Study On The Gas Concentration Measurement Based On The Optical Fiber Cavity Ring-Down Method

It is a great scientific research that the concentration measurement and component analysis of trace gas. The poisonous, harmful and contaminative gas from the petroleum, chemical, coal, space flight and nuclear energy industry is closely linked with human health, environmental protection and ultra-precision machining, although their concentrations are faint. Scientists have obtained many research achievement and acquired social and economic benefits after made a lot of theoretical and experimental studies on this subject for many years. But it is still ineffectual to on-line concentration measurement and component analysis of high sensitivity and precision of trace gas. Following the developing and crossover of spectroscopy, chemistry and material science, and the progress of laser and passive component fabrication technique, it provides an opportunity for solving the problem, and appeared many new technology, such as cavity ring-down, film absorption, and new measurement method based on nonlinear laser modulation and demodulation have come true. In this thesis, theoretic and experimental research have been made on gas concentration measurement used fiber loop cavity ring-down method, with the specific works as follows:The selective absorption theory of gas molecule in near infrared was detailed described; Movement form and spectrum of gas molecule, as well as spectrum characteristic of fundamental, fundamental frequency and combined frequency were analyzed; Theoretic basis of gas absorption measurement was provided; Various gas absorption spectrum was confirmed. Secondly, basic theories of cavity ring-down spectroscopy theory was described; Harmonic detection and cavity ring-down, hence, characteristic parameters were analyzed theoretically. Thirdly, according to coupled wave theory, the main component, DFB-LD, of the system was researched further. Theoretic analysis of coupled wave equation and it's solution were discussed, and the spatial hole burning effect was suppressed. Finally, dynamic characteristics, relaxation oscillation mechanism, and gain-switched theory of DFB-LD were discussed, and the frequency chirp of DFB-LD was eliminated by fiber grating filter. Gas concentration measurement system based on fiber loop cavity ring-down was design, and mathematical model of which was established, besides, the characteristic of resonant cavity and cavity enhancement were analyzed. Secondly, variable gain erbium doped fiber amplifier was proposed, Flattening methods of EDFA based on photonic crystal and genetic algorithms were presented, and the mathematical model of which were given. According to the simulation, which gives evidence of flatness, the gain fluctuation is about 2 dB. Thirdly, experimental system of difference frequency laser based on absorption spectrum were analyzed, and mathematical model of which was established. In the system, difference frequency generation occurred in the PPLN crystal when Nd:YAG of high-power laser with fixed wavelength and tunable diode laser(TDL) operated, so we obtain a wide-range tunable laser, including PPLN crystal, PPLN crystal and TDL. Finally, PDH was used to steady frequency of the wide-range tunable laser.Related experimental study of the system including tunable laser frequency stable, variable gain erbium doped fiber amplifier, variable optical attenuator and gas concentration measurement. Seven different ring-down time of fixed peak wavelength laser measured from different gas cells were obtained, and the absorption characteristics of the same gas with different concentration were compared, finally, the relationship of ring-down time, loop loss, absorption coefficient and gas concentration were given.