Comb-assisted cavity ring-down spectroscopy

This work presents the technological development, the characterization and some first spectroscopic results of a novel spectrometer, conjugating the benefits of optical cavities with the stable, reproducible and controllable spectral structure of optical frequency combs. In particular, two major comb-assisted continuous wave cavity ring-down spectrometers have been developed. In a first implementation, high frequency accuracy and sensitivity are obtained with a comb-locked cavity ring-down spectrometer op- erating in the near-IR. Key feature of the approach is a wide-bandwidth phase lock between a mode of a frequency comb and an extended cavity diode probing laser by the use of an acousto optical frequency shifter in a feed-forward configuration. A second comb-assisted cavity ring-down configuration has also been implemented in a joint collaboration with Laboratoire interdisciplinaire de Physique (LiPhy), where a frequency comb has been coupled to an existing continuous wave cavity ring-down setup. In this case the probing laser is left free-running and its frequency is calibrated against a self-referenced frequency comb by means of a specifically developed counting routine. In both cases the frequency axis accuracy is at the kHz level. This accuracy is combined with an extremely high sensitivity on the vertical scale, permitting the absolute interrogation of several molecular tagets.

Questo lavoro presenta lo sviluppo di due diversi sistemi di spectroscopia di tipo cavity ring-down assistita da pettine di frequenza. In un primo approccio il laser di sonda è agganciato ad un dente del pettine tramite una tecnica innovativa basata sull'utilizzo di un modulatore acusto ottico in una configuarazione di feed-forward. Nel secondo approccio non viene eseguito nessun aggancio di fase e la frequenza del laser di sonda viene contata grazie ad un pettine di frequenza. In entrambi i casi l'accurattezza dell'asse delle frequenze è al livello del kHz. Questa accuratezza è combinata con un'elevatissima sensibilità sulla scala verticale, permettendo l'indagine accurata di transizioni molecolari molto deboli.