Temperature Dependence of Glue-Induced Birefringence in Surface-Attached FBG Strain Sensors

The accuracy of surface attached fiber Bragg gratings (FBG) as strain sensors is affected by glue-induced birefringence, especially for applications in a large temperature range. In this study, we investigated the effects of the gluing technique with a new approach using FBGs in PM fibers that were azimuthally aligned relatively to the surface they were attached to. With this methodology, the glue-induced birefringence induced during the thermal curing process and its temperature dependence was investigated in the range of -30 to 170 degrees C. The used adhesive showed visco-elastic behavior up to 150 degrees C. The glue-induced birefringence increased with decreasing temperature reaching a value in the order of B = 6.10(-5) at -30 degrees C. Measurable reversible stress relaxation processes took place, even at low temperatures below the glue's glass transition temperature, which made the glue-induced birefringence dependent on time, temperature, and its prehistory. The same amount of glue-induced birefringence must be expected when FBGs in single mode fibers with the same gluing technique are used, and have to be taken into account in highly accurate measurements.