title: Rapid, time-division multiplexed, direct absorption- and wavelength modulation-spectroscopy.
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.
authors.affiliation: Physikalisch-Technische Bundesanstalt, Bundesallee 100, Braunschweig 38116, Germany. alexander.klein@ptb.de.
authors.forename: Alexander
authors.initials: A
authors.lastname: Klein
authors.affiliation: Physikalisch-Technische Bundesanstalt, Bundesallee 100, Braunschweig 38116, Germany. oliver.witzel@ptb.de.
authors.forename: Oliver
authors.initials: O
authors.lastname: Witzel
authors.affiliation: Physikalisch-Technische Bundesanstalt, Bundesallee 100, Braunschweig 38116, Germany. volker.ebert@ptb.de.
authors.forename: Volker
authors.initials: V
authors.lastname: Ebert