Cavity-Ringdown Laser Absorption Spectroscopy

(CRLAS) [also written as "Cavity Ring-Down as well as CRDS] was developed by Drs. Anthony O'Keefe and David Deacon in the mid-1980's as a means of measuring very weak molecular absorption signals using a pulsed light source. The technique is founded upon several earlier schemes used to characterize ultra-high reflectivity mirror coatings^1-2. These important papers are rarely recognized in the current chemical literature, but clearly form the basis of several currently used variations of the technique. These earlier techniques were based upon the use of cw-laser sources with very narrow bandwidths, and thus very long coherence lengths associated with the laser radiation.

In these early schemes, the mirrors to be characterized were configured into a two mirror linear (or three mirror ring) cavity with the HR coatings facing each other. The laser light was injected into the cavity through the backside of one of the mirrors and the injection laser quickly (e.g. on the order of nanoseconds) shut off using a Pockles cell. The decay of the light injected into the cavity was monitored using a photodetector behind one of the other mirrors. The RATE of optical decay could be easily related to the mirror reflectivity and the dimensions of the cavity. The major limitation in these early approaches was that the coupling of narrow frequency modes between the laser cavity and the ringdown cavity was somewhat chaotic because the two cavities could drift independently. Because the injected intra-cavity energy built up slowly over many microseconds, the ringdown cavity acted as a classical etalon and the optical buildup was very irregular as the cavities drifted in-and out of resonance. This resulted in a noise level which permitted measurements of mirror reflectivity to ~99.99%.

We recognized that we could achieve two important goals in such measurements by using a pulsed laser source as our injector. First, by using a laser pulse shorter than the ringdown cavity round trip length, we could avoid the spiking amplitude noise resulting from the strong interference effects of a spatially overlapped coherent light beam within the ringdown cavity. Secondly, the use of a pulsed laser (e.g. a pulsed dye laser) provides much greater optical powers resulting in a larger signal emerging from the ringdown cavity. Our first papers introduced the technique as a sensitive means of making spectroscopic absorption measurements³ and, in an obvious extension, as a method of making quantitative trace concentration level analytical measurements⁴. These works established that this new approach was capable of absorption measurements with sensitivities of much better than one part in a million, and that the technique could be used easily to measure quantitatively trace species concentrations in air, electrical plasmas, and even within solid materials⁵. Shortly after these introductory papers we published the first paper (along with our collaborators in the Chemistry Department of U.C. Berkeley) in which we first used the technique to make fundamental absorption measurements of supersonically cooled gas phase copper clusters⁶. Since our first papers in the late 1980's many scientific researchers around the world have employed this technique. Many applications in process control and semiconductor processing are under development.

After a long coating development search we were able to extend the CRLAS technique into the mid-infrared spectral range⁷. This spectral region is of great potential interest for both scientific and applied markets because many molecular species have strong absorptions there. The development was difficult because of several factors. The thick dielectric coatings required are difficult to make with the required surface and thickness precision. Many coating materials being used for infrared dielectric mirrors resulted in highly toxic waste disposal problems, making their cost prohibitive. Los Gatos Research now offers selected mirror coatings over a wide spectral range in the infrared. We have published several studies in which we demonstrate the technique in the infrared, present detailed experimental approaches to the problem, and discuss the unique problems which arise in operation (detector problems, etc.)⁸.

Ringdown Spectral Photography^9-10

We have demonstrated a new technique which combines the CRD detection scheme with the use of a spectrally broad laser pulse, and where the time decaying output of the cavity is spectrally dispersed and recorded by a CCD. The output of the cavity is reflected off a rapidy roatating mirror, onto a grating, and then to a 2-D detector array. One axis of the array records the time decaying amplitude of the signal. The other axis corresponds to the spectral dispersion of the output. Recorded in a single pulse, the data provides wavelength resolved Cavity Ringdown spectra of any intra-cavity absorptions. Using a broad band dye laser, we can produce tens of nanometers of injection bandwidth. The approach has been demonstrated using the weak oxygen b-X (1,0) band as a test absorption gas. The time domain results are plotted below, where the broadband ICOS spectrum is also evident in the intensity (false color) spectrum superimposed upon the CRD results.