Distributed Gas Monitoring System Research Based On Fiber-Loop Ring-Down Method

The application background of this paper is carbon dioxide capture and storage, and it focuses on developing a CO2 leakage monitoring system, which could work in harsh environment and achieve distributed remote real-time on-line gas detection. Investigating existing gas detection technology, analyzing development status and advantages and disadvantages of chemical method, electrochemical method and optical method of CO2 detection, CRDS (Cavity Ring-Down Spectroscopy) is found in gas absorption spectroscopy field and is combined with FOST (Fiber Optic Sensing Technology) to form a new structure, FLRDS (Fiber Loop Ring-Down Spectroscopy). Optical cavity is displaced by fiber loop and gas cell in FLRDS, but measuring the optical pulse attenuation speed causing by gas absorption is same, which is easier to develop distributed remote monitoring system than CRDS.Firstly, DFB (Distributed Feed-Back) laser is selected as system optical resource, researching principle of laser diode and TEC (Thermoelectric Cooler) and testing their performance, completing temperature scan under permanent power laser output mode using laser current and temperature controller, and realizing 10ns pulse width and 1mW amplitude laser pulse output using electric circuit. Secondly, based on temperature scan with permanent power mode laser output system, a CO2 detection experimental platform is set up, which obtain gas absorption line of different concentration, and data processing methods including absorption intensity normalization, spectroscopy correlation shift algorithm, and optimum solution of redundant linear equation are used, which get calibration data error-bar that has about 1% concentration accuracy and less than 2s measurement time. Third is one of the important creative points of this paper. FLRDS is the combination of the FOST and gas measurement technology, but controlling the loop loss is one of the key technical problems to be solved. Based on the former laser pulse output result, a FLRD experimental platform is set up and a fiber amplifier is introduced to prolong RDT (Ring-Down Time), which obtain ring-down signal of different attenuation. Since RDT is one of key points to decide measurement accuracy, peak find processing and exponential decay curve fitting are used to evaluate RDT. According to the SNR of experimental result and loop controlling level, the CO2 concentration measurement accuracy could reach 5%. Finally, in some application environment where fiber cable laying is impossible or laying cost is too high, wireless sensing network is studied and wireless gas detection nod is developed, in which a micro processor control the gas detection and process signal and a wireless transceiver transfer and receive signal, and networking feasibility is analyzed based on system power consumption and cost.The creative points of this paper includes:proposing distributed gas detection based on FLRDS and achieving loop loss compensation and RDT evaluation, which improve measurement accuracy; proposing and achieving correlation shift algorithm and optimum solution of redundant linear equation processing absorption spectroscopy to calculate gas concentration; proposing conception of wireless gas detection sensing network and developing wireless gas detection nod.