Sensitivity Optimization of a Microstructured Optical Fiber Ammonia Gas Sensor by Means of Tuning the Thickness of a Metal Oxide Nano-Coating

In this paper, the influence of the thickness of metallic-oxide coatings, ITO, and SnO2 on the sensitivity of a microstructured optical fiber Fabry-Pérot (FP) has been studied with the aim of developing ammonia gas fiber optic sensors. Also, the distribution of the optical power that can be coupled to the metallic-oxide sensing films is investigated in order to understand how the sensor's sensitivity can be improved; the thickness of the coatings plays a relevant role on the sensitivity and response time. Films with thicknesses between 200 and 850 nm were experimentally examined resulting in an optimal thickness of 625 nm for a SnO2 film. The behavior of the sensors toward different concentrations of ammonia gas from 10 to 130 ppm was analyzed by measuring the phase shifts of the reflected signal using the fast Fourier transform of its optical spectrum. The registered response/recovery times of this sensor are below 90 s.