Tunable Fiber Laser Based Photoacoustic Spectroscopy Technology For Trace Gas Detection

Trace gas detection techniques have been playing an important role in the modern analysis technologies. Trace gas measurements base on photoacoustic spectroscopy (PAS) possess the advantages such as high sensitivity, selective detection, fast response in continuous-flow mode and suitable for online multicomponent trace gas analysis, so it has been widely studied and applied in many fields. In this dissertation, a high performance near-infrared photoacoustic spectrometer for trace C2H2 and NH3 detection is developed. The main research works are outlined as follows.The relationship between photoacoustic signal intensity and photoacoustic cell structure parameters is discussed in detail according to the mechanism of gas photoacoustic effect. Through analyzing the typical structure of the first-order longitudinal resonant cylindrical photoacoustic cell the sound field distribution, photoacoustic cell constant, resonance frequency and quality factor of [100] mode are derived, and three dimension diagrams of three characteristic parameters with photoacoustic cell geometric parameters are numerically simulated. The results show that the maximum photoacoustic signal can be obtained when the microphone is installed in the middle of the cell. Furthermore, the basic consideration for determining the geometric parameters of photoacoustic cell is described.By analyzing the noise sources and signal extraction methods of the PA system, it emphasized that the noise suppression method and the second harmonic detection techniques would be key important factor for improving detection limit sensitivity. Based on the broadened Lorentz shape resulted from the collision of the absorbed gas molecules, a theoretical curve between the ratio of the second harmonic peak value to the width at half maximum and modulation index is given by numerical simulation method, which provides a theoretical basis for the selection of the optimal frequency modulation parameters. Using acetylene gas as an example, the theoretical analysis shows that when the gas concentration varies in the range of ppb-ppm, the change of gas concentration has a very small influence on the width of absorption lines under atmospheric pressure. Therefore, the effect on optimal modulation depth can be ignored. The physical mechanism of light amplification in erbium-doped fiber is described. A tunable ring cavity erbium-dopde fiber laser is designed and developed, which obtained an 3.5 mW,3dB bandwidth less than 0.01nm and spectrum over C+L-band. In order to enhance the laser output power, and to improve the photoacoustic signal and detection sensitivity, a combination of the tunable fiber laser and an erbium doped fiber amplifier (EDFA) is deployed. The EDFA has a double-clading, double-direction pump structure, and high saturated gain output. The saturated output power and the 3dB bandwidth of the EDFA are 500mW and 36nm respectively. Based on all above, a high performance photoacoustic spectrometer for trace gas detection is firstly developed with a first-order longitudinal resonant photoacoustic cell with double absorption optical path, the lock-in amplifier based second harmonic detection technique, a tunable fiber laser and an erbium doped fiber amplifier.The near-infrared absorption spectrums of acetylene and ammonia gases are analysed. An acetylene P9 peak (1530.37nm) and an ammonia transitionpP3(5)a (1531.7nm) are selected for the systerm performance evaluation due to the weak interference with H2O and CO2 absorption at these laser lines. Because NH3 molecule shows polar, which can adsorb on the inner wall of photoacoustic cell, a low surface energy material Teflon is used as cell materials. As a result, the fast detection for NH3 with high sensitivity is achieved.The evaluation of system performance showed that the detection limit sensitivity for acetylene detection is 1.3ppb and the detection limit for ammonia under room temperature and atmospheric pressure is 3ppb; the response time of the Teflon resonant cell for low concentration NH3 is up to 60s approximately under the flow rate of 200SCCM.This thesis developed a sensitive photoacoustic spectrometer based on a tunable fiber laser and an erbium doped fiber amplifier. The system have many potential applications such as environmental air pollution detection, dissolved oil gas analysis for power transformer monitoring and human breath gas analysis in medicine.