Fabrication and characterization of an optical fiber pH sensor using sol–gel deposited TiO2 film doped with organic dyes

Some pH sensors based on optical fibers use organic dyes which are sensitive to the pH levels. Such dyes are deposited over the fiber and they are supported by a matrix, typically SiO2. In the present work there is introduced the fabrication and characterization of a pH sensor based on a multimodal optical fiber that uses a sensing film deposited by the sol–gel technique (immersion method). The TiO2:dye sensing film was obtained from a precursor solution of titanium tetra-isopropoxide (TTIP) doped with different organic dyes: brilliant green, rhodamine 6G, rhodamine B and coumarin. TiO2 was used since it is more resistant and it has a longer lifetime than SiO2 films. Therefore it can be expected than such time would be longer for this kind of sensors. It was found that each dye is sensitive to different pH ranges so that with the combination of all of them it is possible to cover a dynamic range of pH levels that varies from 2 to 12. Furthermore, there was found that the device sensitivity increases with a decrease of the operation wavelength.