Chinese Optics Letters, Volume. 20, Issue 9, 093001(2022)

Reconstruction and fitting of second-harmonic signals by wavelength modulation spectroscopy method based on fast Fourier transform

Linquan Lai1, Yue Chen1, Kongtao Chen1, Jiale Tang1, Kaiwen Yin1, Fuqiang Jia1,*, Dun Qiao2, Yuanlong Fan2, Kang Li2, and Nigel Copner2
Author Affiliations
  • 1Fujian Key Laboratory of Ultrafast Laser Technology and Applications, School of Electronic Science and Technology, Xiamen University, Xiamen 361005, China
  • 2Wireless & Optoelectronics Research & Innovation Centre, Faculty of Computing, Engineering & Science, University of South Wales, Wales CF37 1DL, UK
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    Conventional wavelength modulation spectroscopy (WMS) is vulnerable to the influence of low-frequency noise. Accuracy of the method highly depends on the performance of the costly lock-in amplifier. In this article, we report a new and effective method for reconstructing second-harmonic signals through WMS based on fast Fourier transform (FFT). This method is less disturbed by low-frequency noise because it does not use a low-frequency ramp wave. Formulation and detection procedures were presented. The discrete second-harmonic waveform can be obtained by continuously changing the DC signal and FFT analysis in this method. Second-harmonic waveforms acquired by the two means are generally consistent. The experimental study validates the obtained gas concentration from 5% to 30%, showing a good linear relationship by the proposed method. The maximum relative error on concentration extraction is 2.87%; as for conventional WMS, this value is 4.50%. The developed measurement method may have potential in computed tomography.

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    1. Introduction

    The progress of laser and spectroscopy technology has greatly promoted the development of material detection technology[1,2]. Among them, tunable diode laser absorption spectroscopy (TDLAS) has been widely employed in gas detection across the energy, petrochemical, and mining industries because of its non-intrusiveness and high accuracy[38]. It is developed into two major approaches: direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS). WMS has high sensitivity and signal-to-noise ratio (SNR) through harmonic detection. Numerous measurement results have shown that WMS is more sensitive by one to two orders of magnitude than DAS[9].

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    Linquan Lai, Yue Chen, Kongtao Chen, Jiale Tang, Kaiwen Yin, Fuqiang Jia, Dun Qiao, Yuanlong Fan, Kang Li, Nigel Copner. Reconstruction and fitting of second-harmonic signals by wavelength modulation spectroscopy method based on fast Fourier transform[J]. Chinese Optics Letters, 2022, 20(9): 093001

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    Paper Information

    Category: Spectroscopy

    Received: Feb. 13, 2022

    Accepted: May. 17, 2022

    Posted: May. 18, 2022

    Published Online: Jun. 16, 2022

    The Author Email: Fuqiang Jia (jiafq@xmu.edu.cn)

    DOI:10.3788/COL202220.093001

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