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Sensors 2014, 14(11), 21497-21513; doi:10.3390/s141121497

Rapid, Time-Division Multiplexed, Direct Absorption- and Wavelength Modulation-Spectroscopy

1
Physikalisch-Technische Bundesanstalt, Bundesallee 100, Braunschweig 38116, Germany
2
Center of Smart Interfaces, TU Darmstadt, Petersenstraße 32, Darmstadt 64287, Germany
*
Author to whom correspondence should be addressed.
Received: 25 September 2014 / Revised: 28 October 2014 / Accepted: 10 November 2014 / Published: 14 November 2014
(This article belongs to the Section Physical Sensors)
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Abstract

We present a tunable diode laser spectrometer with a novel, rapid time multiplexed direct absorption- and wavelength modulation-spectroscopy operation mode. The new technique allows enhancing the precision and dynamic range of a tunable diode laser absorption spectrometer without sacrificing accuracy. The spectroscopic technique combines the benefits of absolute concentration measurements using calibration-free direct tunable diode laser absorption spectroscopy (dTDLAS) with the enhanced noise rejection of wavelength modulation spectroscopy (WMS). In this work we demonstrate for the first time a 125 Hz time division multiplexed (TDM-dTDLAS-WMS) spectroscopic scheme by alternating the modulation of a DFB-laser between a triangle-ramp (dTDLAS) and an additional 20 kHz sinusoidal modulation (WMS). The absolute concentration measurement via the dTDLAS-technique allows one to simultaneously calibrate the normalized 2f/1f-signal of the WMS-technique. A dTDLAS/WMS-spectrometer at 1.37 µm for H2O detection was built for experimental validation of the multiplexing scheme over a concentration range from 50 to 3000 ppmV (0.1 MPa, 293 K). A precision of 190 ppbV was achieved with an absorption length of 12.7 cm and an averaging time of two seconds. Our results show a five-fold improvement in precision over the entire concentration range and a significantly decreased averaging time of the spectrometer. View Full-Text
Keywords: trace gas measurement; absorption spectroscopy; TDLAS; WMS; hygrometer; calibration-free trace gas measurement; absorption spectroscopy; TDLAS; WMS; hygrometer; calibration-free
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Klein, A.; Witzel, O.; Ebert, V. Rapid, Time-Division Multiplexed, Direct Absorption- and Wavelength Modulation-Spectroscopy. Sensors 2014, 14, 21497-21513.

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