A Laser-Based Multipass Absorption Sensor for Sub-ppm Detection of Methane, Acetylene and Ammonia

A compact, sensitive laser-based absorption sensor for multispecies monitoring of methane (CH₄), acetylene (C₂H₂) and ammonia (NH₃) was developed using a compact multipass gas cell. The gas cell is 8.8 cm long and has an effective optical path length of 3.0 m with a sampling volume of 75 mL. The sensor is composed of three fiber-coupled distributed feedback lasers operating near 1512 nm, 1532 nm and 1654 nm, an InGaAs photodetector and a custom-designed software for data acquisition, signal processing and display. The lasers were scanned over the target absorption features at 1 Hz. First-harmonic-normalized wavelength modulation spectroscopy (f = 3 kHz) with the second harmonic detection (WMS-2f/1f) is employed to eliminate the unwanted power fluctuations of the transmitted laser caused by aerosol/particles scattering, absorption and beam-steering. The multispecies sensor has excellent linear responses (R² > 0.997) within the gas concentration range of 1-1000 ppm and shows a detection limit of 0.32 ppm for CH₄, 0.16 ppm for C₂H₂ and 0.23 ppm for NH₃ at 1 s response time. The Allan-Werle deviation analysis verifies the long-term stability of the sensor, indicating a minimal detection limit of 20-34 ppb were achieved after 60-148 s integration time. Flow test of the portable multispecies sensor is also demonstrated in this work.