Volatile organic compounds detection with tilted fiber Bragg gratings coated by ZnO nanoparticles

This paper presents a sensor based on a Tilted Fiber Bragg Grating (TFBG) covered with a mesoporous coating consisting of ZnO nanoparticles for the detection of volatile organic compounds. TFBGs are Bragg gratings that are tilted by a small angle inducing a coupling between the forward-going fundamental core mode and the backward-going cladding modes. They present a transmission amplitude spectrum consisting of several tens of resonances, which present their own sensitivity to the surrounding refractive index. Specific sensors can be built by using TFBGs covered with a dedicated coating that changes its refractive index when in contact with target chemical species. This concept was illustrated with ethanol and a mesoporous ZnO coating whose refractive index changes due to the gas adsorption on the ZnO particles. The exposure to ethanol vapors of the covered TFBG yields important modifications of the transmission amplitude spectrum in the range 1510-1590 nm. All cladding mode resonances show a red shift while their peak-to-peak amplitude decreases with increasing ethanol concentration in air. The response, defined as the amplitude or the wavelength change of a resonance peak, is fast (1s), linear, reversible and without hysteresis (red shift of 60 pm/vol% ethanol and -3.3 dB/vol% at 1550 nm).