Desarrollo de sensores de fibra óptica: aplicación de un interferómetro Fizeau al estudio de polímeros

Abstract

In the last decades, the fiber optic sensor has been developed from the experimental stage to practical applications. With the rapid advent of optical networks, the cost of fiber optic sensors has substantially dropped because of the commercially viable key components in fiber optic communications such as light sources and photo-detectors. Being a physic media, fiber optic itself is affected continuously by perturbations of different kinds, suffering geometrical (size, profile) and optical changes (refraction index, modes), that depend on the nature of the parameter to be measured. In communication applications, it is precisely to minimize these effects, but in the case of sensors that response is deliberately enhanced so that the resulting change in the optical radiation can be taken as a measure of the external agent. Under certain design, the fiber itself acts as the light modulator, converting temperatures, stress, strain, rotation or even electric current to its corresponding change in optic radiation. In other cases, extrinsic optical transducers can be also coupled to the fiber system. Fiber optic sensors have inherent advantages which include their ability to be lightweight, of very small size, passive, low-power, resistant to electromagnetic interference, their high sensitivity, their bandwidth, and their environmental ruggedness. The main objective of this thesis is the study of specific fiber optic sensor and its applications to polymer resins. The first two chapters deal with an introduction and a general overview of fiber optic sensors, remarking the main details and properties of them, ranging from those simplest (e.g. intensity based sensors), to the more complicated designs like interferometric and polarimetric types. Chapter 3 describes more deeply the interferometric type sensors, in particular a low finesse Fabry-Pèrot interferometer, also known as Fizeau interferometer, which is unique in its qualities, ease of use and constitutes the heart of this study. In addition, some initial developments and results are showed. Chapter 4 describes several applications of the Fizeau interferometer to the characterization of photo-curing polymer resins. The method presented here is simple and accurate, and allows several measurements that can be summarize briefly as follows: polymer shrinkage of photo-curing resins (for commercial and new BISGMA types); coefficient of thermal expansion, which allowed deducing quantitatively the influence of the exothermic reaction within the photo-curing resins. In Chapter 5 this sensor was used together with an acoustic technique to determine viscouselastic properties. Characterization and measurement of structural changes in the polymerization reaction can be done simply by detecting micrometric vibrations of a cantilever beam with a crack filled with the resin. Finally, Chapter 6 gives a set of basic conclusions in order to make a balance of this work and thus enable new lines to be carried out in next future phase. Appendix sections contain some basic background concepts, descriptions of devices and techniques that were not suitable for inclusion in the main section (or they might be considered very widely spread). i.e.: an introduction to fiber optics, its typical parameters, support circuits and programs codes among the sensors, brief description of polymer molecules and compounds used within the new resins, etc.