Cavity Ring-Down Spectroscopy (CRDS)
We have been using the newly developed spectroscopic method of cavity ring-down spectroscopy (CRDS) to detect trace gases in the atmosphere.
CRDS is an ultrasensitive spectroscopic method that uses an optical resonator (a group of mirrors) to acheive long effective pathlengths (on the order of 10 km) in short base pathlengths (on the order of 50 cm). We have been using a variant of CRDS that uses a continuous wave laser (CW-CRDS). This method was pioneered by many workers, especially Daniel Romanini and Barbara Paldus, and we have simplified the method for use in atmospheric trace gas detection. This research resulted in a publication on our variant method of CW-CRDS [Schulz and Simpson, 1998].
CRDS allows one to detect compounds usually detected by long-path-absorption spectroscopy with removing the averaging over a long path. This advantage is very significant because it allows the sensor to be a truly in-situ detector and makes it portable, allowing detection of compounds using airplanes, ships, etc. It also allows detection of stable gases at high detection rates. This high-data rate feature allows detection of fluxes via the eddy-correlation technique.
Our primary application of the technique is the measurements of reactive radicals in the Arctic. We have detected the nitrate radical (NO3) at low parts per trillion levels in ambient laboratory air [King et al., 1999].
Our experimental setup is illustrated in the following diagram:
An external cavity diode laser (ECDL) emits single-mode light in the 662 nm region which is circularized with an anamorphic prism pair (AP). The circular beam is deflected with an acousto-optical modulator (AOM) that acts as a high-speed switch. In the on state, light excited the ring down cavity (the four mirrors arranged in a bow-tie configuration). When the solid-state avalanche photodiode (APD) detector indicates that the a sufficient level of light has been pumped into the cavity, the AOM is switched off, and the computer records a ring-down transient.
The ring-down transient is an exponentially decaying waveform and the rate of this decay indicates light losses in the ring-down cavity. The cavity losses result from two phenomena, mirror losses (essentially constant) and absorption or scattering of gases and particles within the cavity. A plot of the light loss rate versus laser wavelength is a CRDS spectrum.
Our setup is pictured below.
