| Photoacoustic spectroscopy (PAS), in which the light absorption in analyte is converted into localized heat production and consequent acoustic pressure wave by modulation, is utilized for trace gas detection with a tiny quartz tuning fork (QTF), also known as quartz-enhanced photoacoustic spectroscopy (QEPAS), in this thesis for its high sensitivity, compact size, low cost, and so on.;Beginning with a brief review of the development of PAS, we introduced the basic principles of PAS and QEPAS method in detail. Then a numerical method was developed to simulate the acoustic coupling and QTF vibration in the spectrophone of QEPAS. With varying resonant tube dimensions, unusual acoustic signal evolution and resonance curves were found and analyzed. Finally, a set of parameters of spectrophone was suggested for optimal QEPAS gas detection.;To verify our simulation result, a complete experimental investigation of QEPAS was carried out for acetylene detection by using wavelength modulation method, with the influence of residual intensity modulation considered. Finally, C2H2 detection with a minimum detectable concentration of 2 ppm was achieved by using a 1.53 microm DFB laser with output power of 8 mW.;We proposed a novel evanescent-wave photoacoustic spectroscopy (EPAS) by using a tapered optical fiber. With a high sensitive QTF as the acoustic detector, we achieved a normalized noise equivalent absorption coefficient of 1.5x10-6 cm-1 W/Hz1/2 for acetylene detection with a 1.1-microm-diameter tapered fiber. The EPAS gas sensing method greatly simplifies the QEPAS system and promises a potential multiplexing gas sensing in fiber-optic network.;We also developed an all-optical gas sensing method based on a miniature diaphragm-type extrinsic Fabry-Perot interferometer (EFPI). The F-P cavity of the EFPI is used as the gas cell for both acoustic generation and detection. With a polymer diaphragm of 2.75 mm in diameter and ~2 microm in thickness, an all-optical PAS system was demonstrated for C2H2 detection with a detectable concentration limit of 4.3 ppm with an 8-mW DFB laser. This work might inspire the realization for small-space high sensitive gas detection and all-optical multiplexing gas sensing. |
