Multi-point fiber optic gas sensor systems

Multiplexing is a very important issue for fiber optic sensors. It enhances the system utilization and the competitiveness of fiber sensors against conventional technologies. It is believed that an increasing attention will focus in this area in the coming years. The main objective of this project is to develop and investigate multi-point gas detection systems for quantitative measurement. Two different multiplexing systems for fiber optic gas sensors, one named time-division-multiplexed (TDM) system, the other frequency modulated continuous wave (FMCW) system, are investigated theoretically and experimentally in this project.; Quantitative measurement of gas concentration based on wavelength modulation spectroscopy (WMS) and second-harmonic detection is demonstrated by applying low-frequency wavelength modulation to an external-cavity tunable diode laser. The tunable laser operating at 1.53 μm region is used to detect acetylene (C₂H₂) based on direct absorption through the use of micro-optic cells. A time-division multiplexed three-sensor system with a forward-coupled ladder topology have been implemented and experimentally tested. The experimental system using micro-optic cells of length 25mm have demonstrated sensitivity of 81 ppm/$Hz$. The crosstalk between the sensors was found to be −30 dB. Power budget analysis shows that a sensor network consisting of 90 sensors could be realized with the same multiplexing topology. The performance of such a networked system has been examined in terms of shot noise limit, inter-channel crosstalk and detection sensitivity.; Apart from the TDM techniques, we also applied a frequency modulated continuous wave (FMCW) technique for multiplexing optical fiber gas sensors. The sensor network is again of a ladder topology and is interrogated by the same tunable laser. For the FMCW system, the optical delays of each channel are carefully designed in order to satisfy the basic requirements that will be discussed in the following chapters. The system performance in terms of detection sensitivity and crosstalk between sensors was investigated and found limited by the coherent mixing between signals from different channels. The system performance can be improved significantly by use of appropriate wavelength modulation/scanning coupled with low pass filtering. Computer simulation shows that an array of 37 acetylene sensors with a detection accuracy of 2000ppm for each sensor may be realized. A three-sensor network of ladder topology is experimentally demonstrated for the detection of acetylene gas. A minimum detectable concentration of 270 ppm/$Hz$ is obtained with 25mm gas cells under atmospheric pressure which corresponds to a minimum detectable absorbance of 1.48 ×10−4/$Hz$. The crosstalk between the sensors is below −20 dB.