DECLARATION BY SCHOLAR

In many industrial application fields (oil & gas and high voltage transformers.) the use of conventional electrical temperature sensors (e.g. thermocouples) is forbidden because of harsh environmental conditions or risk of explosions due to sparks. In these fields fiber optic sensor represent a valid alternative as they can withstand severe conditions (high temperature, pressure) and offer a complete electromagnetic immunity. Already available commercial fiber optic temperature sensors are mainly of two types: distributed sensors, that is, they can provide the entire profile of temperature along the entire fiber length (even km) with high spatial resolution (Raman, Brillouin sensor). And local sensors is that, in this case the fiber is used only to transport the optical light but the actual sensor is a material with peculiar thermal properties (fluorescence, absorption) which is place on top of the fiber end face. These commercial systems are still expensive monitoring solutions and their cost can only be sustained mainly by industries such those of the Oil & Gas and High-Voltage Transformers. On the other hand, the Manufacturing Industry (packaging, mechanical, automotive.) is now realizing the importance of implementing predictive maintenance strategies to prevent the occurrence of failure and downtimes in the manufacturing process. The implementation of an effective predictive maintenance requires the installation of monitoring system comprising a plurality of conventional sensors (even temperature sensors) which can provide manifold real-time information on the machine condition. The manufacturing industry is however struggling with the still high costs and complexity of installation of these monitoring solutions based on conventional electrical sensors. In this frame fiber optic sensors can bring a real innovation, due to their minimal invasiveness, electromagnetic immunity and capability to sense manifold environmental parameter with the same transducer, that is, the optical fiber. As mentioned above commercially available fiber optic sensors are however too expensive. In the present work, we have decided to re-consider a coherent approach, that is, interferometric, to fiber optic sensors, to achieve a cost-effective solution that can be proposed also to the Manufacturing Industry. Interferometric technique were already proposed more than 40 years ago, offering very high sensitivity and accuracy, but their actual in-field applicability has been limited by technological issues related to the stabilization of the interferometric working point and polarization fading. In the present work to overcome, the above-mentioned issues a completely passive phase diversity receiver combined with Faraday rotation mirrors, respectively for quadrature point and polarization stabilization, have been exploited to develop a lowcost interferometric fiber optic sensor capable to reliably detect temperature variations in an industrial environment. The present work are divided as follows: in the first chapter, a brief review of the fiber optic sensor peculiar properties is given while in the second chapter a more detailed description is provided related to fiber optic temperature sensors. In the third chapter a theoretical description of the coherent approach to fiber optic sensing is given. In particular, in order to convert temperature measurement which is a quasi-static parameter, in a dynamic measurement, which guarantees a better accuracy, coherent detection is associated with a wavelength modulation, that is, a chirp of the optical laser source. In chapter four different typology of laser source with different chirp characteristics are taken into account and compared to evaluate the optical source that allows better performance in terms of sensitivity and accuracy in coherent detection. Finally, in chapter five, the coherent fiber optic temperature sensor experimental setup is described and preliminary results are provided.