Sensitivity of fiber-optic carbon dioxide sensors utilizing indicator dye

Fiber-optic carbon dioxide sensors with a dip-coated sol–gel film containing indicator dye of thymol blue are prepared and characterized. The sensitive film has both organic and inorganic parts with good gas permeability. The difference between attenuations in N 2 gas and in CO 2 /N 2 mixture gas increases with the increase in the CO 2 concentration in the 0.55–0.7 μm range. The attenuation difference increases linearly with the increase in the logalism of CO 2 concentration, although there is no difference under 0.1% CO 2 gas concentration. The sensitivity of the sensor increases with the increase in the dye concentration. The relative humidity and the film composition have an important effect on the attenuation difference.