Innovative Plastic Optical Fiber Sensors

This thesis describes the development of new types of fiber optic sensors for the measurement of mechanical quantities such as displacement, vibration and acceleration. Also, it describes the realization of specific acquisition systems designed to interrogate the developed sensors. Since optical fibers have been historically associated with high speed telecommunication links because of their very large bandwidth and low attenuation, there is a great interest for their employment in sensor applications. Fiber sensors represent a promising solution in many fields since fibers can be used for the measurement of several quantities, not only mechanical as those investigated in this work, but also chemical with the possibility to detect specific chemical or bio-chemical molecules. Among the physical quantities to be detected, the displacement measurement is required in some applications, especially in structural civil and mechanical fields, where it is possible to evaluate the cracks evolution, providing information about the safety of the structure under monitoring in order to detect eventually risky situations. All the developed sensors are able to measure the displacement along one or two axis, that can be employed also during vibration tests especially at high frequencies, and also acceleration sensors to monitor acceleration at low frequencies. The developed sensors are based on plastic optical fibers instead of the traditional glass fibers, which are traditionally employed in optical communications. This change is related to the aim of realizing sensors maintaining the excellent typical characteristics of the fibers, such as electromagnetic immunity, intrinsically fire safety and flexibility of applications, but with costs comparable to those of commercial electromechanical sensors. Indeed, nowadays, the commercial fiber optic sensors are based on glass fibers because they have very good performance. However, they find limited applications due to the high costs of their complex interrogation systems and also for the procedure required to splice the fibers. On the other hand, plastic optical fibers represent a promising alternative because of their geometrical and optical properties that allow employing low-cost non coherent sources such as LED and also simplifying the procedure for the sensor connection and installation. Therefore, the design of the proposed plastic optical fiber displacement sensors is described with the sensor practical arrangement and the realized prototypes. An acquisition system has been designed and realized to characterize the sensors and the characterization results are also provided. Moreover, the development and the characterization of a plastic optical fiber sensor able to measure the displacement in two directions have been described. The main drawback of the developed sensors are stability issues and for this reason laboratory and in situ-tests have been carried out in order to verify the sensor performance over the time. I The results obtained with the stability tests have highlighted the necessity to develop displacement sensors with increased stability. To this aim, a compensation technique based on two different wavelengths has been developed. The same working principle of the developed displacement sensor has been exploited to realize a fiber vibrometer to be employed during the vibration monitoring for measuring without contact the vibrations of the device under test. The sensor development, a suitable calibration procedure developed to overcome the problem of real targets with a non uniform reflectivity, and the experimental tests have been described. Furthermore, the preliminary results concerning the feasibility study of a plastic optical fiber accelerometer are reported.