| Tunable Diode Laser Absorption Spectroscopy (TDLAS) has been widely used in trace gas detection with the merits of the high sensitivity, high resolution and fast response time. TDLAS as advanced gas detection means, has combined with long path optical multipass cell to get the higher sensitivity detection. Aiming at the requirement of the miniaturization and high precision in gas measuring instruments, a multipass cell based on two cylindrical mirrors has been designed. Combined with the Tunable Diode Laser Absorption Spectroscopy system, the trace detection of methane and carbon dioxide has been realized respectively. The main contents can be generalized as follows:An introduction of the significance of trace gases detected in the atmosphere. The principle of the laser absorption spectroscopy, different spectral lines and the selection of gas absorption line are also described.On the basis of knowing the principle and structure of the traditional multipass cell, a multipass cell based on two cylindrical mirrors has been described, the gold-coated mirrors have a cylindrical focal length of 250 mm and a diameter of 50 mm, a front cylindrical mirror with a central optical aperture of 4 mm diameter, rotation of the cylindrical mirrors and variations of the cell base length have been used to achieve different spot patterns and path lengths. The multipass cell has the characteristics of simple and compact structure.cheaper processing cost, high efficient use of mirrors area, longer optical path lengths at the same base length compared with traditional multipass cell, this can be used to improve the detection limit of the system.A TDLAS system based on cylindrical mirror optical multipass cell has been established. Methane and carbon dioxide direct absorption measurement using the multipass cell with fibre-coupled distributed-feedback laser at 1.65 μm and 1.57 μm have been performed respectively. CH4 minimum detectable concentration of 0.68 ppm (1σ,3.3 s averaging time) and CO2 minimum detectable concentration of 63.3 ppm (1σ,5 s averaging time) has been determined respectively for absorption optical path lengths is 13.8 m, and the detection of atmospheric methane and carbon dioxide has been achieved using the system. |
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