An Extrinsic Fiber-Optic Gas Sensor Employing Distributed Bragg Reflectors

A new concept of an extrinsic fiber-optic gas sensor employing a cavity with distributed Bragg reflectors (DBRs) is proposed and experimentally demonstrated using the wavelength interrogation. The DBR is represented by a one-dimensional photonic crystal (1DPhC) comprising six bilayers of TiO _² /SiO ₂ with a termination layer of TiO ₂ , and the cavity of the adjustable thickness is realized by means of springs separating two parts of the fiber connector adapter to which the DBRs are glued. Two different extrinsic fiber-optic gas sensors are experimentally analyzed with results manifesting different technologies with which the DBRs are manufactured. The transmission spectra of the fiber-optic sensors are measured in visible and near-infrared spectral regions for a humid air, and we revealed, in accordance with the theory, narrow peaks within the 1DPhC band gap due to the cavity modes. The first sensor employs porous DBRs and has a maximum sensitivity at low relative humidities (RHs) of air, and the sensitivity to the RH of air, figure of merit (FOM) and the limit of detection (LOD) reached at 25 %RH values of 0.566 nm/%RH, 0.094 %RH ^-1 and 0.018 %RH, respectively. The second sensor is characterized by the sensitivity increasing with the RH, and the sensitivity and FOM attained at 70 %RH values of 0.109 nm/%RH and 0.017 %RH ^-1 , respectively. An extrinsic fiber-optic gas sensor employing a cavity with DBRs of adjustable thickness has the potential to be used in a great number of gas sensing applications.