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 v arious environments. This sensor offers the possibility to be linked to fib er optics for remote measurements and dangerous locations. it offers multip lexing possibilities and presents perfect immunity to electromagnetic pertu rbations. A fraction of the power transmitted along the core of the fiber p ropagates as an evanescent field in the cladding, so the transmission of th is 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 evane scent field in the cladding interacts with the absorption medium surroundin g the core and the power transmitted by the fiber is attenuated. In this pa per, we describe a simple method for removing the cladding by chemical etch ing with HF. The transmission characteristics of this sensor is analyzed to determine the diameter of the remaining core. Due to the core capillaria t o HF, a tapered profile is produced at the ends and the geometry is optical ly 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 polluta nt. (C) 2000 Elsevier Science S.A. All rights reserved.