A SIMPLE INTRINSIC OPTICAL-FIBER CHEMICAL SENSOR

This paper describes an intrinsic optical-fibre sensor for the measurement of variations of the refractive index of a medium acting as the cladding. A multimode fibre is illuminated with a laser beam incident at an oblique angle so that some of the rays trapped in the core are refracted if the index of the cladding is slightly shifted. Owing to the stability of the light source, a variation of about 2 x 10(-5) in the refractive index is readily detectable. The sensitivity and the linearity of the sensor depend on the angle of incidence of the collimated beam. This set-up can be used for the remote analysis of liquid or polymeric mixtures. The realization of a gas sensor is also presented: the bare core of the fibre is coated with a polymeric film whose refractive index varies during gas absorption. An example of methane detection at concentrations less than the lower explosive limit with a polyoxyethylene film is given. This simple and low-cost sensor can be used for detection of other gases after the preparation of specific sensitive claddings.