Sensitive anti-interference trace methane detection system based on novel hollow-core anti-resonant fiber combined with wavelength modulation spectroscopy.

Optical spectroscopic detection of methane underpins high-precision greenhouse gas quantification, source tracing and climate mitigation research. Nevertheless, traditional optical gas detection systems rely on free-space optics and discrete gas cells, suffering from large volume, complex alignment, and poor anti-interference capability. We fabricated a large-aperture five-tube double-nested hollow-core anti-resonant fiber (HC-ARF) and constructed a gas cell system with integrated gas filling and free-space optical coupling functions. The designed HC-ARF serves as both an optical waveguide and a miniature gas cell, significantly reducing optical loss and system footprint while greatly enhancing gas-light interaction efficiency. We developed an all-fiber system combined with wavelength modulation spectroscopy (WMS). Using a 1654 nm Distributed Feedback Laser (DFB) laser and optimized modulation parameters, we acquired second harmonic (2f) spectra for methane concentration measurement.The system achieves an ultra-low optimum steady-state limit of detection (LOD) of 0.02 ppm with an effective optical path length of 5 m and an averaging time of 40.2 s. The 90% rise response time was only 60 s at 1.8 bar, and the response performance can be further improved by adjusting the gas pressure. Two-day continuous atmospheric methane monitoring verified the sensor's excellent stability and environmental adaptability. Benefiting from its compact structure, high sensitivity and excellent immunity to external interference, this all-fiber sensor holds broad application prospects in atmospheric monitoring and industrial leakage early warning.