Remote flammable-gas sensing using a fluoride-fiber evanescent probe

Evanescent wave spectroscopy at 3.3gm on the surface of the core of an IR transmitting fluoride fibre is used to detect the concentration of flammable gases such as propane or methane in the environment of the fibre. Three probe designs are discussed. In one the gas diffuses through the teflon cladding of a multimode fibre causing an attenuation of the evanescent field and thereby a reduction in transmittance of the fibre. In the second a short section of cladding is removed from multimode fibre to access the evanescent field region. In the third singlemode fibre is used with its cladding reduced locally by polishing. Referencing is carried out at non absorbing wavelengths in the 3.2 to 3.6pm region.