Off-Axis Cavity Ring Down Spectroscopy Based on a Continuous-Wave Optical Parametric Oscillator
Abstract
Continuous-wave cavity ring down spectroscopy (cw-CRDS) is a sensitive absorption technique for trace gas analysis. Although it is highly sensitivity and relatively fast, ring down repetition rate and spectral resolution are limited by the cavity free spectral range (FSR). Normally, the injected beam is mode matched to the lowest transverse electro-magnetic mode (TEM00) of the cavity. Light is coupled into the cavity only when standing wave condition is fulfilled. Scanning of the laser without variation of the cavity length leads to transmission comb where recorded ring down times are separated in frequency by the FSR. Recently Romanini et. al. reported an off-axis (OA) CRDS spectrometer operating in the 766 nm region where the FSR of the cavity was reduced by N = 4 times from the original. In this re-entrant condition the cavity length is chosen to provide degeneracy of transverse modes. If the injection is adequately off-axis the beam returns to the starting point after N round trips. This divides the FSR to N group of degenerated modes which are equally frequency-spaced. We present an OA-CRDS spectrometer (N = 4) based on a continuous-wave optical parametric oscillator (cw-OPO) operating in the mid-infrared region (2.75 - 3.45 μm). The measurement of formaldehyde (H_2CO) using an OA-CRDS spectrometer will be presented.
J. Courtois, A. K. Mohamed and D. Romanini Opt. Express 18, (5), 1 March 2010.- Publication:
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66th International Symposium On Molecular Spectroscopy
- Pub Date:
- June 2011
- Bibcode:
- 2011mss..confETA05P
- Keywords:
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- INFRARED/RAMAN