Fiber Optic Temperature Effects for Compensation of Strain Measurements

Abstract

This work analyzes the applicability of different temperature sensing methods in order to compensate Fiber Bragg Grating based strain measurements in a short segment of a single fiber. The presented methods for temperature measurements relying on the refractive index (including reflectrometry techniques) or on scattering effects have significant disadvantages for simultaneous strain and temperature measurement. Despite this result a method for high resolution distributed temperature measurement is introduced based on spatially distributed fiber Bragg gratings. In contrast to refractive index Methods, doped fibers show promising properties. This work derives a detailed model of doped fiber fluorescence lifetime, leading to the conclusion that the fluorescence spectrum is more suited for temperature sensing. Additionally, a connection between absorption, stimulated emission, spontaneous emission and the measured spectrum is developed based on the fluorescence lifetime model found. The results emphasize the need to treat the measured spectrum as the steady state of a dynamic system rather than an approximated representation of internal absorption and emission spectra. The fluorescence spectrum will be used for temperature sensing by tracking lowpass effects in the spectrum. The optimal doping material is subject to further research and not determined in this work.