Study On The Novel Photosensitive Fibers And The Fiber Bragg Gratings With High Temperature Sustainability

FBG-based sensors for temperature detection have been widely used in industrial fields due to their distinguishing advantages, Such as wavelength-encoded measurement, immunity to electromagnetic interference (EMI), light weight, flexibility and so on. But they are normally suitable for comparatively low temperature range. This is mainly because of their poor sustainability under high temperature. As we all know FBG is not a permanent structure, it will be vanished or washed out when operating for a long time under a sufficient high temperature. So FBGs with high temperature sustainability should be fabricated in order to extend their sensing applications at higher temperature regions.The aim of the work in this thesis is to develop novel photosensitive optical fibers into which strong FBGs with high temperature sustainability can be inscribed by the phase-mask technique with a UV laser source. By co-doping Sb, In, or Bi into germanium doped silica fibers, several new photosensitive fibers have been developed by means of modified chemical vapor deposition (MCVD) method. Strong gratings have been fabricated by exposing pieces of these fibers to the laser emission of an UV excimer laser working at 248 nm. Annealing tests on these gratings confirmed that the gratings were able to survive an annealing temperature of 800℃or even 900℃for 24 hours or more and still retain an observable reflectivity in the reflective spectrum, showing superior high temperature sustainability. The results obtained from these tests fully show the potential of using these gratings in industrial applications.Based on the extensive experimental tests on the photosensitivity of the fibers and on the high temperature sustainability of the gratings, the mechanisms concerned with the decay of the gratings and the photosensitivity of the fibers were further investigated. A novel theoretical model, named cation-hopping, was presented to account for the experimental results obtained. According to this model, the vacancies existing in germanosilicate fibre are important in acquiring a high degree of photosensitivity, and the cations hopping away from the original localized positions during UV emission exposure are responsible for the changes in the refractive index.Based on the combination of cation-hopping model and the so called aging decay model, a cation-oriented trap distribution model is constructed and used to simulate the decay properties of the gratings during high temperature annealing, yielding a good fit to the experimental data. An accurate lifetime of these specially fabricated gratings operating under certain temperature can be predicted by using the cation-oriented trap distribution simulation.As an important application of these strong FBGs with superior high temperature sustainability, heat flux measurement was performed by using the FBG-based fiber optic sensor and the fluorescent lifetime-based fiber thermometer for a comparison.Results thus obtained show the great prospect of using these fiber sensors in actual industrial applications.