Long-distance in-situ near-infrared gas sensor system using a fabricated fiber-coupled Herriott cell (FC-HC) operating within 1.5–2.3 μm

Long-distance field gas sensing in hazardous environment is highly desirable in industry but remains challenging due to ultralow concentration levels of the target gas species. A fiber-coupled Herriott cell (FC-HC) was fabricated for long-distance gas detection, which is composed of a sealed Herriott cell with a dimensional size of 8.3 cm × 9.0 cm × 47.2 cm, a single-mode fiber collimator for input light coupling and a multi-mode optical fiber for output light coupling. The experimentally measured absorption path length of the FC-HC is ∼24.6 m through acetylene (C2H2) detection. Also, this FC-HC is compatible with distributed feedback lasers operating from ∼1.5 μm to ∼2.3 μm. C2H2 and methane (CH4) measurements were carried out using the fabricated FC-HC. The measurement precision of C2H2 is 0.38 parts-per-million in volume (ppmv) with a 0.5 s averaging time, which can be further decreased to 27.20 parts-per-billion in volume (ppbv) with a 70 s averaging time. The limit of detection (LoD) of CH4 is 1.11 ppmv with an averaging time of 0.5 s and can be improved to 0.04 ppmv with an averaging time of 122 s. Field measurement of the sensor system for in-situ CH4 leakage was performed to validate the normal operation of the system. The proposed FC-HC shows potential applications in the field of long-distance gas sensing based on laser absorption spectroscopy.