Abstract
Design and fabrication of a dual spot-ring Herriott cell (DSR-HC) were proposed. The sealed Herriott cell with a dimensional size of 5.5 cm × 9.2 cm × 32.1 cm, possessed two input/output coupling holes leading to two absorption path lengths of ~20 m and ~6 m, respectively. An acetylene (C2H2) sensor system with a double-range was developed using the DSR-HC and wavelength modulation spectroscopy (WMS) technique. A near-infrared distributed feedback (DFB) laser was employed for targeting a C2H2 absorption line at 6521.2 cm-1. C2H2 concentration measurements were carried out by modulating the laser at a 5 kHz frequency and demodulating the detector signal with LabVIEW software. An Allan-Werle deviation analysis indicated that the limit of detection (LoD) for the two absorption path lengths of 20 m and 6 m are 7.9 parts-per-million in volume (ppmv) and 4.0 ppmv, respectively. The DSR-HC concept can be used to fabricate similar cells for single-gas detection requiring two different detection ranges as well as for dual-gas detection requiring different absorption path lengths.
Highlights
Acetylene (C2H2) is colorless, inflammable and explosive at ambient temperature and pressure and is widely used in the industrial field as a fuel and a basic raw material in organic synthesis
When the temperature of the distributed feedback (DFB) laser was set to 23.8 °C, the driving currents corresponding to the wavenumbers are marked by round dots on the absorption curve, which was measured with a Fourier Transform Infrared Spectrometer (FTIR, Thermo Fisher Scientific, model Nicolet Is50 FT-IR).The scan range of the driving current was set to 38−96 mA, corresponding to a wavenumber range of 6520.4−6522 cm−1
A C2H2 sensor system was developed using the dual spot-ring Herriott cell (DSR-HC) with two absorption lengths of ~20 m and ~6 m, respectively, by dividing the laser beam into two parts and injecting the two beams simultaneously into the DSR-HC
Summary
Acetylene (C2H2) is colorless, inflammable and explosive at ambient temperature and pressure and is widely used in the industrial field as a fuel and a basic raw material in organic synthesis. Tunable laser absorption spectroscopy (TDLAS) has been widely used in trace gas detection because of its non-intrusive, sensitive and selective sensing characteristics [5,6,7]. A multi-pass gas cell [10] and a high-fineness cavity [11] are two kinds of gas absorption approaches for enhancing sensitivity based on different principles, which are widely used in TDLAS based gas sensor systems. The detection range of a gas sensor with a certain absorption path length is usually limited to 2-3 orders of magnitude in concentration levels. In order to address this problem, modeling and a new design of a dual spot-ring Herriott cell (DSR-HC) is proposed, which has two independent entrance holes leading to two different absorption path lengths. The proposed Herriott cell model can be used to fabricate similar cells with different dual absorption lengths. Besides the detection of one single gas species with two different detection ranges, such a gas cell can be adopted in dual-gas detection requiring different absorption lengths for a specific sensitivity
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