A compact self-calibration atmospheric CH4 sensor based on NIR laser and new type multi-pass gas cell

Abstract

A near-infrared (NIR) laser-based optical sensor was developed for atmospheric methane (CH4) measurement. The sensor adopted a distributed feedback (DFB) fiber laser operating at ∼ 2334 nm as the light source. A compact new type multi-pass gas cell (MPGC) with 41.5 m effective optical path length (EOPL) and a high-speed data acquisition (DAQ) card with 1 million samples per second (MS/s) sample rate were utilized to improve the system performance. A self-calibration method was used to solve the problem of needing periodic external intervention for concentration calibration which has been encountered in traditional wavelength modulation spectroscopy (WMS) technology-based sensor system. For the selected CH4 molecule absorption line, located at 4284.512 cm−1, a detection limit (three-sigma value (3σ)) of 0.66 parts per million by volume (ppmv) at ambient temperature and pressure was achieved. Allan deviation analysis results indicate that measurement precision of 0.17 ppmv was obtained with a 1-s integration time, which could be further optimized to 0.03 ppmv by averaging up to ∼ 83 s. The CH4 concentration measurement both in the lab and in the field were carried out successively, which demonstrates the stability and robustness of the developed sensor.