RESUMO
Cavity attenuated phase shift Faraday rotation spectroscopy has been developed and demonstrated by oxygen detection near 762 nm. The system incorporates a high-finesse cavity together with phase-sensitive balanced polarimetric detection for sensitivity enhancement and achieves a minimum detectable polarization rotation angle (1σ) of 5.6 × 10-9 rad/âHz, which corresponds to an absorption sensitivity of 4.5 × 10-10 cm-1/âHz without the need for high sampling rate data acquisition. The technique is insusceptible to spectral interferences, which makes it highly suitable for chemical trace gas detection of paramagnetic molecules such as nitric oxide, nitrogen dioxide, oxygen, and the hydroxyl/hydroperoxyl radicals.
RESUMO
Cavity ring-down Faraday rotation spectroscopy (CRD-FRS) is a technique for trace gas measurements of paramagnetic species that retrieves the molecular concentration from the polarization rotation measured as the difference between simultaneously recorded ring-down times of two orthogonal polarization states. The differential measurement is inherently insensitive to nonabsorber related losses, which makes off-resonance measurements redundant. We exploit this unique property by actively line-locking to a molecular transition for calibration-free trace gas concentration retrieval. In addition, we enhance the effective duty-cycle of the system by implementing a Pound-Drever-Hall laser lock to the cavity resonance, which allows for ring-down rates of up to 9 kHz. The system performance is demonstrated by measurements of trace oxygen with a minimum detection limit at the ppmv/âHz-level.