Design challenges of a birefringent FBG optical sensing system

Abstract

Fiber Bragg Gratings (FBGs) have been used and deployed in several applications and industries over the past years. Standard FBG based temperature sensors are sensitive to both strain and temperature and in order to measure temperature, the strain influence needs to be isolated from the FBG by careful transducer design, packaging and calibration of the sensor. Birefringent FBGs such as polarization maintaining FBGs (PM-FBG) that can simultaneously measure strain and temperature have been demonstrated in recent publications. Such sensors exhibit a double FBG response which is polarization dependent and the wavelength peak separation is an important parameter to enable measurements beyond standard FBGs. To achieve the best performance of a birefringent FBG, an optimized interrogation technique that can measure both FBG orthogonal polarization responses with high precision is required. In addition to the need for an optimized interrogator, the selection of sensor inscription method, coating type, mounting technique, and calibration are very important parameters to deliver the best overall system performance. PM-FBG sensors can be multiplexed on a single fiber and offer a simplified installation option without the requirement for complex transducer packaging designs. Here we have developed as part of an ESA GSTP project, a fiber optic sensing system for birefringent FBGs based on a high precision tunable laser interrogator system. We have also evaluated different types of PM-FBG sensors with different coatings and mounting techniques and demonstrated an optical temperature measurement system with an operating temperature range from -20°C to + 80°C using PM-FBGs with improved calibration techniques.