas, P; Del Villar, I; Chibante, R; Rego, G;

Publicação
JOURNAL OF LIGHTWAVE TECHNOLOGY

Abstract
We demonstrated the possibility to inscribe long-period fiber gratings (LPFGs) in a B/Ge codoped fiber by using grating periods shorter than 150 mu m. We also have arc-induced in the SMF 28 fiber an LPFG in the dispersion turning points by using a grating period of 197 mu m. In previous works, the shortest periods were, respectively, of the order of 190 and 320 mu m for the same fibers. To achieve such a considerable reduction in the grating periods which enables access to the higher order cladding modes (higher sensitivity), we have developed a high-voltage power supply that allows for a constant and stable electric current ranging from 10.5 up to 21 mA. Computer simulations were used to identify the cladding mode resonances for each grating inscribed in the different fibers. The fabricated LPFGs were characterized as a function of the external refractive index from 1.33 up to 1.42, and an average refractive index sensitivity of -720 nm/RIU in the 1.33-1.41 range was obtained for a 192-mu m LPFG without further optimization, such as the use of etching or thin films deposition.

2015

Cryogenic Temperature Response of Reflection-Based Phase-Shifted Long-Period Fiber Gratings

Autores
Martins, R; Caldas, P; Teixeira, B; Azevedo, J; Monteiro, J; Belo, JH; Araujo, JP; Santos, JL; Rego, G;

Publicação
JOURNAL OF LIGHTWAVE TECHNOLOGY

Abstract
In this study, we investigated the temperature behavior of phase-shifted long-period fiber gratings (PS-LPFGs) inscribed in two types of optical fiber: B/Ge and SMF28. The experiments were carried out from 5 to 305 K using a superconducting quantum interference device magnetometer. The average temperature sensitivity obtained of -0.43 nm/K for PS-LPFGs inscribed in the B/Ge fiber is one order of magnitude larger than for PS-LPFGs inscribed in the SMF28 fiber, in the 60-240 K range. Values ranging from -0.08 nm/K up to 0.2 nm/K were obtained in the 5-35 K temperature range, which are considerably better than previous results achieved for metal-coated FBGs and also for LPFGs inscribed in a similar B/Ge codoped fiber. Nevertheless, further work is required in order to correctly address sensor reliability.

2014

Optical Fiber Refractive Index Sensor with Reduced Thermal Sensitivity based on Superimposed Long-Period Gratings

Autores
Silva, GE; Caldas, P; Santos, JC; Santos, JL;

Publicação
23RD INTERNATIONAL CONFERENCE ON OPTICAL FIBRE SENSORS

Abstract
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.

2014

All-Fiber Sensor Based on a Metallic Coated Hybrid LPG-FBG Structure for Thermal Characterization of Materials

Autores
Silva, GE; Caldas, P; Santos, JC; Santos, JL;

Publicação
23RD INTERNATIONAL CONFERENCE ON OPTICAL FIBRE SENSORS

Abstract
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.

2014

Reflection-Based Phase-Shifted Long-Period Fiber Grating for Cryogenic Temperature Measurements

Autores
Martins, R; Monteiro, J; Caldas, P; Santos, JL; Rego, G;

Publicação
23RD INTERNATIONAL CONFERENCE ON OPTICAL FIBRE SENSORS

Abstract
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.

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