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Intra-pulse Quantum cascade laser spectrometers
Research Prototype
The Strathclyde Intrapulse QC laser spectrometer has been tested in flight at altitudes up to 1500 m. It will be available for further aircraft testing and is expected to be able to be operated in aircraft such as the NERC FAAM aircraft, as well as in aircraft such as the NERC ARSF aircraft in which it has already been tested.
Contact either Dr Nigel Langford, current head of the QC group n.langford@phys.strath.ac.uk or Professor Geoffrey Duxbury g.duxbury@strath.ac.uk
Emerging Technologies
This technology utilizes a pulsed quantum cascade (QC)infrared laser operated close to room temperature. The spectrometers using pulsed lasers fall into two categories, the inter-pulse and intra-pulse approaches. In the inter-pulse method a short current pulse is used to produce quasi-monochromatic output. The frequency tuning is then produced by using a sub-threshold current ramp. This is often called the inter-pulse method, and has been widely used, particularly by the group of Tittel and Kosterev at Rice University, and that of Zahniser and McManus at the Aerodyne Corporation. This method has been described in detail in several recent reviews and papers. We have concentrated on developing the less commonly described intra-pulse method, since it produces a spectrum which can cover a much wider frequency tuning range, and also allows this complete range to be recorded for each current pulse. In an intra-pulse instrument, a long current pulse is used so that the laser frequency sweeps in frequency during the pulse. A complete spectrum of a frequency micro-window is then recorded during each pulse. This method requires a very fast wide frequency bandwidth infrared detector-amplifier combination, and also a fast and efficient digitiser. The resolution of the intra-pulse spectrometer is not determined by the instantaneous line width of the laser, but by the chirp rate and the Fourier transform, time-frequency, limitation. Apart from the large micro-window available in the intra-pulse method, the other advantage of the use of frequency-chirped pulsed lasers is that it minimises the effects of interference fringes, which occur when multiple pass cells are used for absorption spectroscopy. The portable instrument was developed with funding from the COSMAS programme of the National Environmental Research Council (NERC). This instrument has been tested in a laboratory environment both at the University of Strathclyde, and at the University of Bristol and the University of Bielefeld. The most recent aircraft deployable version of the spectrometer contains a distributed feed-back QC laser operating around a wavelength of 7.84 ┬Ám (a wavenumber of 1275 cm-1) and an astigmatic Herriott cell having a path length of 100 m. The frequency resolution of our system is determined by the changing rate of frequency down-chirp during the pulse. In these experiments it ranged from 0.01 cm-1 at the start of the pulse to 0.005 cm-1 at the end. The intra-pulse scan range is from 2.5 to 4 cm-1. Within this range we can detect ambient concentrations of methane nitrous oxide and water. Owing to the wide scan range we can observe three strong lines of nitrous oxide, three of methane and two of water. The variation of the concentration of methane was observed during a low level flight path of the NERC ARSF Dornier aircraft from Oxford Airport to Haverfordwest, Pembrokeshire, and also in a circuit over Oxfordshire, in October 2006. The signal to noise ratio obtained, once the equipment stabilized, was close to they obtained in a laboratory environment.
Geoffrey Duxbury
University of Strathclyde
Scottish Universities Physics Alliance, Department of Physics, John Anderson Buiding, 107 Rottenrow, Glasgow, G4 0NG, Scotland, UK.
DeveloperPI and developer
Development SectorAcademia
R&D ProgramYes
Years Till Available3-5
Investment Required$100k-$1M
Projected ApplicationAtmospheric Research, Industrial Research
Unit Cost$10k-$100k
Key RisksAvailability of QC lasers and of fast, wide bandwidth (>400MHz) infrared detectors
Ease of Usesingle operator
1. Review of uses of QC laser spectrometers. G. Duxbury, N. Langford, M.T. McCulloch and S. Wright, Chem. Soc. Rev, 34, 1 (2005) 2. First flight of aircraft borne instrument, S. Wright, G. Duxbury and N. Langford, App. Phys. B, 85, 243-249 (2006) 3 Test of sensitivity of the technique for detecting reactive processes. A. Cheesman, J.A. Smith, M.N.R. Ashfold, N. Langford, S. Wright and G. Duxbury, J. Phys. Chem. A 110, 2821 (2006) 4. Recent flights of the instrument in October 2006. K.G. Hay, N. Langford and G. Duxbury, (to be published 2007)
Commercial spectrometers of similar type have been developed by Cascade Technologies, a spin-out company fom the University of Strathclyde, for the detection of smokestack emissions and are currently being tested onboard ships. see www.cascade-technologies.com contact Dr Iain F Howieson Chief Technology Officer Cascade Technologies Unit A, Logie Court Stirling Innovation Park Stirling, FK9 4NF Scotland, UK i.howieson@cascade-technologies.com
12 Oct 2009 16:08