Germanosilicate Fibers And Bragg Gratings : Newer Efforts In Understanding Photosensitivity And Novel Methods For Strain-Temperature Discrimination

The different topics covered in this thesis include photosensitivity in germanosilicate fibers/glasses and application of fiber Bragg grating sensors in simultaneous strain and temperature discrimination. Fiber Bragg Gratings are wavelength dispersive refractive index structures manufactured through ultra-violet (UV) exposure of optical fibers. Their applications range from wavelength division multiplexing filters, dispersion compensators and fiber laser resonators for telecommunication applications to different types of point or distributive sensors for a variety of applications. One aim of this thesis has been to understand the mechanism of photosensitivity in germanosilicate fibers/preforms. Studies undertaken in this part of the thesis include thermal dynamics of Fiber Bragg Gratings and nano-indentation on ultra-violet irradiated germanosilicate glass preforms. An interesting, periodic appearance of a new peak has been observed in the reflected spectrum of Bragg grating inscribed in a germanosilicate fiber during thermal treatment. The new peak occurs on the longer wavelength side of the spectrum during heating and on the shorter wavelength side during cooling, following an identical reverse dynamics. A commercial grating with 99.9% reflectivity also shows a similar decay dynamics. The observed temperature induced distortion in refractive index modulation profile has been understood in the light of compaction-densification model. It is proposed that during the fabrication process of a grating, the modulation in the thermal expansion coefficient brought about by the interference fringes results in a non-uniform expansion throughout the grating length which in turn results in the distortion of the refractive index profile with increase/decrease in temperature. Since the reflection spectrum of a grating can be approximated as the Fourier transform of the refractive index profile, any distortion in the index profile results in the observed anomalous behaviour in the reflection spectrum. Nano-indentation studies have been performed to measure the changes in mechanical properties of a glass preform subjected to different levels of ultra-violet exposure. The results reveal that short term exposure leads to an appreciable increase in the Young’s modulus suggesting the densification of the glass, confirming the compaction-densification model. However, on prolonged exposure, the Young’s modulus decreases, which provides the first direct evidence of dilation in the glass leading into the Type IIA regime. The present results rule out the hypothesis that continued exposure leads to an irreversible compaction and prove that index modulation regimes are intrinsic to the glass matrix. In the second part of the thesis, three different schemes have been proposed for the use of Fiber Bragg Gratings as strain-temperature discriminating sensors: (a) The first method is based on the measurement of the different characteristic wavelength shifts of two types of gratings. Strain and temperature sensitivities of a Type I Bragg grating (G1) in germania doped silica fiber, fabricated under normal conditions, and zero strain, are compared with that of a Bragg grating inscribed under pre-strained condition (G2). Experimental results show that both, strain and temperature sensitivities of G1 and that of G2 are different. Based on this study, we have proposed an approach which enables simultaneous discrimination of axial strain and temperature. (b) In the second method, a single sensing element has been used to encode strain and temperature into an additional parameter other than the wavelength shift. The thermal out-diffusion of germanium from the core of a photosensitive fiber under elevated temperature is exploited to form a Fabry-Perot filter with a single Fiber Bragg Grating. The filter is fabricated using the standard phase-mask technique and one-time exposure. Energy Dispersive X-Ray analysis is used to measure the out-diffusion. The filter is used as a sensor for simultaneous measurement and discrimination of strain and temperature. The proposed technique, where a single grating is used to discriminate the parameters, provides a large advantage over other existing methods. (c) In the third method, a compact design based on cross-wire arrangement of Fiber Bragg Gratings having identical Bragg resonance and different reflectivity is proposed for simultaneously sensing strain (uniaxial) and temperature. Two gratings are assembled orthogonal to each other on an aluminium base. The cross-wire design allows the two sensors to experience the same temperature but different strain. The gratings are identified by their respective reflectivity and, strain and temperature are resolved from the shift in Bragg wavelength. The proposed design exploits the fact that strain is a vector and temperature is a scalar parameter. This sensor has wide industrial application in discriminating strain from temperature effects