Fabrication and characterization of an evanescent wave fiber optic sensor for air pollution control

The fiber optic evanescent wave sensors provide low-cost integrated optics sensors for the monitoring of air pollution and species concentrations in various environments. This sensor offers the possibility to be linked to fiber optics for remote measurements and dangerous locations. It offers multiplexing possibilities and presents perfect immunity to electromagnetic perturbations. A fraction of the power transmitted along the core of the fiber propagates as an evanescent field in the cladding, so the transmission of this fiber depends on the absorption of the cladding. The sensor is obtained by removing a small length of the cladding of a standard commercial single-mode fiber silica/silica and replacing it by an absorbing medium. The evanescent field in the cladding interacts with the absorption medium surrounding the core and the power transmitted by the fiber is attenuated. In this paper, we describe a simple method for removing the cladding by chemical etching with HF. The transmission characteristics of this sensor is analyzed to determine the diameter of the remaining core. Due to the core capillarity to HF, a tapered profile is produced at the ends and the geometry is optically determined. A metallic or polymer layer can be deposited to increase the response of the sensor and adapt it to the detection of a specific pollutant.