Redes de Bragg padrão e regenerada em fibra ótica: produção e análise do envelhecimento térmico para aplicações em baixas e em altas temperaturas

Abstract

It is important that the signal reflected intensity by optical sensors has a good long-term stability, but there are sparse researches in this area. Thus, the main objective of this work is to exhaustively analyze the thermal aging of FBG (Fiber Bragg Grating) and RFBG (Regenerated Fiber Bragg Grating) sensors operated at low and high temperatures, for long periods of time. For the analysis at low temperatures two optical sensors were produced, one of the standard FBG type and the other of the RFBG type, for environment temperature measurement between 0 °C and 40 °C. Weekly, reflection spectra of the two sensors were obtained using an optical sensing interrogator for 151 weeks. With the aid of a multimeter connected to a type-K thermocouple, temperature measurements were obtained as benchmark. When evaluating the reliability (precision and accuracy) of the measurements obtained, no discrepancy greater than 2 °C was observed between the reading of the thermocouple and the FBG and RFBG sensors, eventually being within the error range of the multimeter, which has a resolution of 1 °C associated with the resolution of the optical sensing interrogator, with the exception of just four spot measurements. In addition, it was found that the lifespan of the regenerated sensor (RFBG) is 20% longer than the sensor that was not regenerated (FBG). For the analysis at high temperatures five optical sensors of the RFBG type were produced to measure temperatures between 600 °C and 900 °C. The sensors were exposed to these high temperatures in increasing and decreasing way with steps of 50 °C or 100 °C, remaining 10 days at each temperature level to characterize the gratings at different levels. The reflection spectra of the RFBG optical sensors were obtained daily, during the 10-days cycles at each temperature level or until the total vanishing of the RFBG under test, with the aid of a type-K thermocouple connected to the furnace control as benchmark. By evaluating the influence of temperature on the sensors durability during the thermal aging, the sensors produced were kept in operation between 1 and 11 days at temperatures from 50 °C to 150 °C above their regeneration temperature. Among them, three sensors remained stable 50 °C above their regeneration temperatures for 4 days, 6 days and 10 days at 900 °C levels. With this work, it was possible to increase the knowledge of the encapsulated FBGs and RFBGs operating limits, produced in standard telecommunications fiber (G-652), operating at low temperatures and at high temperatures, contributing to the foundation for new applications related to thermo-optical sensors with FBG and RFBG technology.