Multi-harmonic detection in wavelength modulation spectroscopy systems

Hangauer, Andreas; Chen, Jia; Strzoda, R.; Amann, M.-C.

doi:10.1007/s00340-012-5049-y

Cited by 18 publications

(16 citation statements)

References 14 publications

(17 reference statements)

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“…It does show dramatic improvement in terms of stability with a much longer turn-over time of ~400s. The spectrum fitting shows worse precision than the peak-to-trough measurement, a similar result observed and discussed by Hangauer et al [27]. Notice the measurement was performed under real field conditions in this case.…”

Section: Sensor Performancesupporting

confidence: 80%

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Compact and portable open-path sensor for simultaneous measurements of atmospheric N_2O and CO using a quantum cascade laser

et al. 2012

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A compact and portable open-path sensor for simultaneous detection of atmospheric N(2)O and CO has been developed with a 4.5 μm quantum cascade laser (QCL). An in-line acetylene (C(2)H(2)) gas reference cell allows for continuous monitoring of the sensor drift and calibration in rapidly changing field environments and thereby allows for open-path detection at high precision and stability. Wavelength modulation spectroscopy (WMS) is used to detect simultaneously both the second and fourth harmonic absorption spectra with an optimized dual modulation amplitude scheme. Multi-harmonic spectra containing atmospheric N(2)O, CO, and the reference C(2)H(2) signals are fit in real-time (10 Hz) by combining a software-based lock-in amplifier with a computationally fast numerical model for WMS. The sensor consumes ~50 W of power and has a mass of ~15 kg. Precision of 0.15 ppbv N(2)O and 0.36 ppbv CO at 10 Hz under laboratory conditions was demonstrated. The sensor has been deployed for extended periods in the field. Simultaneous N(2)O and CO measurements distinguished between natural and fossil fuel combustion sources of N(2)O, an important greenhouse gas with poorly quantified emissions in space and time.

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Section: Sensor Performancesupporting

confidence: 80%

“…In WMS, different methods [27] have been used to retrieve the gas concentration from the harmonic signals. A simple way is to measure and calibrate the central peak amplitude (or peak-to-through height) of the 2nd harmonic signal, which is proportional to the gas concentration in an optical thin condition.…”

Section: Data Retrievalmentioning

confidence: 99%

Compact and portable open-path sensor for simultaneous measurements of atmospheric N_2O and CO using a quantum cascade laser

et al. 2012

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“…The basic principles of dTDLAS and WMS are well described in literature [ 16 – 20 ]. For reasons of continuity they will be briefly discussed and important points for this paper will be emphasized.…”

Section: Spectroscopic Techniquesmentioning

confidence: 99%

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

Klein

Witzel

Ebert

2014

Sensors

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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.

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“…However, many of the proposed methods [18][19][20][21][22] cannot effectively suppress the effect of laser intensity fluctuations because the laser should scan across the entire absorption feature. Hangauer et al [23] put forward a method that reconstructed the laser transmission (used to infer the absolute line shape) by employing the harmonics at a single wavelength. This method can effectively eliminate the effect of laser intensity fluctuation.…”

Section: Introductionmentioning

confidence: 99%

Calibration-free wavelength modulation for gas sensing in tunable diode laser absorption spectroscopy

et al. 2014

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A method is proposed that employs the ratios of the 2nd and 4th harmonics at the line center to measure line width under high absorption. The measured line width is then applied in the ''calibration-free 2f/1f'' formula to detect gas pressure and concentration. To verify this method, the transitions of CO 2 at 6,982.0678 cm -1 and H 2 O at 6,979.2995 cm -1 are selected to measure the line width, gas pressure, and concentration in laboratory and field environments, respectively. Experimental results are very consistent with the expectations.

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Multi-harmonic detection in wavelength modulation spectroscopy systems

Cited by 18 publications

References 14 publications

Compact and portable open-path sensor for simultaneous measurements of atmospheric N_2O and CO using a quantum cascade laser

Compact and portable open-path sensor for simultaneous measurements of atmospheric N_2O and CO using a quantum cascade laser

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

Calibration-free wavelength modulation for gas sensing in tunable diode laser absorption spectroscopy

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