| The development of trace gas detection is very important for ambient air monitoring, human or animal and plant physiological state diagnosis, and long-range detection of explosives. Since photoacoustic spectroscopy with high sensitivity, dynamic range, detector response has nothing to do with the incident wavelength and other characteristics, has long been one of the most important development direction of trace gas detection technology. Over the years, in order to improve the sensitivity of photoacoustic spectroscopy, the improved photoacoustic cell design and more sensitive microphones had been used. Emerging quartz enhanced photoacoustic spectroscopy, represents a new development direction of photoacoustic spectroscopy. In this paper, quartz enhanced photoacoustic spectroscopy principle was studied; a simple fiber optical path QEPAS system was presented. In the open air environment, using water vapor in the air as the trace gas to be detected, the QEPAS experiment was completed. The performance of the two type's optical structure QEPAS system was evaluated, using the obtained experimental data.The principle of QEPAS involves infrared gas absorption, wavelength modulation and demodulation principle and implementation method and photothermal signal generation and detection of acoustic. The infrared gas absorption line intensity, linear function and line width calculation method was given. On the basis of Fourier analysis, a theoretical analysis of the wavelength modulation and demodulation process was given. A brief introduction of the photoacoustic signal generation is given starting from the cyclical energy change. Connection and difference of QEPAS and traditional laser pas were given. The signal characteristics of QEPAS were analyzed.In a QEPAS system, quart tuning fork plays a dual effect of photoacoustic energy accumulation and photoacoustic signal detection. Starting from the piezoelectric effect and inverse piezoelectric of quartz crystal, the mechanism of the quartz tuning fork as the energy accumulator and the acoustic sensor was analyzed. The quartz tuning fork feature test circuit was built. Using the feature test circuit, the feature test experiment had been done and the resonant frequency f0, Q Value and the equivalent resistance R had been obtained.Only the symmetric vibration of a TF (i.e., the two TF prongs bend in opposite direction) is piezoelectrically active. Hence the excitation beam should pass through the gap between the TF prongs for efficient excitation of this vibration. Based on this, a fiber optical path QEPAS program was proposed, the feasibility on the optical was analyzed, and the corresponding experimental system was built. The collimation converging experimental program also was given in this paper. In order to improve the system sensitivity, micro resonator was generally used in collimation converging QEPAS system. From the acoustic point of view, a brief discussion of the constraints during the use of micro resonator was given. The theoretical length values of micro resonator tubes with different diameter pipe sizes were given, under the experimental condition.The DFB laser temperature characteristics and current characteristics test experiments had been completed. According to the measured parameters of DFB laser and the absorption line spectra parameters, the specific form of modulation signals was ultimately determined. In the open air environment, used water vapor in air as the trace gas to be detected, the QEPAS experiment was completed. According to the experimental data, the achieved sensitivity of fiber optical path QEPAS system is 7.15×10-7 cm-1·W/Hz1/2. The collimation converging optical path QEPAS system used 0.9mm and 1.2mm internal diameter of resonate tube is 2.40×10-6 cm-1·W/Hz1/2 and 3.83×10-6cm-1·W/Hz1/2 respectively.
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