Gas sensing inside hollow-core fibers

Laser-based spectroscopy in the infrared region is critical for applications requiring trace-level gas detection with high selectivity. While multi-pass cells are traditionally employed to enhance sensitivity by extending the optical path length, their bulkiness, mechanical sensitivity, and alignment challenges limit their practicality. Hollow-core fibers (HCFs), particularly antiresonant HCFs (ARHCFs), provide an interesting, compact alternative to multi-pass cells. In the past few years ARHCFs have proven effective for absorption spectroscopy in visible, near-infrared, and mid-infrared regions. This work presents a novel reflective, dual-pass sensing configuration utilizing ARHCFs for gas detection. In this setup, a 60-cm-long ARHCF serves as both a gas cell and a sampling probe, with laser light entering and exiting from the same side. The reflective configuration eliminates the need for additional tubing, minimizing dead volume and reducing the sample volume required. Additionally, the effective path length is doubled, enhancing detection sensitivity while maintaining a rapid sensor response time of ~2.5 seconds. Spectra recorded near 3270 nm using wavelength modulation spectroscopy demonstrated the system’s capability to detect methane (CH₄) concentrations with high selectivity and sensitivity.