Hence, the WMS-2 f/1 f method is favorable for trace gas detection. By normalizing the second harmonic signal by the first harmonic signal which is simultaneously recorded (i.e., WMS-2 f/1 f), the influence of optical power fluctuations can be largely eliminated. However, the 2 f signal magnitude will be influenced by the laser power fluctuations in addition to being sensitive to the photodetector shifts. The WMS method with second harmonic signal (2 f) detection (WMS-2 f) has been widely used for trace gas sensing. Note that WMS method overcomes the baseline fitting issue and improves the detectivity by shifting the detection bandwidth to higher frequencies (twice the modulation frequency). Additionally, the detection limit of DAS is limited due to low-frequency noises. However, the unexpected laser power fluctuations induced by laser systems and particles scattering/absorption from the target environment will definitely introduce unwanted interferences and extra uncertainties. By baseline fitting to the transmitted absorption profile, the gas concentration can be quantified. DAS is a straightforward technique for gas detection. #COVID 19 MULTIPASS PORTABLE#Flow test of the portable multispecies sensor is also demonstrated in this work.įor LAS-based sensors, direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS) are two typical measurement schemes. 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. The multispecies sensor has excellent linear responses (R 2 > 0.997) within the gas concentration range of 1–1000 ppm and shows a detection limit of 0.32 ppm for CH 4, 0.16 ppm for C 2H 2 and 0.23 ppm for NH 3 at 1 s response time. First-harmonic-normalized wavelength modulation spectroscopy ( f = 3 kHz) with the second harmonic detection (WMS-2 f/1 f) is employed to eliminate the unwanted power fluctuations of the transmitted laser caused by aerosol/particles scattering, absorption and beam-steering. The lasers were scanned over the target absorption features at 1 Hz. #COVID 19 MULTIPASS SOFTWARE#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 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. A compact, sensitive laser-based absorption sensor for multispecies monitoring of methane (CH 4), acetylene (C 2H 2) and ammonia (NH 3) was developed using a compact multipass gas cell.
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