title: High-speed off-axis Cavity Ring-Down Spectroscopy with a re-entrant configuration for spectral resolution enhancement.
abstract: Monitoring of changing samples by Cavity Ring-Down Spectroscopy (CRDS) is possible using fast frequency scans of the laser and/or the cavity resonance. Mode-matched cavity excitation improves performance of fast CRDS but data-points result separated by the cavity Free Spectral Range (FSR): low pressure samples demand long cavities. We demonstrate fast CRDS with off-axis injection of a "re-entrant" resonator yielding FSR/N data-points separation. Our N = 4 short-cavity setup is found to perform well compared with other fast-CRDS implementations. Interestingly, the intrinsic chirped ringing affecting ring-down signals in mode-matched fast-CRDS disappear with off-axis injection. This is due to a fine splitting of the re-entrant-cavity degenerate groups of modes by astigmatism.
authors.affiliation: Office National d'Etudes et de Recherches Aérospatiales, Chemin de la Hunière, 91761 Palaiseau Cedex, France. jeremie.courtois@onera.fr
authors.forename: Jérémie
authors.initials: J
authors.lastname: Courtois
authors.forename: Ajmal Khan
authors.initials: AK
authors.lastname: Mohamed
authors.forename: Daniele
authors.initials: D
authors.lastname: Romanini