Hua XIA(夏滑), Guo-jie TU(涂郭结), Tao PANG(庞涛),Zhi-rong ZHANG(张志荣), Bian WU(吴边), Feng-zhong DONG(董凤忠)
（Anhui Institute of Optics & Fine Mechanics, Chinese Academy of Sciences, Anhui Provincial Key Laboratory of Photonic Devices and Materials, Hefei 230031, China）
Abstract-Combination of Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique and multipass cell is an attractive approach for ultrahigh sensitive detection of trace gases. Theoretically，based on Beer-Lambert law, the longer optical path length and the larger gas absorption, the lower concentration gas could be detected. However, lower radiation intensity and inevitable etalon fringe resulted from multiple reflections would greatly weaken the Signal-to-Noise Ratio (SNR) and thus an expected ultrahigh sensitive detection system is difficult to achieve. In order to fully make use of the advantages of TDLAS and multipass cell, the base length and the total optical path length of the multipass cell are needed to be carefully balanced. Furthermore, the harmonic signals contaminated by various noises are processed with wavelet transform method. As a demonstration of this method, few low concentrations of gas CO in N2 are measured employing TDLAS technique and a novel sealed multipass cell with total optical length of 114 m. The detection limit is about 5×10-6（volume ratio）, which is one order of magnitude better than earlier noise reduction.
Key words-absorption spectroscopy, TDLAS, multipass cell, etalon fringe, wavelet transform, detection limit
Manuscript Number: 1674-8042(2011)04-0402-04
doi: 10.3969/j.issn.1674-8042.2011.04.023
References
[1] J. U .White, 1942. Long optical paths of large aperture. Opt.Soc.Am., 32: 285-288.
[2] D. R. Herriott, H. J. Schulte, 1965. Folded optical delay lines. Appl. Opt., 4: 883-889.
[3] J. B. McManus, P. L. Kebabian, 1995. Astigmatic mirror multipass absorption cells for long-path-length spectroscopy. Appl. Opt. 34(18):3336-3348.
[4] P. Werle, F. Slemr, 1991. Signal-to-noise ratio analysis in laser absorption spectroscopy using optical multipass cell. Appl. Opt., 30(4): 430-434.
[5] W. S. Qian, 2000. Efficient reduction of fringe noise in trace gas detection with diode laser multipass absorption spectroscopy. Appl.Phys., 39(7A): 4034-4040.
[6] Hua Xia, Feng-zhong Dong, Guo-jie Tu, et al., 2010. High sensitive detection of carbon monoxide based on novel multi-pass cell. ACTA Optical Sinica, 30(9): 2596-2601.
[7] P. W. Werle, P. Mazzinghi, 2004. Signal processing and calibration procedures for in situ diode-laser absorption spectroscopy. Spe Acta Part A, 60(8-9): 1685-1705.
[8] K. Kerner, S. M. Rochester. Variable free spectral range spherical mirror Fabry-Perot interferometer, 2011-06-06.http://budker.berkeley.edu/papers/pdfs/katie(SFPI).pdf.
[9] S.Mallat,1989.Multiresolution approximation and wavelet orthonormal bases of L2.Trans AMS, 315(1): 69-87.
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