Fiber optic fluorescence sensors based on sol-gel entrapped dyes

Many fiber optic sensors for chemical species rely on a change in the fluorescence characteristics of a dye in response to its chemical environment. Such sensors often require the complex immobilization of the fluorescent species on or near the fiber surface. A novel process, whereby a highly transparent porous glass can act as a support for sensor dyes, is presented. The sol-gel process for low-temperature fabrication of porous glass has been used as a means of entrapping chemically-sensitive dyes for fiber optic sensor applications. As a proof of principle, fluorescein dye was used to investigate the performance of a pH sensor. Because of its potential for distributed sensing and planar integrated optics sensors, evanescent-wave rather than direct excitation of fluorescence was used. A short length of unclad fiber was coated with a thin layer of porous cladding within which the pH-sensitive dye is entrapped. The sensor displayed maximum sensitivity in the range of pH 3.5 to pH 6.5 with a response time of less than 5 seconds. The suitability of the technique for distributed and integrated-optic sensing of gaseous and liquid species is discussed.