Zoek in de site...

Cavity Ring Down Spectroscopy (CRDS)

The development of cavity ring-down spectroscopy (CRDS) started in 1988 with a paper published by Anthony O'Keefe and David Deacon [1] in which they adapted a technique to characterize high-reflective mirrors, and used this to measure the small absorption by gases in the visible region. A short laser pulse is coupled into the cavity and is reflected back and forth. Each time the light is reflected a small amount (1-R) leaks out, leading to an exponential decay of the pulse energy in the cavity. This small amount is measured with a fast detector and recorded using a fast data acquisition card. The high sensitivity in CRDS results from the long path lengths of typically several kilometers that can be obtained. CRDS is based on the measurement of the exponential decay of light within the cavity rather than the absolute absorbance [2]. The temporal behavior of the intensity I(t) on the detector is given by:

I(t)=I0e-t/τ, with τ(w)=d/c[(1-R)+α(w)d]

With I0 the initial intensity falling on the detector and τ, the decay constant is called the ring-down time, and is defined as the 1/e decrease of the exponential signal. The ring-down time can be written as a function containing the reflectivity of the mirrors (R), the distance between the mirrors (d), and α(w) the absorption coefficient. The ring-down time for an empty cavity, τ0 for whichα(w)=0, depends mainly on mirror losses and various optical phenomena like scattering and refraction. In the visible and near-infrared ring-down times up to a few hundred microseconds can be reached while in the mid-infrared, decay times are more modest (<13 µs) as the reflectivity is typically less than 99.99%. The absorption coefficient can be calculated from both ring-down times of an empty cavity and filled with an absorber, and is given by:

α(w)=1/c[1/τ(w)-1/τ0(w)]

CRDS

Schematic representation of CRDS

A continuous wave variation of the Cavity Ringdown Spectroscopy (cw CRDS) techniquewas introduced by Romanini et al. in 1997 [3]. Mürtz and coworkers [4] for the first time adapted this technique to the mid-infrared, by using narrow linewidth CO2 laser, emitting light at 10 mm, for ethylene (C2H4) down to a fraction of parts-per-billion by volume (ppbv) detection.

Advantages of a high power (more than 100 mW), together with a narrow linewidth and operation within 3-4 mm region, have made an OPO a promising base for cw CRDS technique. In 2002 in the work of Popp et al. [5] it was described the first cw CRDS trace gas detector employing a cw OPO. They demonstrated a suitability of using an OPO for the ethane (C2H6) measurements at 300 pptv detection limit.

1. O'Keefe, D.A.G Deacon, Cavity ring‐down optical spectrometer for absorption measurements using pulsed laser sources, Review of Scientific Instruments 59 (1988) 2544 -2551.
2. G. Berden, R. Peeters, G. Meijer, Cavity ring-down spectroscopy: Experimental schemes and applications, International Reviews in Physical Chemistry 19 (2000) 565-607.
3. D. Romanini, A.A. Kachanov, N. Sadeghi, E Stoeckel, "CW cavity ring down spectroscopy," Chem. Phys. Lett. 264, 316-322 (1997).
4. M. Mürtz, B. Frech, and W. Urban, "High-resolution cavity leak-out absorption spectroscopy in the 10-mu m region," Appl. Phys. B 68, 243-249 (1999).
5. A. Popp, F. Müller, F. Kühnemann, S. Schiller, G. von Basum, H. Dahnke, P. Hering and M. Mürtz, "Ultra-sensitive mid-infrared cavity leak-out spectroscopy using a cw optical parametric oscillator," Appl. Phys. B 75, 751-754 (2002).