Application of White Noise Perturbation in Wavelength Modulated Off-Axis Integrated Cavity Spectroscopy
WANG Jing-jing1, 2, DONG Yang2, TIAN Xing2, CHEN Jia-jin2, TAN Tu2, ZHU Gong-dong2, MEI Jiao-xu2, GAO Xiao-ming1, 2*
1. School of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei 230031, China
2. Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
Abstract：Off-axis integrated cavity output spectroscopy is an important method for trace gas detection, and its measurement limit is affected by residual cavity mode noise and background noise. In this paper, the residual mode noise in the output spectrum of OA-ICOS is reduced by injecting radio frequency (RF) white noise into the modulation current of the laser. Meanwhile, the influence of background signal is suppressed by wavelength modulation technology, and the signal-to-noise ratio of the OA-ICOS based methane sensor is further improved. Firstly, the influence of RF white noise with different power on the absorption spectra of methane in air is studied in detail, and the corresponding optimal modulation amplitude is calculated thought analyzing the linewidth of the absorption spectra. Subsequently, the influence of RF white noise with different power on 2f signal is studied. The results show that the baseline noise and 2f signal amplitude decrease simultaneously with the increase of RF noise power. By analyzing the signal-to-noise ratio of several sets of 2f signals, it is determined that the best power of RF white noise to improve the signal-to-noise ratio of the system is -25 dBm. Then, the relationship between methane concentration and the 2f signal in different concentration ranges was studied. The results show that the methane concentration is linear with 2f amplitude in the concentration range of less than 1.0×10-6, and the methane concentration is in a curve relationship with the 2f signal in the concentration range of 0.1~2.2×10-6. Subsequently, the stability of the system was investigated by measuring 2.2×10-6 methane for a long time and Allan analysis of variance. The system has good stability, the optimal integration time is up to 1 250 s, and the corresponding minimum detectable limit is 1.2×10-9. Finally, the methane concentration in the air out our laboratory was measured by using the built methane gas sensor for two days and nights. The results show that the diurnal variation of methane concentration is falling during the day and rising at night. The fluctuation range of methane concentration is 2.02×10-6~2.3×10-6, and the average concentration is 2.14×10-6. This study provides a certain reference value for the application of off-axis integral cavity output spectroscopy in the measurement of the trace gas. It is of great significance for the development of high-precision in-situ trace gas measuring instruments.
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