| The subject of this paper is supported by the National Natural Science Foundation of China, and the project name is “Infrared methane sensing properties change model and self-adaptive detection mechanism under deep mining environment”(Grant No.: 61307124).Detection of trace gases plays an important role in industrial process control, environmental protection, production safety, national defense as well as other fields. Gas sensors based on laser absorption spectroscopy(LAS) have advantages of high sensitivity, high selectivity, etc. Because of its unique structure, hollow-core photonic band-gap fiber(HC-PBF) can provides an ideal place for the interaction between infrared light and target gas. Therefore, HC-PBF has been attracted widespread attentions, especially in nonlinear optics, gas sensing, trace gas detection and other research areas.From the perspective of theoretical and experimental, using hollow-core photonic band-gap fiber, a gas sensor system based on laser absorption spectroscopy has been researched and designed in this paper.The main contents of this work are as follows:1. The basic properties of PCF were calculated and analyzed theoretically. Using the plane-wave expansion method,the conditions of photonic band-gap in 2-D photonic crystal were computed. And then, the fundamental properties of photonic crystal fiber were analyzed, including the mode field distributions in HC-PBF, the coupling efficiencies of the optical transmission between HC-PBF and single-mode fiber(SMF). These lay a theoretical foundation for the research and design of a gas sensor using photonic crystal fiber.2. Slow-light effect on the enhancement of absorption spectroscopy was investigated for a gas sensor using HC-PBF. By using microfluidic technology, we proposed a kind of tunable HC-PBF for gas sensing, and investigated its slow-light properties including dispersion and group index of optical field modes. Then, by using finite element method, we computed the absorption enhancement factor resulting from the slow-light of HC-PBF for a target gas with a complex refractive index, emphasizing the effect of slow-light on spectroscopic gas sensing. Moreover, we took acetylene(C2H2) and ammonia(NH3) as practical sensing examples to explain how to tune its mode to the targeted absorption line.3. Using HC-PBF as the interaction place between gas and light, a gas sensor system based on LAS was researched and designed, including laser driver circuit, fibered-based optical path, and harmonic signal extraction circuit. First, a near-infrared distributed feedback(DFB) laser was selected as the light source, which was driven by a self-developed driver based on proportion integration differentiation(PID) algorithm to achieve wavelength modulation on the laser; secondly, an HC-PBF and two single-mode fibers were adopted as the optical path for gas sensing. Finally, a digital lock-in amplifier was developed to extract the second harmonic signal from the absorption signal.4. Taking C2H2 as targeted gases and 1534.099 nm as its targeted absorption line, experiment were carried out using the sensing system. The gas sensor system using HC-PBF was established. According to the national standard, standard gas samples within the concentration range of 500~2600 ppmv were prepared, and measurements of C2H2 concentration were performed, and the second harmonic signals were extracted to characterize gas concentration. A good linear relationship was observed between the amplitude of the amplitude of the second harmonic signal and C2H2 concentration. |
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