Rapid, time-division multiplexed, direct absorption- and wavelength modulation-spectroscopy.

Alexander Klein,Oliver Witzel,Volker Ebert

doi:10.3390/s141121497

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.

Highlights

Tunable diode laser absorption spectroscopy (TDLAS) is an outstanding technique for concentration and temperature measurements of gaseous species in scientific as well as industrial applications
Direct TDLAS and wavelength modulation spectroscopy (WMS) are the preferred techniques used in laser absorption spectroscopy
The signal to noise ratios (SNR) for the WMS-2f/1f measurement is defined as the peak to peak difference divided by the standard deviation of the residual

Summary

Introduction

Tunable diode laser absorption spectroscopy (TDLAS) is an outstanding technique for concentration and temperature measurements of gaseous species in scientific as well as industrial applications. It is used in combustion diagnostics where its minimally invasive, sampling-free approach and the high temporal resolution are especially appreciated [1,2,3,4,5]. TDLAS is used in environmental and process monitoring for absolute and calibration-free detection of multiple gas species, e.g., H2O, CO2 or CH4 [6,7,8,9]. The dTDLAS approach has the advantage to be calibration-free [10]

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