|Title:||Quartz-enhanced photoacoustic spectroscopy and its association with fiber-optic devices|
Hong Kong Polytechnic University -- Dissertations
|Pages:||xviii, 190 p. : ill. ; 30 cm.|
|Abstract:||Photoacoustic spectroscopy (PAS), in which the light absorption in analyte is converted into localized heat production and consequent acoustic pressure wave by modulation, is a powerful tool for trace gas detection. Among various acoustic detection techniques, quartz-enhanced photoacoustic spectroscopy (QEPAS), in which a tiny quartz tuning fork (QTF) is employed as acoustic transducer via piezoelectric effect, shows a number of advantages, including high sensitivity, compact size, low cost, and so on, over other methods. Beginning with a detailed review of the development of PAS, we mainly focused on the gas detection based on QEPAS method and its association with fiber-optic devices in this thesis. The basic principles and theoretical descriptions of QEPAS, including gas absorption line, acoustic generation, resonance and loss mechanism in a cylindric cavity, resonance model of QTF, and wavelength modulation (WM) method, were introduced to provide a theoretical basis for the whole thesis. We developed a numerical model with finite element method to simulate the multiple physical processes in the spectrophone of QEPAS. The model was calibrated by experimental results and found to be reliable. The position of the laser beam was optimized to be 0.6 mm down from the opening of the QTF gap. With varying resonant tube dimensions, unusual acoustic signal evolution and resonance curves were found. The results were explained by the acoustic coupling and QTF vibration. A set of parameters of spectrophone was suggested for optimal QEPAS gas detection. The numerical model provides an efficient approach in better understanding the acoustic coupling and energy conversion in QEPAS, and demonstrates a theoretical guidance for the spectrophone optimization. The detailed model setup procedure might be also instructive in the simulation of multiple physical processes. An experiment of QEPAS gas sensing was carried out by using a near-IR DFB laser. The acoustic signal was amplified by a pair of rigid tubes with optimized dimension and accumulated in a resonant QTF. Considering the property of diode laser, the influence of residual intensity modulation (RIM) on WM was analyzed theoretically and compared with experimental results. It shows that the acoustic signal profile is deformed and an extra phase delay is induced for optimal acoustic detection. By using a DFB laser with an output power of 8.44 mW and a wavelength of 1.53 μm, acetylene detection with a minimum detectable concentration limit of 2 ppm was achieved, corresponding to a normalized noise equivalent absorption (1σ) coefficient of 6.16×10⁻⁸ cm⁻¹ W/Hz ¹'².|
In order to simplify the optical collimating/focusing components in conventional open-path QEPAS, we proposed a novel evanescent-wave photoacoustic spectroscopy (EPAS) by using a tapered optical micro/nano fiber. As the diameter of an optical fiber is reduced to the scale of wavelength or even smaller, a considerable portion of light propagating outside of the fiber can be used to generate an acoustic wave by photoacoustic effect. By using a high sensitive QTF as the acoustic detector, a tapered optical fiber with a diameter of 1.1 μm, passing through the middle of the gap between two prongs of QTF, was demonstrated for EPAS acetylene detection with a comparable normalized acoustic signal with bare-QTF based QEPAS. The normalized noise equivalent absorption coefficient was estimated to be 1.5×10⁻⁶ cm⁻¹ W/Hz¹'² and possible enhancement approaches were suggested. 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, and the diaphragm is served as acoustic responser. With a polymer diaphragm of 2.75 mm in diameter and ~ 2 μm in thickness, an all-optical PAS system was demonstrated for acetylene detection with a detectable concentration limit of 4.3 ppm at atmospheric pressure by using an 8-mW DFB laser. The method incorporates the acoustic generation and detection in a same optical device for the first time, to the best of our knowledge, and pushes the sensor size into millimeter scale. This work might inspire the realization ways for all-optical multiplexing gas sensing.
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