Development and measurement of a near-infrared CH4 detection system using 1.654 μm wavelength-modulated diode laser and open reflective gas sensing probe

A near-infrared (NIR) methane (CH4) detection system based on tunable diode laser absorption spectroscopy (TDLAS) was experimentally demonstrated, which adopts a distributed feedback (DFB) laser and an open reflective gas sensing probe. An analog proportion-integration-differentiation (PID) based laser temperature control & modulation module and an orthogonal lock-in amplifier module were self-developed. The laser's temperature fluctuation can be limited within the range of −0.02–0.02 °C, and good stability was observed through 50-min monitoring on the emitting wavelength of the laser. For the developed orthogonal lock-in amplifier, the maximum relative errors on amplitude extraction are less than 0.35% for the 1f harmonic signal and 0.71% for the 2f harmonic signal. Gas detection experiments were conducted to derive the relation between harmonic amplitudes and gas concentration. According to the detection results on the 2 × 104 parts per million (ppm) CH4 sample for a period of 1000 s, the related detection precision is estimated to be −92–118 ppm. The relative detection error is −1–2.3% within the whole detection range of 0–5 × 104 ppm. Based on the Allan deviation at an integral time of 1 s, the limit of detection (LoD) is decided to be 29.52 ppm with a path length of 40 cm, indicating a minimum detectable column density of ∼12 ppm m. Compared with our previously reported NIR CH4 detection system, this system exhibits some improvements in both optical and electrical structures, including the analog temperature controller with less software consumption, more accurate lock-in amplifier, simple and reliable open reflective sensing probe, etc.