| Tunable diode laser absorption spectroscopy (TDLAS) is one of the most widely used gas detection techniques and has been widely used in many fields such as environmental monitor, industrial process and biological medicine with the merits of its high sensitivity, high selectivity, and fast responsibility. However,some intrinsic properties of single mode diode lasers can impair their spectroscopic applicability. For example, the central wavelength of the single mode diode laser might change according with thermal or mechanical fluctuations, and so on. To cope with these problems, a novel method combines tunable multi-mode diode laser absorption spectroscopy, optical correlation spectroscopy, and wavelength modulation spectroscopy (TMDL-COSPEC-WMS) is developed and used for the detection of carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) in this dissertation.Firstly, we demonstrate the measurements of CO2 concentrations using a system employing a tunable multimode diode laser at 1570nm based on the TMDL-COSPEC-WMS. The absorption lines of CO2 around 1570nm are suitable for WMS-2f detection by multimode diode laser because they are discrete and the emission spectrum of the laser overlaps well with the absorption lines of CO2 in this region. The useful signal pairs are selected based on high signal to noise ratio and correlation coefficient. The CO2 concentration in measurement cell is derived from the relation between the normalized WMS-2f signal peak heights of the measurement and reference signals. Beside that, another TMDL-COSPEC-WMS system is also designed and demonstrated for the concentration measurements of oxygen using A-band absorption lines of oxygen around 760nm.The reliability will be enhanced when the gas concentration measurement employing multi signals, but the sensitivity will be shift caused by the competition of the multiple modes. In order to increase the sensitivity of the system employing TMDL-COSPEC-WMS, a Herriott multi-pass absorption cell with the path length of 100 m discrete lens-type is used as a gas absorption cell. Herriott cells are used widely due to some evident advantages such as easy to use, robust, adequate for all wavelength, and immune to the environment, etc. The optical-path enhanced TMDL-COSPEC-WMS system is used to measure CH4by employing Fabry-Perot (FP) type TMDL at 1675 nm as the light source. Additionally, another system based on TMDL-WMS and Herriott cell is also developed to detect the CO by a stable FP type TMDLnear 1570 nm.At present, the typical rapid-tuning range of the telecommunications quality single-mode DFB diode laser commercially available is 0 to 2 cm-1. Hence few of the sensors enable simultaneous measurements of multiple species and flow parameters along one light of sight by using a single DFB diode laser. Here we report on the first application (to our knowledge) of an extended-wavelength (2.33 μm) multi-mode diode laser for simultaneous measurement of the concentrations of CH4 and CO in the ambient air. The chosen wavelength corresponds to an atmospheric transmission window where the absorption of the main interfering species is weak. The high sensitive and stable system used in chapter 4 is also employed for the simultaneous detection of CH4 and CO. The accuracy, precision, and stability of the TMDL-COSPEC-WMS system are also analyzed to confirm the potential of the system for simultaneous measurements of CH4 and CO in atmosphere.In the gas detection by using TMDL-COSPEC-WMS, there is no need to calibrate the characterization and stabilize the frequency of the lasers. So the TMDL-COSPEC-WMS systems have higher reliability and stability and are immune to the variation of the environment. The sensitivity can be enhanced by combining the multi pass cell and harmonic detection technique. All the advantages promote the application and commercialization of the system in environmental monitoring and surveillance. |
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