Phthalocyanine based optical fiber sensors

Among the different gas detection technologies, optical systems based on optical fibers can be very useful in specific applications like the monitoring of road tunnels or wide spaces. Various techniques exist which are almost all based on the deposition of a sensitive layer reacting with the gases, changing its optical properties (complex refractive index). Two kinds of sensors can be distinguished: extrinsic sensors where the sensing element is not part of the optical fiber (remote spectroscopy of a chemochromic membrane) [1] and intrinsic sensors based on a coating directly deposited on the fiber. In that case, refractive index changes of the sensitive layer induce changes of the transmission of light. A lot of different interrogation technologies were used [2]. Phthalocyanines are good materials for gas sensing [3]. These organic molecules are known to be sensitive to oxidizing or reducing gases at ppm concentrations and have been proposed as the chemically active component of both conductive and optical gas sensors. Another major advantage is their remarkable chemical and thermal stability. Among the different phthalocyanines, Lanthanide bisphthalocyanine (LnPc2) complexes with a “doubledecker” structure attracted a lot of attention because of their electrochromic properties leading to strong optical changes in the visible and NIR range [4-6]. Another advantage is that the gasochromic effect can be observed in the telecom wavelength range [1300-1600 nm], which allows then to use standard and low cost equipment. It is also well known that the coating method has a big impact on the response time and that the gas desorption can be rather slow at room temperature. Heating the substrate is not practical with optical fibers and therefore optical alternatives need to be found. This paper will describe the use of lanthanide bisphthalocyanines for optical gas sensing, in particular for NO2 and the various interrogation and coating methods