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 anew 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 °C. The used adhesive showed viscoelastic behavior up to 150 °C. The glue-induced birefringence increased with decreasing temperature reaching a value in the order of B = 6 · 10-5at -30 °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.