A NEW BROADBAND CAVITY ENHANCED FREQUENCY COMB SPECTROSCOPY TECHNIQUE USING GHz VERNIER FILTERING.

We present a new approach to Cavity Enhanced - Direct Frequency Comb Spectroscopy where the full emission bandwidth of a Titanium:Sapphire laser is exploited at GHz resolution. The technique is based on a low-resolution Vernier filtering obtained with an appreciable --actively stabilized-- mismatch between the cavity Free Spectral Range and the laser repetition rate, using a diffraction grating and a split-photodiode footnote{Rutkowski it{et al}, Opt. Lett., 39(23)2014}. This particular approach provides an immunity to frequency-amplitude noise conversion, reaching an absorption baseline noise in the 10$^{-9}$,cm$^{-1}$ range with a cavity finesse of only 3000. Spectra covering 1800 cm$^{-1}$ ($sim$ 55 THz) are acquired in recording times of about 1 second, providing an absorption figure of merit of a few 10$^{-11}$,cm$^{-1}$/$sqrt{Hz}$. Initially tested with ambient air, we report progress in using the Vernier frequency comb method with a discharge source of small radicals. vspace{2cm