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: 2.19). 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.

0 Bookmarks
 · 
113 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We demonstrate the performance of a compact and robust gas sensor based on a pulsed distributed feedback quantum cascade laser (QCL, laser pulses 30 ns, repetition rate 1 MHz) in combination with off-axis integrated cavity output spectroscopy (OA-ICOS) in an improved 3 mirror configuration. The room temperature laser (wavelength 915 cm−1) was wavelength tuned over ∼0.25 cm−1 with a 4 kHz repetition frequency. A detection limit of 10 ppbv (part-per-billion volume) for regular OA-ICOS is demonstrated for ethylene in 2 min averaging time, which is equal to a noise equivalent absorption sensitivity (NEAS) of 1 × 10−8 cm−1 Hz−1/2. An improved three mirror configuration OA-ICOS scheme showed a four times increase in sensitivity as compared to standard OA-ICOS, resulting in a NEAS of 2.5 × 10−9 cm−1 Hz−1/2. The sensor was used for ethylene detection from apples, stored under controlled atmosphere conditions.
    Sensors and Actuators B Chemical 11/2014; 203:311–319. DOI:10.1016/j.snb.2014.06.122 · 3.84 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The mid-infrared wavelength region near 8 μm contains absorption bands of several molecules such as water vapor, hydrogen peroxide, nitrous oxide, methane and acetylene. A new laser absorption sensor based on the ν4 band of methane and the ν4+ν5 band of acetylene is reported for interference-free, time-resolved measurements under combustion-relevant conditions. A detailed line-selection procedure was used to identify optimum transitions. Methane and acetylene were measured at the line centers of Q12 (1303.5 cm−1) and P23 (1275.5 cm−1) transitions, respectively. High-temperature absorption cross sections of methane and acetylene were measured at peaks (on-line) and valleys (off-line) of the selected absorption transitions. The differential absorption strategy was employed to eliminate interference absorption from large hydrocarbons. Experiments were performed behind reflected shock waves over a temperature range of 1200–2200 K, between pressures of 1–4 atm. The diagnostics were then applied to measure the respective species time-history profiles during the shock-heated pyrolysis of n-pentane.
    Journal of Quantitative Spectroscopy and Radiative Transfer 01/2015; 24. DOI:10.1016/j.jqsrt.2015.01.009 · 2.29 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Detection of explosives is an emerging task for maintaining civil security. Optical methods and especially tunable diode laser spectroscopy are discussed as means for providing fast and reliable data. Selective and sensitive detection is possible in the midinfrared spectral region; however, until recently, small and easy to operate laser sources were not readily available for applications outside the laboratory. The situation changes with the maturation of quantum cascade lasers (QCLs). We present detection methods based on photofragmentation and subsequent midinfrared detection of the fragments for the detection of nitrogen-based explosives. For this type of explosive, the very low vapor pressure makes the use of direct spectroscopic techniques extremely difficult, since the equilibrium concentrations are in the ppb to ppt range. Peroxide-based explosives like triacetone triperoxide possess a much higher vapor pressure, making direct absorption spectroscopy and also a quartz-enhanced photoacoustic spectroscopy sensor possible. The progress and challenges of the application of QCLs, also with respect to interferences with other molecules present, are discussed.
    Optical Engineering 11/2010; 49(11):1126-. DOI:10.1117/1.3498771 · 0.96 Impact Factor

Full-text

Download
78 Downloads
Available from
May 29, 2014