Automated high-resolution frequency-stabilized cavity ring-down absorption spectrometer

We describe a frequency-stabilized cavity ring-down absorption spectrometer with automated scanning capabilities. The system utilizes the comb of ring-down cavity resonances as frequency markers for spectral scans. The comb is actively stabilized with respect to a frequency-stabilized reference laser, and the continuous wave, single-mode probe laser frequency is in turn actively stabilized with respect to resonances of the ring-down cavity frequency comb. Ring-down spectra are obtained by locking the external-cavity diode laser probe to sequential cavity resonances (separated by the ring-down cavity free spectral range), and the ring-down cavity frequency comb is translated by controlled amounts using acousto-optic frequency shifting methods for fine frequency steps less than the free-spectral range. A computer-controlled method for automating frequency tuning and probe laser locking to the cavity resonances is described, and high-resolution near-infrared (vicinity of 10 712 cm(-1)) absorption spectra of water vapor are presented. A frequency resolution of approximate to1 MHz and minimum measurable line intensity of 10(-27) cm(2) cm(-1) molecule(-1) are demonstrated. (C) 2005 American Institute of Physics.