Research On Optical Fiber Loop Cavity Ring-down Spectroscopy Gas Concentration Measuring Method And Experiment

Optical fiber sensing technology is rapidly developed with the development of the optical fiber communication technology in the 1970s. It has lots of advantages, such as, anti-electromagnetic interference, simple structure and so on. However, the existing sensitivity of gas concentration detection sensors is limited by absorption path length and fluctuating of light. Researching on a novel optical fiber gas sensor becomes more and more important. The early cavity ring-down spectroscopy makes use of a resonant cavity which composed of two pieces of high reflective mirror to measure the quantity that to be measured. This technology makes absorption path length increased, which becomes has an advantage of it. However, it still has some deficiencies. Optical fiber loop ring-down spectroscopy is a new kind of optical fiber cavity ring-down technology. Compared with cavity ring-down system, it imitates the concept of ring-down, uses optical fiber loop as the ring-down cavity, which replaces two high reflectivity mirrors. It has many advantages. For example, it can be immune from light source disturbance, has high sensitivity, simple structure and so on.In this paper, an optical fiber based on fiber loop ring-down spectroscopy is proposed. The main research contents are as follows:(1) The theory of gas molecular absorption and the detection principle of optical fiber ring cavity ring-down spectroscopy absorption are stated and analyzed. Device components are researched and the processing method of finding pulse peak signal is analyzed and selected.(2) An optical fiber based on fiber loop ring-down spectroscopy is proposed. And here, we solved two key issues, one is the selection of coupler, we used the splitting ratio of couplers with 10:90 at last; another is optimization methods of finding ring-down peak signal, here, we use both linear smoothing filtering method and multi-point averaging. At the same time, the frequency of the pulse is 6.65kHz, duty cycle is 3%, the length of the gas cell is 25cm, the fiber loop length is 3000m and reasonable distribution system, etc.(3) An optical fiber based on fiber loop ring-down spectroscopy system is set up. Preliminary measurement experiments of the acetylene gas concentration are realized. The superiority of the system is verified. From the experimental data, the conclusion is that gas resolution is up to 1000Oppm, sensitivity is 0.327μs/ppm, stability is 1.02%, the repeatability error is 2.37% maximum absolute error of the system is 0.12% and greatest error is 6.0%.(4) Analyzing the measurement results, including loss analysis, the validation of system superiority, the relationship between gas concentration and ring-down time, evaluation of ring-down time, and evaluation of measurement error, etc.