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

S. Welzel, G. Lombardi, P.B. Davies, R.A.H. Engeln, D.C. Schram, J. Röpcke

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Achieving the high sensitivity necessary for trace gas detection in the midinfrared mol. fingerprint region generally requires long absorption path lengths. In addn., for wider application, esp. 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 thermoelec. (TE) cooled quantum cascade lasers (QCLs) and detectors. We have investigated the sensitivity limits of an entirely TE cooled system equipped with an .apprx.0.5 m long cavity having a small sample vol. of 0.3 l. With this spectrometer cavity enhanced absorption spectroscopy employing a continuous wave QCL emitting at 7.66 micro m yielded path lengths of 1080 m and a noise equiv. absorption of 2 * 10-7 cm-1 Hz-1/2. The mol. concn. detection limit with a 20 s integration time was found to be 6*108 mols./cm3 for N2O and 2 * 109 mols./cm3 for CH4, which is good enough for the selective measurement of trace atm. constituents at 2.2 mbar. The main limiting factor for achieving even higher sensitivity, such as that found for larger vol. 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 abs. internal absorption calibration, in theory inherent for CRDS expts., are not achievable.
Original languageEnglish
Article number093115
Pages (from-to)093115-1/15
JournalJournal of Applied Physics
Issue number9
Publication statusPublished - 2008


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