Valmir de Oliveira

Bragg gratings in standard nonhydrogenated fibers for high-temperature sensing

Fiber Bragg gratings engraved in standard telecommunications-grade single-mode fibers without previous hydrogen loading show enhanced thermal stability for high-temperature measurements up to 800 °C. The reflectivity decay at that temperature is adequate for industrial applications with a weekly change of sensing heads.

Production of optical notch filters with fine parameter control using regenerated fiber Bragg gratings.

A new method to create a controlled notch filter for attenuation of signals based on regenerated fiber Bragg gratings is experimentally demonstrated. A fine adjustment of the notch depth is achieved by controlling the time of regeneration phenomenon in strongly saturated Bragg gratings written in standard ITU G.652 single-mode fiber. This method can used to produce tailored notch filters for several photonic applications, such as optical RF filtering, subcarrier processing or radio over fiber systems.

Fiber Bragg grating sensing applications in temperature monitoring of three-phase induction motors

This paper presents an analysis of simultaneous temperature and electrical parameters measurements in a three phase induction motor (TIM). The electrical parameters, namely voltage and current in the motor windings, were measured with Hall effect sensors and the temperature in the stator slots were monitored using fiber Bragg grating sensors (FBG). In the performed temperature analysis, two sets of sensors were used, containing four FBG in each fiber. Eight FBG sensors were uniformly distributed in the TIM stator. A special encapsulation of the sensor was elaborated to minimize mechanical disturbances in the fiber due to motor operation. Tests were performed with motor running at no load and with intentional voltage disturbances in the windings produced at every 15 minutes periods. It was observed that the temperature distribution is not uniform in the TIM stator either with or without a balanced voltage in the windings. A mean level increase of 0.5°C in temperature was registered in all of the sensors when there is a lack of balance in the voltage windings.

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.

Regeneration of Bragg gratings in polarization maintaining optical fibers

We report the formation of thermal regenerated Bragg gratings in different types of polarization maintaining fiber optics. Experimental results about the modification of the polarized reflection bands as a function of the temperature, before and after the grating regeneration, are presented.

A technique to package Fiber Bragg Grating Sensors for Strain and Temperature Measurements

This paper reports an effective method of packaging a fiber Bragg grating (FBG) for the simultaneous measurement of temperature and strain. The technique consists of embedding two fiber Bragg grating sensors inside a polymeric material with different geometrical characteristics. The mechanical and thermal characterizations of the optical transducer were performed. Then a matrix equation used measurements of the wavelength shifts from the two sensors and yielded information about the temperature and strain coefficients. The preliminary results demonstrate the feasibility of the encapsulation technique allowing measurements of temperature and strain in smart structures and harsh environments. The experimental procedure provides robustness to the sensor and the matrix equation approach has the potential to determine simultaneously the strain and temperature coefficients.

Thermal monitoring of the diamond deposition process using regenerated FBG

In this work, Bragg gratings were used to monitor, in real time, the temperature during the diamond deposition process obtained by hot filament chemical vapor deposition (HFCVD). Due to the instability of uniform fiber Bragg gratings (FBGs) to high temperatures, regenerated fiber Bragg gratings (RFBGs) were utilized. The results reveal that the diamond deposition process occurs at about 832 ºC (in the experimental conditions used in this case). The work also suggests that RFBGs are suitable devices to monitor processes which, as in the diamond deposition, the temperature is a critical parameter, and where other traditional methods have a little feasibility due to the own experimental setup or samples type under study.

Monitoring the junction temperature of an IGBT through direct measurement using a fiber Bragg grating

This paper proposes a new technique to monitor the junction temperature of an insulated gate bipolar transistor (IGBT) through direct measurement using an optical fiber sensor mounted on the chip structure. Some features of the sensor such as electromagnetic immunity, small size and fast response time allow the identification of temperature changes generated by the energy loss during device operation. In addition to the online monitoring of the junction temperature, results show the thermal characteristics of the IGBT, which can be used to develop an accurate model to simulate the heat generated during the device conduction and switching processes.

Strongly regenerated Bragg gratings in standard single-mode fibres

We report the formation of strongly regenerated Bragg gratings in standard, telecommunications grade, fibre optics. The fibres are Hydrogen loaded for several days and the inscription of the seed grating is done using 248 nm UV light. Seed gratings are not fully saturated. When heated to 800 °C-1000 °C the seed grating is regenerated; and the new grating presents strong reflection spectrum.

Multiplexed FBG Optical Instrumentation Using an FPGA-Based System

An FBG interrogation system was developed. Data processing algorithms were implemented by FPGA. It was tested by monitoring the fabrication of an evanescent-field sensor, which is then applied in an refractive index sensing experiment.