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

Abstract

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.

Infrared Non‐Invasive Exhaled Biomarker Sensing: A Review

“…[ 85 ] Minimum detection limit of 0.23 ppm for detection of ammonia has been explored with InGaAs photodetector and DFB laser and demonstrated 1 s response time. [ 133 ] TDLAS detector with DFB laser at 1.51 µm has been demonstrated with effective ammonia detection limit of 0.16 ppm and response time of 184 s. [ 134 ] However, TDLAS with QCL at 10.3 µm has been demonstrated with the minimum detection limit of 8.3 and 30 ppb at 30 and 1 s response time respectively. [ 135 ] The in situ measurement of ammonia has been explored with LPAS and QCL showed detection limit of 7 ppb at 9.06 µm with response time of 100 s. [ 136 ] In contrast, LPAS with QCL at 10.34 µm has been demonstrated with 74 ppb detection limit at environmental atmosphere.…”

“…[ 157 ] InGaAs photodetector along with DFB has been explored for detection of methane with the minimum detection limit of 0.32 ppm at 1 s response time. [ 133 ] Methane with minimum detection limit of 78 ppb and the response time of 0.8 s was reported with TDLAS‐based detector alongside multipass cell (Figure 4c). [ 86 ] Pyroelectric‐based NDIR detector with incandescent lamp has been demonstrated for methane detection with the 1.7 ppm detection limit and 4 s response time at long optical pathlength (25 cm).…”