Remote multipoint fiber optic gas sensors using an FMCW technique and a tunable laser

The cost per sensing point may be reduced by networking a number of gas sensors that shares the same tunable laser and/or the same signal processing electronics. In this paper we report on the use of a frequency modulated continuous wave (FMCW) technique for addressing the remote optical fiber gas sensor arrays. The sensor network is of a ladder topology and is interrogated by a tunable external-cavity semiconductor lasers. The system performance in terms of detection sensitivity and crosstalk between sensors is investigated. By using appropriate wavelength modulation/scanning coupled with low pass filtering, the coherent interferometric noise can be reduced greatly. Computer simulation shows that an array of 20 acetylene (C₂H₂) gas sensors with 2000ppm (2.5cm gas cell, or 50ppm.m) detection sensitivity for each sensor may be realized. A two-sensor acetylene gas detection system is experimentally demonstrated with detection sensitivity of 165ppm/√Hz (2.5cm gas cell or 4ppm.m/√Hz) and crosstalk of -25dB.