Since 1994, we have been developing and applying variants of the ultrasensitive absorption spectroscopy technique of cavity ring-down spectroscopy (CRDS). Early applications included the study of predissociation of electronically excited states of small molecules and radicals such as HNO, FCO, SH, S2,ClO and IO, and some of this work paved the way for atmospheric sensing of trace radicals such as the halogen monoxides by CRDS. More recently, we have applied near-IR and mid-IR CRDS methods to sensing of trace atmospheric constituents, and have extended the use of CRDS to the study of optical properties of single aerosol particles. Our aerosol studies are carried out in collaboration with Prof J.P. Reid and are described in a separate section of this site.
A review of the use of continuous wave (cw) diode lasers in CRDS and cavity enhanced absorption methods, written by members of the Bristol group and colleagues from Oxford University (M. Mazurenka et al., Ann. Rep. C 101, 100 (2005)), appeared in the RSC journal Annual Reports C. The cover picture shows various modes of a high finesse cavity such as is used in CRDS experiments.
Cover of the 2005 issue of Annual Reports C showing the mode structure in an optical cavity used for CRDS.
CRDS based methods are now being widely developed for trace gas sensing in atmospheric, medical and industrial chemistry applications because of their capability for quantitative and calibration-free retrieval of absolute concentrations at mixing ratios in the parts per billion to parts per trillion range. The methods make use of a high-finesse optical cavity to establish absorption pathlengths of many kilometres in a compact instrument. We have been at the forefront of development of such methods using modern, compact and low power solid state lasers. The Bristol group was the first to report atmospheric sensing of NO2 and CH4 using diode laser based CRDS instruments operating in the visible and near-IR, and has since shown that mid-IR quantum cascade lasers can be efficiently coupled to an external optical cavity by a simple optical feedback design. This advance opens up applications of CRDS in the mid-IR “fingerprint” region for trace sensing of small organic and inorganic compounds in ambient air, breath samples for medical diagnosis, and industrial process gases. The figures below show a schematic diagram of the V-shaped cavity used for optical feedback cavity enhanced absorption spectroscopy (OF CEAS) in the mid infra-red, and a sample spectrum of air at wavelengths around 7.8 µm. The observed spectral lines are assigned to methane and nitrous oxide.
The V-shaped cavity design used for OF-CEAS with a quantum cascade laser. The laser is injected into the cavity from the right side of the figure.
Mid-IR optical-feedback CEAS spectrum of N2O (320 pb) and CH4 (1.8 ppmv) bands in ambient air. The spectrum was obtained using a cw quantum cascade laser.
Some of our work in development of mid IR spectroscopy techniques with QCLs is in collaboration with Cascade Technologies Ltd. We are also grateful to he Natural Environment Research Council (NERC) and the Analytical Trust Fund for financial support.
Representative publications:
A quantum cascade laser based optical feedback cavity-enhanced absorption spectrometer for the simultaneous measurement of CH4 and N2O in air, D. Hamilton and A.J. Orr-Ewing, Appl. Phys. B 102, 879-890 (2011)
Cavity ring-down and cavity enhanced spectroscopy using diode lasers, M.I. Mazurenka, A.J. Orr-Ewing, R. Peverall and G.A.D. Ritchie, Annual Reports C 101, 100 – 142 (2005).
Mid-IR ethane detection using difference frequency generation in a quasi-phase matched LiNbO3 waveguide, R. Grilli, L. Ciaffoni, G. Hancock, R. Peverall, G.A.D. Ritchie and A.J. Orr-Ewing, Applied Optics, 48, 5696-5703 (2009).
