In cavity ring-down spectroscopy (CRDS) a sample chamber is constructed with highly reflective mirrors at each end.  A laser is aligned in the 
chamber so that a light pulse will be reflected between the mirrors many times, with a small amount of radiation leaking out through the end mirror on 
each reflection.  This leads to an exponential decay of the signal intensity in the chamber over time.  A detector outside the chamber measures the decay
in light intensity that comes through the end mirror, and a computer is used to fit this signal to an exponential function.  If an absorbing species is present 
in the cavity, the signal decays faster, and this change in decay rate is easily detected.  Thus a spectrum can be obtained by measuring the decay rate 
of the laser signal as a function of its frequency.  This leads, with the help of  a computer, to an absorbance spectrum.
	Because the laser traverses the (usually gas phase) sample thousands of times at each frequency in a CRDS experiment, the effective path lengths that 
are obtained are often on the order of kilometers.  This leads to extremely high sensitivity for this technique so that normally forbidden transitions may be 
observed.  In addition, the spectra of weak isotopic species can be observed in natural their abundance.  The CRDS technique is very versatile since the major 
requirements are simply a tunable laser and high reflectivity mirrors in the frequency region of interest.  The technique has been applied in the UV, visible and 
infrared regions.  Although the original CRDS spectrometers utilized pulsed lasers, continuous wave (CW) lasers have also been implemented more recently.  
Some links to more detailed sites related to CRDS are given below.

Bristol CRDS site
A good site describing both the CRDS technique and some interesting applications, includes pictures and 
a good animation.

Bananas, Explosives and the Future of Cavity Ring-Down Spectroscopy
An article in Optics and Photonics News, including references, describes the technique and analytical 
applications.

Berkeley CRDS site
A site associated with the Saykally group at UC Berkeley, contains more pictures, links and references.

Los Gatos Research
A company that specializes in mirrors and other equipment for CRDS, also includes references and other info.



Last updated: 6/16/04