Direct-frequency-comb spectroscopy by a scanning Fabry-Pérot microcavity resonator

We present a direct-frequency-comb spectroscopy method able to resolve the spectral comb structure with an instrumental frequency resolution limited only by the coherence properties of the comb source itself. The proposed technique, based on a 50 000-finesse scanning Fabry-P\'erot microcavity resonator combined with a low-resolution diffraction grating, is characterized using a 250-MHz Er-fiber frequency comb demonstrating an ultimate frequency resolution as low as 50 kHz over an optical bandwidth up to 4 THz and with tens-of-milliseconds measurement times.