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


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.

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    • "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]. "
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