Study of stress relaxation in UV regenerated fiber Bragg gratings

Harsh environmental sensing using fiber Bragg grating (FBG) technologies is a rapidly growing field of research. A new generation of devices that operate in extreme environments (particularly those operating above 800°C) including environments with additional challenges such as radiation and intense optical fields (e.g. high power lasers), must be capable of withstanding gradual annealing and degradation, or aging, over time while preserving their intrinsic advantages (multiplexing capabilities, electromagnetic immunity, low intrusivity, mechanical reliability) in comparison to thermocouples. Of particular local relevance these include live diagnostics with optical sensor embedded into high-temperature composites for aircraft, helicopters and space infrastructure, temperature and pressure sensing in increasingly deeper and hotter oil bores, and the monitoring and mapping of temperature distribution at power plants, furnaces and chemical reactor. Much of the answer is related to inherent glass structure and properties, particularly viscous flow, chemical migration and stress relaxation. Thermal stability may therefore be achieved by judicious application of glass preparation, glass annealing and glass laser irradiations. FBG regeneration process [1] is the only current approach that can enable photonic technologies to operate in such harsh environments at elevated temperatures. In this paper we investigated the mechanisms of FBG thermal regeneration and in particular glass relaxation processes using micro-Raman, surface topography measurements and SEM.