Measurement of methane concentration with wide dynamic range using heterodyne phase-sensitive dispersion spectroscopy
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摘要:
为实现痕量甲烷气体的宽动态范围高灵敏度检测,本文开展了双边带拍频抑制模式的外差相敏色散光谱技术研发,研究了电光调制器工作特性以及偏置电压调控方法,对比了抑制与非抑制模式下的色散相位谱轮廓与信噪比,并对检测性能(如线性动态检测范围)进行了系统研究。基于近红外分布式反馈激光器和电光调制器,搭建了外差相敏色散甲烷气体检测系统,通过探索和分析电光调制器的最佳工作区间,实现了双边带拍频抑制进而得到了大幅值、高信噪比的色散相位信号。测量了典型高频(1.2 GHz)强度调制下甲烷/氮气标气的色散相位信号,获取了色散相位信号峰峰值随气体浓度的变化规律。同时开展了波长调制光谱技术实验,对两种技术的线性度、检测动态范围和对光功率波动的抗干扰性能进行对比研究。最后,通过测量不同浓度的标气验证了该系统在宽动态、快速时间响应下的性能。所开发的基于外差相敏色散光谱技术的甲烷检测系统具有线性度高(
R 2 = 0.9999),动态检测范围宽(38.5 ppm~40%),且对光功率波动免疫性高的显著优势。本文研发的基于外差相敏色散光谱技术的气体检测技术在宽动态范围检测和实际现场检测应用领域具有广阔的前景。Abstract:In this paper, we developed a dual-sideband beat-suppressed heterodyne phase-sensitive dispersion spectroscopy (HPSDS) for sensitive detection of trace gases across a wide dynamic range and explored the operational characteristics of the electro-optic modulator (EOM) and bias voltage control methods under sideband suppression mode. The dispersion phase spectral profiles and the corresponding signal-to-noise ratios in both suppression and non-suppression modes were compared before a comprehensive evaluation of the detection performance. A HPSDS-based detection system was developed based on a near-infrared distributed feedback laser and an EOM. The suppression of the dual-sideband beat was achieved by exploring and analyzing the optimal operational range of the EOM, leading to the optimization of dispersion phase signals with increased amplitude and high signal-to-noise ratio. The dispersion phase signals under typical high-frequency (1.2 GHz) intensity modulation were recorded for different standard methane/nitrogen mixtures. The relationship between the peak-to-peak values of the dispersion phase signals and the varied gas concentrations was then summarized. Meanwhile, wavelength modulation spectroscopy (WMS) experiments were conducted; subsequently, the HPSDS and WMS techniques’ performances were compared in terms of linearity, dynamic detection range, and immunity to optical power fluctuations. Finally, the HPSDS-based system's performance under a wide dynamic range and rapid time response was verified by measuring different concentrations of standard gases. Experimental results indicate that the HPSDS technique exhibits high linearity (
R 2 = 0.9999), a wide dynamic range (38.5 ppm to 40%), and remarkable immunity to optical power fluctuations. The dual-sideband-beat-suppression-HPSDS-based methane sensor developed in this paper shows great potential for applications involving wide dynamic range detection and on-site practical trace gas detection. -
图 4 (a)实验测得的EOM输出光功率与其偏置电压的关系;(b)调制频率为1200 MHz,在抑制模式与非抑制模式下测得的10% CH4的色散相位信号(T = 298 K, P = 1 atm, L = 20 cm);(c)双边带拍频抑制后的拍频信号频谱图;(d)无双边带拍频抑制时的拍频信号频谱图
Figure 4. (a) The measured output power of EOM as a function of bias voltages; (b) measured dispersion phase signals of 10% CH4 with or without dual-sideband beat suppression at the modulation frequency of 1200 MHz (T = 298 K, P = 1 atm, L = 20 cm); (c) the frequency spectrum of the beat note signal with dual-sideband beat suppression; (d) the frequency spectrum of the beat note signal without dual-sideband beat suppression
图 6 (a)不同CH4浓度下的HPSDS峰峰值、WMS-2f和WMS-2f/1f信号值; (b) 0.1%~0.8%浓度范围内的WMS-2f , WMS-2f/1f信号值-浓度线性关系
Figure 6. (a) HPSDS peak-to-peak values, WMS-2f and WMS-2f/1f signals at different CH4 concentrations; (b) linear relationship between WMS-2f, WMS-2f/1f signals and concentration within the range of 0.1% to 0.8% CH4, respectively
图 7 (a) 15分钟连续测量10% CH4的结果浓度分布图;(b) 测量结果的频率分布直方图及高斯曲线拟合曲线;(c) HPSDS,WMS-2f和WMS-2f/1f 信号的Allan方差分析
Figure 7. (a) Diagram of concentration distribution results for continuous measurement of 10% CH4 for 15 minutes; (b) frequency distribution of the measured concentration and the Gaussian profile fitting; (c) Allan deviation analysis of HPSDS, WMS-2f and WMS-2f/1f signals
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