Article

Trace gas measurements using optically resonant cavities and quantum cascade lasers operating at room temperature

INP Greifswald, 17489 Greifswald, Felix-Hausdorff-Str. 2, Germany
Journal of Applied Physics (Impact Factor: 2.18). 12/2008; 104(9):093115 - 093115-15. DOI: 10.1063/1.3008014
Source: IEEE Xplore

ABSTRACT

Achieving the high sensitivity necessary for trace gas detection in the midinfrared molecular fingerprint region generally requires long absorption path lengths. In addition, for wider application, especially for field measurements, compact and cryogen free spectrometers are definitely preferable. An alternative approach to conventional linear absorption spectroscopy employing multiple pass cells for achieving high sensitivity is to combine a high finesse cavity with thermoelectrically (TE) cooled quantum cascade lasers (QCLs) and detectors. We have investigated the sensitivity limits of an entirely TE cooled system equipped with an ∼0.5 m long cavity having a small sample volume of 0.3 l. With this spectrometer cavity enhanced absorption spectroscopy employing a continuous wave QCL emitting at 7.66 μ m yielded path lengths of 1080 m and a noise equivalent absorption of 2×10-7 cm -1  Hz -1/2 . The molecular concentration detection limit with a 20 s integration time was found to be 6×108 molecules / cm 3 for N 2 O and 2×109 molecules / cm 3 for CH 4 , which is good enough for the selective measurement of trace atmospheric constituents at 2.2 mbar. The main limiting factor for achieving even higher sensitivity, such as that found for larger volume multi pass cell spectrometers, is the residual mode noise of the cavity. On the other hand the application of TE cooled pulsed QCLs for integrated cavity output spectroscopy and cavity ring-down spectroscopy (CRDS) was found to be limited by the intrinsic frequency chirp-
of the laser. Consequently the accuracy and advantage of an absolute internal absorption calibration, in theory inherent for CRDS experiments, are not achievable.

Download full-text

Full-text

Available from: Richard Engeln
  • Source
    • "During the last decade different laser spectroscopic techniques, such as tunable diode laser absorption spectroscopy (TDLAS) and cavity ring-down spectroscopy (CRDS), have been extensively used to measure the amount fraction of different gas species [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]. Each technique has its own merits, consequently their choice depends on the individual field of application [12] [17]. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Employing direct absorption spectroscopy and using a spectrometer comprising a single-pass and a multipass white cell, we probed the R(12) line of carbon dioxide (CO 2) in the combination band around 2 μm. Gravimetric gas standards containing CO 2 , between 300 and 60 000 μmol mol −1 (0.03% to 6%), in N 2 were quantified by means of the TILSAM method. The spectrometric results were compared with the gravimetric reference values. We describe our implementation of the 'Guide to the Expression of Uncertainty in Measurements' to infrared laser-spectrometric gas analysis. Data quality objectives are addressed by uncertainty and traceability flags. Uncertainty budgets are presented to show the quality of the results and to demonstrate software-assisted uncertainty assessment. The relative standard uncertainties of the spectrometrically measured CO 2 amount fractions at, e.g., ambient levels of 360 μmol mol −1 and at exhaled breath gas levels of 50 mmol mol −1 were 1.4% and 0.7%, respectively. Our detection limit was 2.2 μmol mol −1 . The reproducibility of individual results was in the ±1% range. Furthermore, we measured collisional broadening coefficients of the R(12) line of CO 2 at 4987.31 cm −1 . The relative standard uncertainties of the measured self-, nitrogen-, oxygen-and air-broadening coefficients were in the ±1.7% range.
    Full-text · Article · Jan 2013 · Measurement Science and Technology
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Mid infrared (MIR) absorption spectroscopy between 3 and 20 µm, known as Infrared Laser Absorption Spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas and for trace gas analysis. The increasing interest in molecular processing plasmas has lead to further applications of IRLAS. IRLAS provides a means of determining the absolute concentrations and temperatures of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Since plasmas with molecular feed gases are used in many applications such as thin film deposition and semiconductor processing this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes and for trace gas analysis. The aim of the present contribution is threefold: (i) to report on selected studies of the spectroscopic properties and kinetic behaviour of the methyl radical, (ii) to review recent achievements in our understanding of molecular phenomena in plasmas and the influence of surfaces, and (iii) to describe the current status of advanced instrumentation for quantum cascade laser absorption spectroscopy (QCLAS).
    Full-text · Article · Jan 2009 · Proceedings of SPIE - The International Society for Optical Engineering
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A continuous wave external cavity quantum cascade laser (EC-QCL) operating between 1872 and 1958 cm−1 has been used to make rotationally resolved measurements in the fundamental band of nitric oxide at 140 mTorr, and the ν2 band of water at atmospheric pressure. These measurements demonstrate the advantages of wide tunability and high resolution of the EC-QCL system. From direct absorption spectroscopy on nitric oxide a laser bandwidth of 20 MHz has been deduced and a sensitivity of 8.4×10−4 cm−1 Hz−1/2 was achieved. Wavelength modulation spectroscopy using current modulation enhances the sensitivity by a factor of 23 to 3.7×10−5 cm−1 Hz−1/2.
    Full-text · Article · May 2009 · Applied Physics Letters
Show more

Similar Publications