Development of Innovative Low Cost POF Sensors for Monitoring Chemical and Environmental Quantities

This work aims at the design and development of innovative sensing systems based on optical technology. Plastic Optical Fiber (POF) can be successfully employed for the development of highly sensitive and selective devices to be used in monitoring environmental and chemical quantity. This thesis presents the implementation of a prototype POF sensor which is able to detect low concentrations of hydrogen fluoride (HF) vapors. The sensor is based on plastic optical fibers whose surface is modified in such way to make it sensitive to the presence of specific contaminants (below to 0.1 ppm of HF). The approach proposed for the realization of the sensors foresees the surface modification of the plastic optic fiber in two steps: (i) etching of the cladding in an organic solvent (such as ethyl acetate); (ii) deposition of the sensitive film (glass-like) able to react with HF by using PECVD (Plasma Enhanced Chemical Vapor Deposition) and/or plasma sputtering. The fluoride ions attack the glass-like film and alter the transmission capability of the fiber so that the detection simply requires a LED and a photodiode. The sensor exploits a cumulative response which makes it suitable for direct estimation of the total exposure to the fluoride ions. This thesis describes the prototype sensor details by defining the way to prepare the plastic fiber, the design of the sensor assembly, the measurement set-up arrangement, and the sensor characterization. These sensors can be used both in civil and industrial fields, but have been designed to, i.e. the monitoring of the gas mixtures in the RPC (Resistive Plate Counters or Resistive Plate Chamber) which are employed detector in high energy physics experiments running at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland. This thesis also introduces a sulfide gas sensor, based on POF, that is able to monitor the low concentrations sulfide vapors (H_2S). This sensor is particularly suited for extensive use in the cultural heritage conservation monitoring, such as for precious artifacts in museum showcases, and can be used also for monitoring people exposed to unhealthy atmospheres. Also these sensors can be produced by depositing a sensing thin film onto the fiber surface and measuring the light transmittance change because of its reaction with the gas. The fiber sensors are made sensitive to sulfide vapors by deposition of an silver nanostructured sensing film by DC sputter coater. This thesis contains the design of the sensor assembly, the measurement set-up arrangement, and the sensor characterization.