High-resolution cavity leak-out absorption spectroscopy in the 10-μm region

We have examined a novel approach to obtain molecular absorption spectra with high resolution and high sensitivity in the 10-mu m region. An external high-finesse cavity is excited on a single fundamental mode with a narrow-linewidth CO2 laser. After excitation, the laser power is turned off for a short time and the subsequent decay of the radiation stored in the cavity is observed via detection of the light leaking out through one of the cavity mirrors. Measurement of the decay time allows one to determine the photon losses und thus to detect weakly absorbing species inside the cavity. Since the cavity is frequency-locked to the laser the decay time can be probed with a high repetition rate, basically limited by the sampling rate of the analog-to-digital converter. This approach is closely related to cavity ring-down spectroscopy with pulsed lasers, but exhibits several advantages concerning spectral resolution and detection sensitivity. As a practical example we demonstrate monitoring of trace amounts of ethylene. Using R = 99.5% mirrors we achieve a detection limit of 1 ppb ethylene (integration time: 100 s) corresponding to absorption losses of 3 x 10(-8)/cm. Further improvement is feasible when mirrors with higher reflectivity become available.