4  Cavity ring-down and cavity enhanced spectroscopy using diode lasers

Abstract

Continuous wave (cw) diode lasers are increasingly being used as light sources in the visible and near-IR regions of the spectrum for cavity ringdown spectroscopy (CRDS) and cavity enhanced absorption spectroscopy (CEAS); the latter technique is also widely known as integrated cavity output spectroscopy (ICOS). The very high sensitivities to weak absorptions that are possible with cw CRDS and CEAS, coupled with the quantitative nature of the absorption measurements, are enabling a rapidly expanding range of applications. We review the benefits and practical implementation of these techniques; methods of data analysis for extraction of quantitative absorption data; the sensitivities of cw CRDS and CEAS, and how they might be optimised; and applications of cw CRDS and CEAS in molecular spectroscopy, atmospheric chemistry, plasma and flame chemistry, analytical science, and medical diagnosis via breath analysis. The development of CRDS and CEAS techniques exploiting cw diode lasers and, very recently, high luminosity light-emitting diodes, has stimulated a wealth of highsensitivity measurements. Highlights include quantitative measurement of various ultra-trace gases such as: NO 3 , NO 2 and ethene in ambient air samples; CO 2 isotopologues, ethane and other organic compounds in human breath samples; and excited electronic states of N 2 and O 2 in plasmas and discharges. Exciting developments include wavelength extension into the mid-IR and UV regions, and use of novel locked-cavity techniques to increase data acquisition rates and sensitivities.

Trace species detection in the near infrared using Fourier transform broadband cavity enhanced absorption spectroscopy: initial studies on potential breath analytes

“…acquiring and processing the interferogram). Baseline spectra are acquired by evacuating the optical cell before and after each set of measurements, and the cavity enhanced absorption spectra are obtained by treating the data according to the following equation [26]:…”

Near-infrared broad-band cavity enhanced absorption spectroscopy using a superluminescent light emitting diode

“…20 Background spectra are acquired by pumping down the vacuum vessel, and the wavelength dependent mirror reflectivity was determined by comparing the single pass data with those of the cavity enhanced data.…”

A cavity ring down/cavity enhanced absorption device for measurement of ambient NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub>

“…The sliding mean of 64 ring-down profiles (each 10 000 data points) was transferred to a PC at 3 Hz. The effective time resolution was ≈5 s. The cavity loss due to absorption at 662 nm is calculated from the change in ring down constant in the presence of an absorber and converted to a concentration of NO 3 using the effective cross section of NO 3 at the experimental temperature and the established relation (Berden et al, 2000;Mazurenka et al, 2005).…”

“…It is operated either in "cavity ringdown" (CRD) mode or "cavity enhanced absorption" mode (CEAS), which is closely related to "integrated cavity output spectroscopy" (ICOS). The basic principles of CRD and CEAS are described in recent, comprehensive review articles (Berden et al, 2000;Mazurenka et al, 2005). Here, we present a detailed description of the prototype device, results of laboratory characterisation, first measurement of N 2 O 5 (+NO 3 ) mixing ratios outside the Max Planck Institute building in Mainz and some results from the first deployment in a multi-instrument NO 3 /N 2 O 5 intercomparison campaign.…”