Active Methane Imaging Sensor Based on WMS-2f/1f and Mechanical Scanning

2026-04-01

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Jiang, Yicong

Dong, Jiacheng

Miao, Jingwen

Zhu, Shouzheng

Wang, Senyuan

Qi, Hongxing

Liu, Shijie *

Li, Chunlai

Chen, Yuwei

DOI：10.3788/LOP251575

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Objective Methane (CH4) is a hazardous, flammable, and explosive gas with significant greenhouse potential. It readily ignites in air upon exposure to open flames, underscoring the critical importance of monitoring leaks to ensure public safety and support climate governance. CH4 possesses a global warming potential (GWP) 27.9 times that of carbon dioxide over a 100-year period, designating it as a critical target for climate change mitigation. Effective monitoring of CH4 emissions is therefore essential for risk reduction in industrial settings and addressing environmental challenges. Conventional passive optical imaging techniques, such as optical gas imaging cameras, are constrained by environmental variables including background radiation and meteorological conditions, limiting their capability for quantitative measurement. Although active detection methods offer improved accuracy, they often lack robustness over long distances or on low-reflectivity targets. This paper introduces a novel active CH4 imaging sensor system that combines wavelength modulation spectroscopy with first harmonic normalized second harmonic detection (WMS-2f/1f) method and synchronized mechanical scanning, overcoming limitations inherent in both active and passive CH4 detection approaches. Methods The system employs a distributed feedback (DFB) laser operating at 1653.74 nm, corresponding to a strong CH4 absorption line located at 6046.94 cm(-1), as documented in the HITRAN database. This wavelength selection minimizes spectral interference from water vapor and carbon dioxide, ensuring high specificity in CH4 detection. The system incorporates a laser rangefinder and a pan-tilt unit capable of two-dimensional mechanical scanning across 360 degrees horizontally and 120 degrees vertically. Multi-sensor data fusion is utilized to integrate angular position, distance, and CH4 concentration inversion, enabling reconstruction of detailed two-dimensional CH4 distribution images. Key signal processing techniques include WMS-2f/1f normalization for enhanced accuracy and a Savitzky-Golay filter to improve signal-to-noise ratio (SNR). The optical assembly within the scanning head comprises a DFB laser, fiber collimator, Fresnel lens, and an InGaAs photodetector. Experimental validation includes calibration within a gas cell at eight concentration levels from 1000 & times;10(-6)& centerdot;m to 15000 & times;10(-6)& centerdot;m. CH4 plume imaging is further evaluated using Lambertian targets with reflectivity between 10% and 90%, demonstrating accurate CH4 mapping at distances of 10 m and 30 m under varied conditions. Results and Discussions Calibration tests using a CH4 gas cell demonstrate a strong linear response between the 2f/1f signal amplitude and CH4 concentration (R-2=0.9976), confirming the system's measurement reliability. Imaging performance is evaluated at 10 m and 30 m range using Lambertian targets with reflectivities of 10%, 50%, and 90%. At 10 m range, the system accurately reconstructs spatial concentration distributions of CH4 gas bags (20%?100% volume fractions) while preserving structural details across all reflectivity levels. At 30 m range, spatial detail is reduced, but the system still clearly identifies methane gas contours against high-reflectivity backgrounds (50%?90%). Even under low-reflectivity (10%) conditions, the developed prototype still enables decent qualitative methane imaging. Quantitative analysis further supports these findings: at 10 m range, R-2 values between measured and actual concentrations are 0.95, 0.97, and 0.99 for 10%, 50%, and 90% reflectivity, respectively. At 30 m range, the corresponding R-2 values are 0.82, 0.93, and 0.98, demonstrating reliable linear fitting (R-2 >= 0.83) even under challenging conditions. Absolute quantification tests at 10 m show average relative errors of 3.17%, 5.50%, and 11.89% for 90%, 50%, and 10% reflectivity, respectively. The system's quantification accuracy remains within 15% across all scenarios, affirming its robustness and suitability for static CH4 measurements. Conclusions This study presents an active CH4 imaging sensing system that synergistically combines WMS-2f/1f with synchronized mechanical scanning. The system achieves a linear fit coefficient of R-2 no less than 0.83 under demanding conditions (30 m range, 10% reflectivity) and keeps quantification accuracy within 15% even at low Lambertian reflectivity. Based on a DFB laser and a single-point detector, the instrument offers a favorable balance between performance and cost, showing strong potential for industrial applications. Its portable design supports mobile deployment and real-time monitoring, addressing vital needs in CH4 leak detection and climate mitigation strategies. Future work will prioritize dynamic plume imaging and absolute concentration quantification to further advance industrial applicability.

Keywords:

methane leakage detection

WMS-2f/1f

active imaging

mechanical scanning

non-contact detection

Active Methane Imaging Sensor Based on WMS-2f/1f and Mechanical Scanning

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