Mode-by-mode optical feedback: cavity ringdown spectroscopy

Abstract

We describe a new continuous wave cavity ringdown spectroscopy (cw-CRDS) approach using an extended cavity diode laser (ECDL) optically self-locked to a high finesse cavity including an intracavity glass plate under the Brewster angle. Low noise, mode-by-mode absorption spectra are recorded at a high acquisition rate (laser frequency scan greater than 400 GHz/s) and covering four orders of magnitude in absorption coefficient. Sampling spectra with the fixed high finesse cavity frequency comb provides high precision frequency markers. An original scheme for the laser beam shut-down, based on signal shape analysis and the diode laser injection current control, is presented. This scheme avoids any supplementary switching device. To retrieve ringdown processing at a kilohertz rate several exponential decay fit algorithms are compared. Performances of this new scheme are demonstrated with the observation of very weak lines of the oxygen B-band around 680 nm. Atmospheric spectra of isolated lines averaged for less than 10 s show a baseline noise of 5×10-10 cm-1 and a single point minimum detectable absorption loss over a one-second measurement interval of 2×10-10 cm/\(\sqrt{\text{Hz}}\) is obtained.