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

Abstract

Röpcke, J. (2008). Trace gas measurements using optically resonant cavities and quantum cascade lasers operating at room temperature. Journal of Applied Physics, 104(9), 093115-1/15. [093115]. DOI: 10.1063/1.3008014 General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. 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 ͑TE͒ cooled quantum cascade lasers ͑QCLs͒ and detectors. We have investigated the sensitivity limits of an entirely TE cooled system equipped with an ϳ0.5 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 m yielded path lengths of 1080 m and a noise equivalent absorption of 2 ϫ 10 −7 cm −1 Hz −1/2 . The molecular concentration detection limit with a 20 s integration time was found to be 6 ϫ 10 8 molecules/ cm 3 for N 2 O and 2 ϫ 10 9 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 ͑CRDS͒ 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.

Direct and wavelength modulation spectroscopy using a cw external cavity quantum cascade laser

“…Furthermore such cw devices are attractive sources for cavity enhanced spectroscopy techniques because unlike pulsed QCLs, they do not suffer from the fast frequency chirping, which has so far prohibited the use of pulsed QCLs in combination with optical cavities. 1 The frequency chirping also limits the resolution of direct absorption spectroscopy studies with pulsed external cavity ͑EC͒-QCLs, but an advantage is that their application with long path cells such as Herriott cells is not hampered by fringing effects. [2][3][4][5][6] True cw sources are generally limited however, in terms of both the wavelengths commercially available ͑ϳ4-10 m͒, and their tuning range ͑of the order of a few cm −1 ͒.…”

Laser-spectrometric gas analysis: CO2–TDLAS at 2 µm

“…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].…”

Stability of widely tuneable, continuous wave external-cavity quantum cascade laser for absorption spectroscopy

“…Currently Fourier transform infrared (FTIR) absorption spectrometers are primarily used for this, but are hampered by low intensity infrared light sources. The high power of the cw EC-QCL laser combined with a wide wavelength scan may allow faster temporal response and higher absorption sensitivity whilst using advanced laser based techniques such as wavelength/frequency modulation spectroscopy [14,15] as well as with high-finesse cavity-enhanced absorption spectroscopy [8,16]. From the limited number of publications it is unclear how well these novel cw EC-QCLs are suitable for such applications.…”